Category Archives: Insects

Arthropods vs. Cane Toads

Cane toads are toxic because their bodies are loaded with cardiac glycosides, deadly toxins that can stop a predator’s heart. Because the toads are non-native in Australia, the native Australian carnivores aren’t adapted to dealing with them. For some, this is very bad news: freshwater crocodiles, monitor lizards, and pythons have all experienced population declines since the introduction of cane toads in 1985 (Smith and Phillips 2006).

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A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Not all is lost, however. It turns out that cardiac glycosides are only toxic to a very narrow group of animals: vertebrates. Predatory insects, arachnids, and other invertebrates have no trouble at all with the poison. Furthermore, a study published earlier this year (Cabrera-Guzmán et al. 2015) revealed that many are more than capable of tackling amphibian prey.

Cane toads start their lives as tadpoles, small and innocent, but plenty toxic enough to kill a hungry frog or fish. In Australia, some of their top predators are giant water bugs and water scorpions. Both are insects (not scorpions) that use tube-like mouthparts to inject acid and digestive enzymes into their prey, dissolving them from the inside out. When the tadpole’s innards are sufficiently liquefied, the insects slurp them up like an amphibian milkshake.

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A water scorpion lying in wait for prey. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A young dragonfly's mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

A young dragonfly’s mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

Dragonfly larvae are also aquatic and predatory, but instead of using acid, they have extendable mouthparts. These are built like the robotic arm on an automatic garbage truck, but less cumbersome and more of a surgical, spring-loaded instrument of death. Most dragonfly larvae eat other insects, like mosquito larvae, but the largest species can easily overpower a small fish or tadpole.

Diving beetles join in the fun. Their mouthparts are less exciting, more like a typical beetle’s with sharp, biting mandibles. What makes them special is speed: their bodies are constructed like those of sea turtles. Like sea turtles, diving beetles are hard-shelled, streamlined, and aquadynamic. Unlike sea turtles, diving beetles use this form to swim after tadpoles that aren’t quite fast enough to escape.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

Diving beetle larvae are just as fierce, and they too have been observed feeding on cane toad tadpoles. Unlike their parents, larvae are long-bodied, with curved, needle-like jaws which they use to inject digestive enzymes into their prey (like the water bugs).

Any cane toad tadpoles that survive this massacre can metamorphose into toadlets, but until they reach their adult size (4-6 inches) they are still at the mercy of their invertebrate predators.

Experiments and observations in the field (Cabrera-Guzmán et al. 2015) have revealed that crayfish are efficient predators of eggs, tadpoles, toadlets, and even adult toads. Australia is home to 151 species of crayfish, including several of the largest species on earth. The spiny crayfish (Euastacus) in particular, some of which can grow to more than a foot in length, prey not only on toadlets but also on full-sized, adult cane toads.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

There are, believe it or not, spiders that specialize in running out over the surface of the water to snatch aquatic insects, tadpoles, and small fish. They are the fishing spiders (large ones are sometimes called dock spiders). Experiments have shown that when fishing spiders inhabit a pond, up to 1 in every 4 tadpoles ultimately becomes spider food (Cabrera-Guzmán et al. 2015).

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

Finally, ants. In the cane toad’s native range of tropical Latin America, meat ants are a major predator. When a toad is attacked, it often stays still, relying on poison for protection. Ants take advantage of this strategy, swarming over the toad’s body and stinging it to death with poisons of their own. Meat ants (Iridomyrmex) and their relatives also live in Australia, and they have been seen dragging the dismembered remains of cane toads back to their nests.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

I’m sorry to say giant centipedes did not make the list of cane toad predators in Australia, but I should mention that the Caribbean giant centipede (Scolopendra alternans) has been observed to prey on native cane toads (Carpenter and Gillingham 1984).

Whether bird-eating spiders, bat-snatching centipedes, or tadpole-chasing water bugs, invertebrates that prey on vertebrates are always fascinating. It’s more common than you might think! I’ll conclude by mentioning Epomis, an unusual genus of ground beetles. Both the beetle larvae and adults are specialist amphibian-eaters, and tackle frogs and toads many times their own size.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

I won’t say any more, since Epomis expert Gil Wizen has already written a fantastic blog post about these beetles, complete with videos of predation in action! I encourage you to check it out here.

Cited:

Cabrera-Guzman E., M.R. Crossland, and R. Shine. 2015. Invasive cane toads as prey for native arthropod predators in tropical Australia. Herpetological Monographs 29(1): 28-39.

Carpenter C.C. and J.C. Gillingham. 1984. Giant centipede (Scolopendra alternans) attacks marine toad (Bufo marinus). Caribbean Journal of Science 20: 71-72.

Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.

Wizen G. and A. Gasith. 2011. Predation of amphibians by carabid beetles of the genus Epomis found in the central coastal plain of Israel. ZooKeys 100: 181-191.

Nicotine, a Natural Insecticide

Nicotine, the addictive agent in cigarettes, comes from the leaves of tobacco plants (Nicotiana). Plants, of course, do not manufacture nicotine as a favor to smokers, but for their own benefit: nicotine is one of the most powerful insecticides in the world, highly effective at stopping hungry, leaf-munching pests in their six-footed tracks.

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Tobacco flowers and leaves. Photo by Joachim Mullerchen, licensed under CC BY 2.5.

Nicotine is a neurotoxin, and to understand how it works, you’ll need to understand a few things about the nervous system. Nerves are simply long, thin cells that run through your body, carrying signals as electrical impulses. Here’s the problem: at the junction between two nerve cells, or between a nerve and a muscle cell, there’s a space across which electrical impulses cannot travel. This space is called the synapse.

To keep the message going, the signal-sending nerve cell sends out an army of molecules called neurotransmitters, who boldly drift across the synapse like astronauts from space-ship to satellite. When a neurotransmitter arrives at the receiving nerve or muscle cell, it enters through a tube-shaped receptor molecule.

There are many kinds of neurotransmitters, each with its own unique receptor. Take acetylcholine, which carries signals from nerve cells to muscle cells. When you recoil from a hot stove, it’s acetylcholine that tells your muscles to get moving.

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A synapse between two nerve cells. Figure in public domain.

If a poison kept all your acetylcholine receptors closed, you would be paralyzed: your muscles wouldn’t get any signals from the nervous system. If, on the other hand, the toxin kept receptors constantly open, your muscles would be constantly trying to move. You would go into convulsions, unable to control your body. Eventually you would exhaust yourself and die.

