Month: February 2020

Silk moth caterpillars like this Automeris species are spectacular denizens of the rainforest.

With the return of warm weather, leaves, and insects still weeks away here in the DMV, it’s time to travel to tropical rainforests in Costa Rica to see what’s up with some of our southern neighbors. First stop is the rainforest near the village of Santo Domingo bordering the Savegre River in Costa Rica. Scrambling across the ground near the base of a tree was a magnificent caterpillar. At first glance the identity of this beauty had me stumped, but after picking it up fiery stings to my finger and palm refreshed my memory of its true identity. This caterpillar is a member of the silk moth clan in the genus Automeris. The remarkable color pattern of this extraordinary larva leads me to believe its identity is Automeris metzli, a creature found from Mexico to Ecuador and also on the island of Trinidad, where it munches leaves of oak and less commonly Erythrinaand coconut.

Don’t let these amazing spines and beautiful colors fool you. Handling this lovely caterpillar could result in a spicy and memorable surprise.

False eyespots on the hind wing may help beautiful male (top) and female (bottom) Automeris moths gain protection from hungry predators. These two are Io moths, Automeris io.

You might think that a very large (this one was several inches long) tasty caterpillar would attract the attention of hungry predators. But Automeris caterpillars have a clever defense. Lining their sides and backs are spines loaded with venom. These spikey armaments are called urticating spines. Upon contact by a predator or overcurious human these spines release venom, causing a painful and relatively long lasting sting. For most people this sting resolves without complication but for some it may cause a serious allergic reaction. For me, well, getting up close and personal with this creature was worth some minor discomfort. Urticating spines are employed for defense by several families of moths, including flannel moths and saddleback caterpillars we met in