That’s how nicotine works (Zevin et al. 1998). In small doses, like in a cigarette, it works as a mild stimulant, keeping a few more receptors open than usual. In massive doses, like when a caterpillar eats a tobacco leaf, it works like a doorstop, keeping all acetylcholine receptors wide open. The unfortunate insect convulses, contracting all its muscles simultaneously until it runs out of energy and expires.

For many years, farmers used nicotine as a pesticide, spraying it over their crops to poison any hungry insects. However, nicotine is quite toxic to mammals, including humans. It isn’t sold as a pesticide anymore in the U.S. or Europe, replaced by neonicotinoids. The new pesticides are similar to nicotine, highly effective, and work in the same way, but are safer for people (not insects, of course).

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The tobacco hornworm. Photo by Tom Murray, used with permission.

There are insects that eat tobacco leaves, and those insects have acquired an immunity to nicotine. The best-known example is the tobacco hornworm (Manduca sexta), a big, fat, bright green caterpillar that ultimately transforms into a hawkmoth. In fact, these caterpillars have evolved the ability to store nicotine in their own bodies, making themselves toxic to caterpillar-eating predators. Experiments have shown that wolf spiders normally avoid tobacco hornworms, but if the caterpillars are fed a diet lacking in nicotine, the spiders attack without hesitation (Kumar et al. 2013).

Cited:

Kumar P., S.S. Pandit, A. Steppuhn, and I.T. Baldwin. 2013. Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46’s role in a nicotine-mediated antipredator herbivore defense. Proceedings of the National Academy of Sciences U.S.A. 111(4): 1245-1252.

Zevin S., S.G. Gourlay, and N.L. Benowitz. 1998. Clinical pharmacology of nicotine. Clinics in Dermatology 16(5): 557-564.

Malaria, Climate Change, and the Next Top Model

by Joseph DeSisto

Both malaria and climate change are complex global problems that scientists are working hard to understand. Malaria, a mosquito-borne disease, kills roughly 600,000 people every year, mostly in Africa. Attempts to control malaria have been massive and relatively well-funded, but the disease continues to pose a serious threat to human life in the tropics. Climate change has no body count (yet), but will dramatically change our planet, altering weather patterns, sea levels, and human life.

These two problem have another thing in common: scientists use mathematical models to try and predict how they will change in the future. For example, climate scientists use past and current data on temperature, carbon emissions, and many other factors, to predict how the earth’s climate will change. Most models anticipate a global temperature increase of 2̊ to 6̊  by the year 2100 (e.g., Paaijmans et al. 2014). Those predictions may not be perfectly accurate, but that is the nature of predictions, and in the absence of time travel, it’s the best we can do.

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Anopheles minimus, a malaria-transmitting mosquito. Photo by James Gathany, in public domain.

Malaria, the microbes that cause it, and the mosquitos that carry it, are all affected by temperature. Scientists have tried developing models to predict how malaria infection rates will change as the world gets hotter. In general those predictions have been bleak: models tend to show an increase in the amount of people affected by malaria (e.g. Pascual et al. 2006, Paaijmans et al. 2014), on the basis that warmer temperatures are most hospitable to malaria-carrying mosquitos. They also show malaria spreading to areas that used to be too cool, like South Africa and the southeastern United States. Yet other models show a decrease in malaria on a global scale (Gething et al. 2010), and some suggest an increase in some areas but a decrease in others (Rogers and Randolph 2000).

Before looking forward, it’s important to look back – how has malaria changed in the last few years? The maps below show malaria infection rates (the percentage of people infected) across Africa, in 2000 (left) and 2010 (right). Darker colors mean more malaria.

 

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Photo by Noor et al. (2014), licensed under CC BY 3.0.

The two maps may not look all that different, but look closely. The red arrow points to West Africa, where the darkest area has grown smaller. Malaria is still common there, but has declined. In Kenya and Tanzania (purple arrow), malaria has declined even more. Even though efforts to completely eradicate malaria have failed, improved mosquito control programs have been successful in reducing the threat of malaria in these regions of Africa.

A model is basically an equation, and in the case of malaria it serves to calculate R0: the basic reproduction number. The basic reproduction number is the answer to the question, for each person already afflicted with malaria, to how many other people will their infection spread? This gives us an idea of how many people are likely to contract malaria in a given year.

For example, if R0 is 15, that means every person with malaria is likely to pass it on to fifteen other people – that’s about the R0 value for measles. If R0 is 0.5, then on average, half of all malaria patients will pass their infection on to another. For all diseases, when R0 is less than 1.0, the disease will decline, and if R0 is greater, the disease will increase in the population. Very high values of R0 can lead to pandemic.

Malaria is a complicated disease, affected by all three players: humans, microbes (more on them later), and mosquitos. In turn, each of these players are affected by temperature, moisture, population density, and other local conditions. As a result, malaria’s basic reproductive number varies depending on where you are. In 2007, a group of scientists attempted to calculate R0 for 121 different human populations in Africa (Smith et al. 2007). The study had two important results.

First, R0 varied wildly across Africa. Although the average value was near 115, many populations had values below 10 and many more had values over 1,000. Second, some R0 were extremely high, approaching 10,000. Remember what this means – on average, each person with malaria has the potential to transmit their malaria to 10,000 other people!

In some cases the R0 value was greater than the human population, suggesting that everyone in the population had malaria when of course this was not the case. Is there a problem with the model? The scientists in question didn’t think so. Instead they pointed out that although malaria’s R0 is much higher than for most other diseases, malaria also takes much more time to spread (on average, 200 days per generation) because of its complex life cycle.

The parasites that cause malaria are single-celled, apparently simple, but with strange and complicated lives. I say parasites, plural, because there are at least five species that cause malaria, but all are protozoans in the genus Plasmodium. The cycle, of course, has no “beginning,” but we will start with the oocyst, a kind of “egg sac” containing multiple Plasmodium cells and surrounded by a protective membrane.

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A Plasmodium sporozoite (purple). Image by Ute Frevert; false color by Margaret Shear, licensed under CC BY 2.5.

The oocyst lives in the body of a female Anopheles mosquito, and by the time the mosquito lands for a blood meal, the membrane bursts. Single-celled Plasmodium parasites surge forth, down the mosquito’s mouthparts, and into the blood-stream of an unsuspecting human. The freed cells are called sporozoites; they are worm-like and active, and waste no time snaking their way through the host’s body until they reach the liver.

Here the parasites eat, grow, and change shape. They divide and transform into masses of globular cells, similar to oocysts, once again enveloped in a bag-like protective layer. As time goes on, liver becomes tiresome, and the parasites crave blood. The bag of cells bursts, and the Plasmodium cells return to the blood stream. Now they have a new mission: find a red blood cell.

Red blood cells are hollow and doughnut-shaped, like inflatable inner tubes. You use them to transport oxygen and carbon dioxide into and out of your body, with every breath. The blood cell’s membrane is thin, almost fluid, and easy for a Plasmodium cell to invade. Once inside, the parasite has two options. It can grow and divide, forming a new mass of cells (like the oocyst). If it does so, these cells will ultimately break out of their shelter to find new red blood cells of their own. More ambitious parasites, however, refrain from dividing. Instead they metamorphose, transforming into either male or female cells.

For the Plasmodium life cycle to complete, a second mosquito is required. The new mosquito lands on a malarial host, sucking up blood and with it, lots of red blood cells. Some of these are empty, but others, if Plasmodium is lucky, contain either male or female hitchhikers. In the mosquito’s digestive system, red blood cells burst open to release their passengers. The freed Plasmodium cells, male and female, meet and unite. Once they do, they are able to multiply and grow into a multi-celled oocyte, full of wriggling sporozoites ready to enter a new human host at the mosquito’s next meal.

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A mosquito in the genus Anopheles, capable of transmitting malaria. Photo by Jim Gathany, in public domain.

For this system to work, mosquitos not only have to be infected with Plasmodium, but sporozoites have to be fully developed and ready to pounce when opportunity (i.e., a bite) comes along. Lots of mosquitos need to be biting people, so that at least some will slurp up the male and female cells when they are ready. Mosquitos need to have reasonable lifespans – those that meet their end in a frog’s belly or a spider’s web are of no use to Plasmodium. The mosquitos, in turn, have requirements of their own: they need optimal growing temperatures, pools of water in which to lay their eggs, and plenty of warm-bodied hosts from which to drink.

All of these factors (and many more) are variables that might appear in an equation to calculate R0. Smith and colleagues (2007) used them to calculate the R0 for all those 121 African populations. More recently, a group of scientists and mathematicians got together to predict how R0 will change with the earth’s climate (Ryan et al. 2015).

In general, both mosquitos and Plasmodium can only develop at temperatures between 63̊ and 93̊ F. That’s good, useful information, but not enough — each of Smith’s variables (mosquito life span, number of bites per person, etc.) is affected by temperature, but each in a slightly different way. Only by combining all of these factors, and considering how each will change under future climate conditions, can we accurately predict how the threat of malaria will change over time. That’s what Ryan and colleagues did, and published this month in Vector-Borne and Zoonotic Diseases.

It turns out that if you combine all those variables, you get a much more complex picture of how climate change and malaria interact. Although some areas will get warmer and more suitable for malaria, others will actually get too hot, so malaria will decline.

This particular study shows a decrease in malaria in West Africa, but an increase in East Africa. In other words, the hot-spot for malaria will shift east over the next six decades.

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From Ryan et al. (2015), licensed under CC BY 4.0.

In the maps above, purple indicates the greatest increase in malaria transmission rates, while the palest tone indicates a decline in malaria.

Caution is important. When different models produce vastly different results, it usually means that some of those models are better than others. This model happens to consider more factors than many others, which suggest it may be more accurate, but as I have tried to show, malaria is a complex disease that will be affected by temperature in complex, hard-to-predict ways.

Having an idea of what the future might hold can inform us – where should we concentrate efforts to control malaria? Where will malaria pose the greatest threat to human health? This latest model suggests our focus will have to shift as Plasmodium, mosquitos, and malaria follow their optimal temperatures in an eastward march across Africa.

Cited:

Gething P.W., D.L. Smith, A.P. Patil, A.J. Tatem, R.W. Snow, and S.I. Hay 2010. Climate change and the global malaria recession. Nature 465(7296): 342–346.

Noor A.M., D.K. Kinyoki, C.W. Mundia, C.W. Kabaria, J.W. Mutua, V.A. Alegana, I.S. Fall, and R.W. Snow. 2014. The changing risk of Plasmodium falciparum malaria infection in Africa: 2000-10: a spatial and temporal analysis of transmission intensity. The Lancet 383(9930): 1739-1747.

Paaijmans K.P., J.I. Blanford, R.G. Crane, M.E. Mann, L. Ning, K.V. Schreiber, and M.B. Thomas. 2014. Downscaling reveals diverse effects of anthropogenic climate warming on the potential for local environments to support malaria transmission. Climate Change 125: 479-488.

Pascual M., J.A. Ahumada, L.F. Chaves, X. Rodó, and M. Bouma. 2006. Malaria resurgence in the East African highlands: temperature trends revisited. Proceedings of the National Academy of Sciences U.S.A. 103(15): 5829–5834.

Rogers D.J. and S.E. Randolph. 2000. The global spread of malaria in a future, warmer world. Science 289: 1763–1766.

Ryan S.J., A. McNally, L.R. Johnson, E.A. Mordecai, T. Ben-Horin, K. Paaijmans, and K.D. Lafferty. 2015. Mapping physiological suitability limits for malaria in Africa under climate change. Vector-Borne and Zoonotic Diseases 15(12): ahead of print.

Smith D.L., F.E. McKenzie, R.W. Snow, and S.I. Hay. 2007. Revisiting the basic reproductive number for malaria and its implications for malaria control. PLOS Biology 5(3): e42. doi: 10.1371/journal.pbio.0050042

The Scaly Crickets

by Joseph DeSisto

Among many new species named today, some of the most unusual were three new crickets from Southeast Asia (Tan et al. 2015). These crickets belong to the obscure and poorly-known family Mogoplistidae, cousins to the more recognizable (and audible) field crickets (Gryllidae). They look like field crickets too, except that their bodies are covered in scales.

A scaly cricket (Arachnocephalus vestitus). © Entomart.

A scaly cricket (Arachnocephalus vestitus). © Entomart.

When you touch a butterfly’s wings, you might notice a fine, powdery substance rubbing off on your fingers. The powder is made up of microscopic scales, which cover the wings of butterflies and moths. Scales give the wings their color, but they also provide insulation and protect the wings during flight. Perhaps most importantly, scales can fall off and make the wings slippery. This allows butterflies and moths to evade a careless hand as easily as a wet bar of soap.

The scientific name for butterflies and moths is Lepidoptera, which translates to “scaly wing” — scales are one of the most important features defining the group. However, many other groups of insects also have scales. Mosquitoes and silverfish have them, and so do scaly crickets.

The scales of a scaly cricket (Ornebius formosanus). Figure from Yang and Yen (2001), licensed under CC BY 2.0.

The scales of a scaly cricket (Ornebius formosanus). Figure from Yang and Yen (2001), licensed under CC BY 2.0.

Cricket scales, like those of butterflies and mosquitoes, are microscopic, powder-like, and easily shed. To really appreciate their beauty, a scanning electron microscope is needed. The first look came in 2001, when Yang and Yen published the first high-resolution images of cricket scales.

Aside from being scaly, scaly crickets aren’t all that unusual. They are adaptable, able to eat decaying plants as well as other insects, and they tend to live in moist sandy habitats. No scaly crickets are capable of flight, and females lack wings entirely, but the males do have small wings which they rub together to make chirping sounds (Love and Walker 1979). Click on the audio file below to listen to an amorous male scaly cricket (recorded by Thomas J. Walker).

Of the three new species, two were found in the Sakaerat Biosphere Reserve, in Thailand. This reserve consists mainly of high-altitude dry forest, with a few grasslands, and is home to many endangered species including tigers and giant black squirrels.

The third cricket is native to Pulau Ubin, an island off the coast of Singapore. Pulau Ubin is one of the last remaining wild areas in the already tiny country. Singapore’s government has been eager to develop portions of the island, but in recent years tourism has become more profitable. Fear of losing foreign visitors has encouraged officials to protect, rather than level, valuable habitat. For now, the status of the new scaly crickets appears secure, but in rapidly urbanizing Southeast Asia, nothing is certain.

A scaly cricket (Mogoplistes brunneus). © Entomart.

A scaly cricket (Mogoplistes brunneus). © Entomart.

Cited:

Love R.E. and T.J. Walker. 1979. Systematics and acoustic behavior of scaly crickets (Orthoptera: Gryllidae: Mogoplistinae) of eastern United States. Transactions of the American Entomological Society 105:

Tan M.K., P. Dawwrueng, and T. Artchawakom. 2015. Contribution to the taxonomy of scaly crickets (Orthoptera: Mogoplistidae: Mogoplistinae). Zootaxa 4032(4): 381-394.

Yang J. and F. Yen. 2001. Morphology and character evaluation of scales in scaly crickets (Orthoptera: Grylloidea: Mogoplistidae). Zoological Studies 40(3): 247-253.

Two New Species of Ant-Decapitating Fly

by Joseph DeSisto

An ant-decapitating fly. Photo by Scott Bauer, in public domain.

An ant-decapitating fly. Photo by Scott Bauer, in public domain.

Today saw the description of two new species of South American flies, both fire ant parasites that decapitate their victims. The two new species were discovered in Brazil and Argentina, associated with fire ant mounds in their native territory (Plowes et al. 2015).

Ant-decapitating flies, as might be expected, have unusual and macabre life histories. The adults are tiny, just a few millimeters in length, with the general appearance of fruit flies. Instead of hovering around rotten bananas, however, female ant-decapitating flies hang around ant mounds. When the time is right, a fly soars down to meet her victim, using a hooked, needle-like ovipositor to inject an egg into the ant’s head (Porter 1998).

With egg laid, her work is done. The fly departs, ant still intact and seemingly healthy.

A fly attacking a fire ant, hoping to lay its egg on the ant's head. Photo by Sanford Porter, in public domain.

A fly attacking a fire ant, hoping to lay its egg on the ant’s head. Photo by Sanford Porter, in public domain.

Not all is well, however. From the fly’s egg emerges a maggot that, as it grows, eats away at the inside of the ant’s head. At the same time, the maggot secretes chemicals that cause the ant to go mad, fleeing its colony and finding shelter in moist leaf litter. With its host almost spent, the maggot severs the ant’s head, and forms a cocoon or pupa inside the now-hollow shell. Some weeks later, a new fly emerges and takes off in search of a new ant for her offspring.

This all sounds very sinister, but it also could be very useful to humans. As it happens, many of these flies specialize in decapitating fire ants and, in enough numbers, can seriously impact fire ant colonies. Now that invasive fire ants are well-established in the southern U.S., the Department of Agriculture is looking to ant-decapitating flies to control the ants’ march north.

The remains of a fly's victim. Photo by Sanford Porter, in public domain.

The remains of a fly’s victim. Photo by Sanford Porter, in public domain.

Whether the two new species of ant-decapitating fly will be useful in controlling fire ants remains to be seen. Multiple fly species have already been introduced in states like Texas (Gilbert and Patrock 2002) and Alabama (Porter et al. 2011), where fire ants are a serious problem for agriculture and human health. In these states the flies have become established and already have a tangible impact on fire ant populations.

However, not all species fare equally well. In order to be useful in controlling fire ants, the flies must be able to adapt to the southern U.S. climate, as well as all the new predators they may not have faced in South America.

Plowes and colleagues (2015) suggest that many more unknown species of ant-decapitators live in remote regions of South America. Discovering new species may help scientists figure out which flies will be most successful at colonizing the U.S., and which species will have the biggest impact on fire ant populations.

Cited:

Gilbert L.E. and R.J.W. Patrock. 2002. Phorid flies for the biological suppression of imported fire ant in Texas: region specific challenges, recent advances and future prospects. Southwestern Entomologist Supplement 25: 7-17.

Plowes R.M., P.J. Folgarait, and L.E. Gilbert. 2015. Pseudacteon notocaudatus and Pseudacteon obtusitus (Diptera: Phoridae), two new species of fire ant parasitoids from South America. Zootaxa 4032(2): 215-220.

Porter S.D., L.F. Graham, S.J. Johnson, L.G. Thead, and J.A. Briano. 2011. The large decapitating fly Pseudacteon litoralis (Diptera: Phoridae): successfully established on fire ant populations in Alabama. Florida Entomologist 94(2): 208-213.

Porter S. D. 1998. Biology and behavior of Pseudacteon decapitating flies (Diptera: Phoridae) that parasitize Solenopsis fire ants (Hymenoptera: Formicidae). Florida Entomologist 81(3): 292-309.

Poisonous Frogs, Beetles, and Birds

by Joseph DeSisto

Meet the golden poison frog of Colombia’s coastal rain forests. This frog, one of nearly 200 species of poison frogs, is by far the most toxic. A single frog packs enough poison to kill 10,000 mice, or 10 or more humans (Myers et al. 1978).

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

The golden poison frog (Phyllobates terribilis). Photo by Brian Gratwicke, licensed under CC BY 2.0.

For the golden poison frog and its close relatives in the genus Phyllobates, batrachotoxin is the weapon of choice. Batrachotoxin acts on the nervous system, opening up the membranes of nerve cells so they can no longer carry signals to and from the brain. Death comes from paralysis, which leads to heart failure.

The golden poison frog was only discovered in 1971 when scientists found them around an indigenous Colombian (Emberá Chocó) village (Myers et al. 1978). The Emberá use the frogs to lace poison darts, with which they hunt game in the surrounding forest. The frog-handlers were careful to cover their hands with leaves, with good reason. Scientists who touched the frog felt a strong burning sensation, and they stressed in their initial description that:

The new species is potentially dangerous to handle: One freshly caught frog may contain up to 1900 micrograms (µg) of toxins, only a fraction of which would be lethal to man if enough skin secretion came into contact with an open wound.”(Myers et al. 1978, pp. 311)

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

The black-legged poison frog (Phyllobates bicolor), closely related to terribilis but not quite as toxic. Photo by Drriss and Marrionn, licensed under CC BY-NC-SA 2.0.

It is important, as the frog’s discoverers remind us, “to be cautionary, not alarmist” (Myers et al. 1978, pp. 340). Even though in theory these frogs are dangerous, there is no record of a person ever being killed by one. Although the poison can go through a person’s skin, it seldom does so in enough quantity to injure. The frogs do not bite. So if you see golden poison frogs while exploring in Colombia, do not panic, but be wary. However delicious and lemon-drop-colored they may seem, definitely don’t try eat them.

Golden poison frogs are sometimes sold in the pet trade, since they lose their poison after being taken out of the wild. This is probably because frogs get batrachotoxin from their food: soft-winged beetles that make the toxin themselves (Dumbacher et al. 2004). In captivity, frog-keepers give their pets a blander diet of crickets and fruit flies, which don’t contain batrachotoxin.

An example of a soft-winged beetle, in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

An example of a soft-winged beetle, not the same species, but in the same family as those eaten by poison frogs. Photo by Udo Schmidt, licensed under CC BY-SA 2.0.

Soft-winged beetles are found all over the world, especially in the tropics, but so far only a few other animals are known to eat them and use their batrachotoxins. Three of these are poison frogs, all found in a small rain forest region of Colombia. The others are birds. Yes, there are poisonous birds, and even though this blog is explicitly not about birds or mammals, I’m going to break that rule today.

The hooded pitohui (Pitohui dichrous) -- both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The hooded pitohui (Pitohui dichrous) — both males and females are brightly colored. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The toxic birds are all found in New Guinean rain forests, and most belong to a group of insect-eaters called pitohius (pronounced PI-to-hooies). Pitohui birds are related to the orioles and blackbirds found in more temperate climes. Shown above is the hooded pitohui, first found to be poisonous when a bird researcher handled one and left with a tingling, burning sensation in his hand.

Later study showed that the hooded pitohui, along with two other related species, has feathers laced with batrachotoxins (Dumbacher et al. 1992). More than a decade later, the same scientists demonstrated that toxin-wielding pitohui birds eat soft-winged beetles, and that these same beetles are loaded with batrachotoxin (Dumbacher et al. 2004). Despite being toxic, the birds are not nearly as dangerous as golden poison frogs, and there is little risk to a careful handler.

The variable pitohui (Pitohui kirhocephalus), not as showy as its cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

The variable pitohui (Pitohui kirhocephalus), not as showy as its hooded cousin, but toxic all the same. Photo by Katerina Tvardikova, licensed under CC BY-NC-SA 3.0.

Several more birds are now known to use batrachotoxins, and all are found in New Guinea (Weldon 2000). Many of them have similar red-and-black color patterns. By having similar colors, multiple bird species can work together to “educate” predators who might not be aware of the poisonous feathers (Dumbacher and Fleischer 2001). To make matters even more interesting, the toxins in bird feathers apparently serve as a repellent to parasitic lice (Dumbacher 1999).

We may continue to learn more about these amazing birds and their lives, or we may not. Most of these birds are becoming rarer and rarer as New Guinean rain forest is slashed and burnt, tilled and grazed into nothing.

I’ve written several articles about poisons and venoms: click here to learn about brown recluse venom and here to learn about tetrodotoxin, a poison used by many fish as well as newts, snails, and blue-ringed octopuses.

Darren Naish, writer of the superb science blog Tetrapod Zoology, writes often about birds. Click here for one of his articles on a poisonous New Guinean species. Note that the article is not on his most recent blog site, which is updated regularly at the first link to Scientific American.

To learn more about the relationship between lice and toxic pitohui birds, click here to read an excellent article by Bianca Boss-Bishop on the aptly-named blog Parasite of the Day.

Cited:

Dumbacher J.P. 1999. Evolution of toxicity in Pitohuis: I. effects of homobatrachotoxin on chewing lice (order: Phthiraptera). The Auk, 116: 957-963.

Dumbacher J.P., A. Wako, S.R. Derrickson, A. Samuelson, and T.F. Spande. 2004. Melyrid beetles (Choresine): a putative source for the Batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. Proceedings of the National Academy of Sciences U.S.A. 101(45): 15857-15860.

Dumbacher J. P., B.M. Beehler, T. F. Spande, H. M. Garra¡o, and J.W. Daly. 1992. Homobatrachotoxin in the genus Pitohui: chemical defense in birds? Science 258: 799-801.

Dumbacher J.P. and R.C. Fleischer. 2001. Phylogenetic evidence for colour pattern convergence in toxic pitohuis: Müllerian mimicry in birds? Proceedings of the Royal Society of London B 268(1480): 1971-1976.

Myers C.W., J.W. Daly, and B. Malkin. 1978. A dangerously toxic new frog (Phyllobates) used by Emberá Indians of western Colombia, with a discussion of blowgun fabrication and dart poisoning. Bulletin of the American Museum of Natural History 161(2): 311-365.

Weldon P.J. 2000. Avian chemical defense: toxic birds not of a feather. Proceedings of the National Academy of Sciences U.S.A. 97(24): 12948-12949.

Silver and Green

by Joseph DeSisto

Sunlight in the western U.S. can be intense, and all the more so when it catches the shimmering green carapace of a jewel scarab:

A Beyer's jewel scarab from Arizona. Photo by Joseph DeSisto.

A Beyer’s jewel scarab from Arizona. Photo by Joseph DeSisto.

The regal purple legs of this beetle identify it as Beyer’s jewel scarab (Chrysina beyeri). Beyer’s jewel scarabs spend most of their lives as white grubs in rotting logs, where they slowly eat their way through the wood. When they finally emerge as adult beetles, they eat oak leaves.

Although nearly a hundred jewel scarab species are found in Mexico and Central America, only four are known from the U.S.A., all in the southeastern portion of the country. Probably the most visually stunning is the so-called glorious jewel scarab (Chrysina gloriosa), with a lime-green exoskeleton sporting thick stripes of metallic silver.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

The glorious jewel scarab, also from Arizona. Photo by Joseph DeSisto.

There is something truly awe-inspiring about a beetle in which you can see your own reflection. We know that, of course, the glorious jewel scarab is not beautiful for our own gratification, but what adaptive purpose could metallic stripes possibly fulfill?

There are likely two. First, the glorious jewel scarab feeds on the leaves of juniper trees, which are common in the beetle’s high elevation habitats in Arizona and Texas. Juniper trees have tiny leaves along narrow branches, and thin strips of sun can create a shimmering effect at the right time of day. The beetle’s shining armor may actually serve as camouflage, by mimicking the narrow rays of sunlight that flicker through the trees.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

A glorious jewel scarab on its host, juniper. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Another Arizona insect uses the same trick. The royal moth’s caterpillar (Sphingicampa) is large and green, with a red stripe on each side and metallic spines jutting out of its back. Yet on their host plants, these caterpillars are well camouflaged. How? Because the plants they live on (locust, acacia, and others) all have small leaves with tiny spaces between them. The spines on royal caterpillars mimic the spaces between the leaves, and the light that flows through them.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

A Sphingicampa caterpillar, showing off its metallic spines. Photo by Joseph DeSisto.

The other reason for metallic strips is less obvious. It turns out that jewel and a few other scarab beetles can be so shiny because their exoskeletons reflect a unique kind of light, called circularly polarized light. To understand that, we need to understand a bit of physics.

What we call light is actually the product of tiny packets of energy called photons, travelling through space at the speed of … light. Photons travel in waves, undulating up and down or side to side. Normally, each photon has a wave pointed in its own direction, but sometimes, all the waves are oriented the same way. In other words, all the waves are on the same plane:

When light is polarized, all photons have wavelengths with the same orientation. Figure in public domain.

When light is polarized, all photons have wavelengths with the same orientation. The red wave shows the path of photons in an example. Figure in public domain.

If all the waves are on the same plane, the light is said to be polarized. Polarized light is fairly common in nature — although humans cannot identify polarized light, many insects such as bees use it to orient themselves with respect to the sun. This allows them to navigate between flowers, and to and from their hives.

Circularly polarized light, on the other hand, is extremely rare. In this situation, waves are still on the same plane as each other, but the plane rotates. If you could see the photons as they traveled directly towards you, it would look like a single photon is moving in a spiral, but in fact many photons are moving together, each at slightly an angle to its neighbor:

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

In circularly polarized light, the plane on which the waves are travelling rotates. Figure in public domain.

I won’t belabor this — physics is not my strong suit and I would rather omit details than risk getting things wrong. The point is, it takes a very special and rare kind of surface to reflect light in this way — the surface of a jewel scarab.

That’s pretty cool by itself, but it also happens that the same beetles are one of the few animals that can see circularly polarized light. There are lenses that allow us to see the same light, and if we look at a glorious jewel scarab through one of those lenses, it will appear not green and silver, but black (Sharma et al. 2009).

This can help otherwise well-camouflaged jewel scarabs find mates. Experiments with glorious jewel scarabs have shown that they are attracted to and will fly towards circular polarized light (Brady and Cummings 2010). Closely related, but less flashy beetles show no such preference. So while a predator looks at a beetle-filled juniper tree and sees nothing but leaves and shimmering sunlight, beetles can easily spot each other as big, black, radiant spots in a green world.

The Beyer's jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

The Beyer’s jewel scarab. Photo by Robert Potts, licensed under CC BY-NC-SA 3.0.

Cited:

Brady P. and M. Cummings. 2010. Differential response to circularly polarized light by the jewel scarab beetle Chrysina gloriosa. The American Naturalist 175(5): 614-620.

Vivek S., M. Crne, J.O. Park, and M. Srinivasarao. 2009. Structural origin of circularly polarized iridescence in jeweled beetles. Science 325: 449-451.

Living Illusions

by Joseph DeSisto

Good camouflage requires more than just color. Millions of years of natural selection have favored birds that can easily identify a brown moth on a brown background, but some species are a little more sophisticated. Lappet moths hide on tree bark – their odd shape, combined with mottled color, helps break up their outline, so visual predators such as birds have a hard time recognizing them as moths.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

A lappet moth (Phyllodesma americana) from Arkansas. Photo by Marvin Smith, licensed under CC BY-ND-NC 1.0.

These moths lay their eggs on trees including birch, oak, poplar, willow, and many others. The caterpillars munch on leaves by night, hiding on twigs and bark by day. They are also well-hidden, but because they have to be able to live on a variety of different trees, each of which has a differently-colored bark, lappet caterpillars don’t have a color that matches a particular background. Instead they, like their parent moths, have bodies with distorted outlines, specifically a lateral fringe of long hairs.

On bark, this helps a caterpillar “merge” with the bark on which it rests. On a twig, maybe not so much:

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

A resting lappet caterpillar on one of its favorite hosts, scrub oak. Photo by Joseph DeSisto.

Animals that depend on camouflage have to stay very still to avoid detection, but if they are spotted, staying still quickly becomes futile. Many animals use color to startle predators as a backup plan, the best-known example being the red-eyed tree frog. At rest, the frogs appear a solid leafy-green, but if disturbed, they quickly open their eyes. The sudden appearance of two giant, bright red eyes can be enough to startle a predator, which might give the frog time enough to make a hasty escape.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

The iconic red-eyed tree frog (Agalychnis callidryas). Photo by Carey James Balboa, in public domain.

Lappet caterpillars have a similar but more elaborate trick. If a potential predator (or human finger) brushes against a caterpillar, it first flexes its body so the hairs on its back part. This reveals two striking, blood-red bands, a warning this caterpillar might be toxic.

Photo by Joseph DeSisto.

The lappet caterpillar has only to flex its body to reveal these striking bands. Photo by Joseph DeSisto.

Let’s say this doesn’t work, and the bird isn’t intimidated. If a bird’s beak (or my tweezers) pinches the caterpillar, it bends its head back over its body, revealing legs surrounded by pitch-black spots:

Photo by Joseph DeSisto.

This caterpillar doesn’t like me very much. Photo by Joseph DeSisto.

Still not startled? The lappet caterpillar has one last show. In desperation, it falls from its perch in the trees and, on hitting the ground, flops upside-down, revealing a bright yellow-and-black belly:

Photo by Joseph DeSisto.

Yellow and black are universal warning signs — many toxic animals share these colors. Photo by Joseph DeSisto.

If this doesn’t work, the caterpillar is pretty much toast. For all its show, the lappet caterpillar isn’t poisonous – at worst, some of its hairs are mildly irritating to the skin. Like the red-eyed tree frog, it relies entirely on visual illusions to ward off predators. It sounds risky, but it’s worked at least some of the time for millions of years.

How Poisons Work: Tetrodotoxin

by Joseph DeSisto

This is the first of a series of short articles, each featuring a different type of poison or venom used by animals.

Poisons and venoms are some of the most complex substances in nature, often containing hundreds of different chemicals, each with a particular purpose. As technology advances, scientists have begun to look at some of these chemicals, usually proteins, and try to figure out what they do.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

The deadly southern blue-ringed octopus (Hapalochlaena maculosa). Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

In some cases, the results give us a new perspective on an animal’s biology: rattlesnake venom, for example, contains a unique protein that allows the snake to track its prey after the initial strike. In other cases, we discover potentially useful surprises: giant centipede venom contains a single protein that inhibits pain in mice, using the same chemistry as morphine, but with greater efficiency. However, regardless of whether the results are useful to us, studying nature’s chemical weapons gives us a whole new appreciation and understanding of the creatures that wield them.

Tetrodotoxin is the poison of choice for a variety of animals, especially in the ocean. These include blue-ringed octopuses, cone snails, moon snails, certain angelfish, some ribbon worms, a handful of amphibians, and the puffer/triggerfish order Tetraodontiformes, for whom the toxin is named. Many of the animals that use tetrodotoxin are brightly colored, a warning to passers-by. Would-be predators, save the immune and the unlucky, heed this warning well.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

An inflated pufferfish (Diodon holocanthus). Photo from Williams et al. (2010), licensed under CC BY 2.5.

Tetrodotoxin, sometimes abbreviated TTX, is a complex molecule that prevents its victims’ nerves from functioning properly. This eventually leads to paralysis, and death comes when the muscles that control breathing no longer receive signals from the nervous system. Basically, you suffocate. Humans can get TTX poisoning when they eat improperly-prepared pufferfish, but stings from blue-ringed octopuses and cone snails (both from the southwestern Pacific) are also a possibility if you are foolish enough to pick them up.

Although many animals use TTX, none of them actually manufacture the toxin themselves. Instead bacteria generate the toxin, to their host’s benefit. In exchange, the bacteria are allowed to live safely within their host’s body — the bacterium Pseudoalteromonas tetraodonis, for example, makes its home within the livers and skins of pufferfish.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

A toxic moon snail (Naticarius orientalis) from East Timor. Photo by Nick Hobgood, licensed under CC BY-SA 3.0.

The rough-skinned newt, from British Colombia and the western United States, has the bacteria needed to make TTX for itself — as a result, this newt has few natural predators. Two animals, however, have managed to work around and even benefit from the rough-skinned newt’s toxins, all without toxin-producing bacteria of their own.

The only animal capable of eating an adult rough-skinned newt is the garter snake. After ingesting the newt, any other predator would almost certainly die, but a few populations of garter snakes have evolved an immunity to TTX. What’s more, the snakes are able to sequester the toxins within their own bodies, so that the garter snakes themselves become poisonous (Williams et al. 2004).

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

The extremely toxic rough-skinned newt (Taricha granulosa). Photo by Rennett Stowe, licensed under CC BY 2.0.

Although many snakes inject toxins with their fangs, and so are venomous, garter snakes that eat rough-skinned newts are the only snakes that can truly be considered poisonous. The difference is that venomous animals have to inject their poisons into predators or prey, via fangs or stingers, while poisonous animals are laced with their chemical weapons and are dangerous to eat.

The other TTX-robber is, unexpectedly, a caddisfly. Caddisflies are insects whose larvae resemble caterpillars, but live underwater and surround themselves with cases made from pebbles, twigs, and other debris. Most are scavengers, eating decomposing plant matter and tiny invertebrates, while the flying adults are short-lived and do not feed.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A Limnephilus caddisfly larva, in its protective case made of twigs. Photo by Tom Murray, used with permission.

A few predatory species in the genus Limnephilus, however, have developed an unusual appetite for rough-skinned newt eggs, which also develop in the water. These eggs are loaded with TTX, and the toxin-resistant Limnephilus larvae manage to eat so many that even the caddisfly adults are toxic (Gall et al. 2012).

Even though many marine invertebrates contain tetrodotoxin, we still don’t know how many contain the bacteria that make it, versus how many, like the caddisfly, “steal” the toxin from their TTX-laced prey. Certain sea slugs, for example, are often washed on shore where they are eaten by beach-combing scavengers such as dogs. In New Zealand, several dogs have died after eating sea slugs that contained TTX (McNabb et al. 2009), and in Argentina, a population of the same slugs appears to have become invasive (Farias et al. 2015). Yet we still do not know whether they have their own TTX-generating bacteria.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

A toxic sea slug, Pleurobranchaea meckelii. Photo from Wägele and Klussmann-Kolb (2005), licensed under CC BY 2.0.

Experiments have shown that certain populations of sea slugs have TTX, while others do not (Khor et al. 2014) — just like the garter snakes of North America. Does this mean the sea slugs are obtaining TTX from some unknown, toxic prey item? The same researchers conducted a survey of all the marine invertebrates that these slugs might be eating, testing everything for TTX. The only positive result was a toxic species of sand dollar, but the sand dollars didn’t produce nearly enough TTX to explain the huge amounts found in sea slugs.

Ignorance has consequences, and there is still plenty of exploring left to do. The slugs may in fact make tetrodotoxin themselves (using bacteria). A more enticing possibility, and just as likely, is that there are still more toxic animals on the sea floor, waiting to be discovered.

One of the very first articles I wrote for this site featured the same sea slugs mentioned in this one, but that was when my only readers were a few sympathetic family members. If you’re curious about sea slugs, and/or you want to know the difference between a nudibranch and a pleurobranch, you can read that article here.

I also recently wrote about how rattlesnake’s use their venom to track once-bitten prey (mentioned in the second paragraph of this article). To learn more about that, click here. It’s amazing, I promise.

Cited:

Farias, N.E., S. Obenat, and A.B. Goya. 2015. Outbreaks of a neurotoxic side-gilled sea slug (Pleurobranchaea sp.) in Argentinian coasts. New Zealand Journal of Zoology, published online: DOI: 10.1080/ 03014223.2014.990045.

Gall, B.G., A.N. Stokes, S.S. French, E.D. Brodie III, and E.D. Brodie Jr. 2012. Predatory caddisfly larvae sequester tetrodotoxin from their prey, eggs of the rough-skinned newt (Taricha granulosa). Journal of Chemical Ecology 38(11): 1351-1357.

Khor, S., S.A. Wood, L. Salvitti, D.I. Taylor, J. Adamson, P. McNabb, and S.C. Cary. 2014. Investigating diet as the source of tetrodotoxin in Pleurobranchaea maculata. Marine Drugs 12(1): 1-16.

McNabb, P., L. Mackenzie, A. Selwood, L. Rhodes, D. Taylor, and C. Cornelison. 2009. Review of tetrodotoxins in the sea slug Pleurobranchaea maculata and coincidence of dog deaths along Auckland Beaches. Prepared by Cawthron Institute for the Auckland Regional Council Technical Report 2009/108.

Williams, B.L., E.D. Brodie Jr., and E.D. Brodie III. 2004. A resistant predator and its toxic prey: persistence of newt toxin leads to poisonous (not venomous) snakes. Journal of Chemical Ecology 30(10): 1901-1919.

Deadly Caterpillars

by Joseph DeSisto

Today’s article is about the hemileucines, caterpillars with venom-injecting spines. While most have harmless, if painful, stings, a few have venom powerful enough to kill an adult human. But before things get too dark, let’s take some time to appreciate some harmless little beasties:

First-instar caterpillars of the io moth (Automeris io), munching on some oak leaves. Photo by Shawn Hanrahan, licensed under CC BY-SA 2.5.

First-instar caterpillars of the io moth (Automeris io), munching on some oak leaves. Photo by Shawn Hanrahan, licensed under CC BY-SA 2.5.

These are the newly-hatched caterpillars of the io moth, found in eastern and central North America. As caterpillars, they like to move and feed in groups, which provides some protection against predators. The moth, with a wingspan approaching four inches, looks like this:

An adult io moth, ready to take flight and find a mate. Photo by Patrick Coin, licensed under CC BY-SA 2.5.

An adult io moth, ready to take flight and find a mate. Photo by Patrick Coin, licensed under CC BY-SA 2.5.

Not bad, right? Sadly, for all its showy appearance, io moths don’t have mouthparts and cannot feed as adults. A lucky moth dies when its energy reserves run out after about a week, if it can manage to avoid being snapped up by bats, spiders, and other hunters.

This has its benefits — a moth with no appetite can spend its time and energy mating and laying eggs as much as possible. It also has consequences — the caterpillars, to prepare for the most important week of their lives, have to eat a lot. Within a few weeks of devouring as much greenery as physically possible, an io caterpillar can go from being a half-inch-long worm to a nearly three-inch-long monstrosity, brilliant green with red and white racing stripes:

An io moth caterpillar, almost ready to spin a coccoon. Photo by Tim Lethbridge, licensed under CC BY-ND-NC 1.0.

An io moth caterpillar, almost ready to spin a coccoon. Photo by Tim Lethbridge, licensed under CC BY-ND-NC 1.0.

See the spiky, poisonous-looking tufts growing out of the caterpillar’s back and sides? Io caterpillars are indeed capable, and more than willing, to deliver a painful sting. If you brush up against these spines, the tips will break off and start to inject venom. It’s not nice, but not much worse than an encounter with stinging nettle plants which, incidentally, uses many of the same toxins. The strategy is shared by other hemileucine caterpillars, among which are some of the largest and most conspicuous moths in North America.

Southern Brazil has its own hemileucines, including members of the genus LonomiaLonomia moths are large and attractive, if perhaps a bit less flashy than their northern brethren:

An adult Lonomia moth from Brazil. Photo by Benjamint, licensed under GFDL 1.2.

An adult Lonomia moth from Brazil. Photo by Benjamint, licensed under GFDL 1.2.

Their caterpillars, too, are less conspicuous, which is why they sometimes wind up stinging people. A single sting contains only a miniscule amount of venom — not nearly enough to do any real harm. The problem comes from the fact that hemileucine caterpillars tend to feed in groups. If a person accidentally brushes up against 20 or more Lonomia at the same time, the result can be severe.

Unlike the io caterpillar, Lonomia has venom designed to do more than just irritate the skin. As soon as it enters the bloodstream, anticoagulants work to prevent the blood from clotting, while other proteins punch holes in the victim’s blood vessels. The result is violent internal bleeding. In the worst cases, death is usually the result of internal bleeding in the brain (Pinto et al. 2010).

Each of these spines contains a tiny amount of venom. Photo from Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Each of these spines contains a tiny amount of venom. Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Over the last few decades, cases in southern Brazil have been on the rise, and scientists at the Butantan Institute in São Paulo have been studying the molecular aspects of Lonomia venom. The most important result is that Brazilian hospitals now have access to an antivenom that quickly halts the venom’s action.

To North Americans a venomous caterpillar, even a deadly one, might seem like an esoteric problem. Yet in parts of southern Brazil, the caterpillar causes more medical emergencies than any snake, spider, or scorpion (Pinto et al. 2010). The Brazilian Ministry of Health estimated that in 2008, for every 100,000 people in the caterpillar’s range, there were eight stings. Meanwhile, incidents have become more common as rain forest has been replaced by fruit tree plantations, which happen to provide ideal habitat for Lonomia, and the stage for deadly encounters.

A cluster of Lonomia obliqua caterpillars. Photo from Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

A cluster of Lonomia obliqua caterpillars. Photo from O Centro de Informações Toxicológicas de Santa Catarina, in public domain.

Cited:

Pinto A.F.M., M. Bergerm J. Reck, R.M.S. Terra, and J.A. Guimaraes. 2010. Lonomia obliqua venom: In vivo effects and molecular aspects associated with the hemorrhagic syndrome. Toxicon 56(7): 1103-1112.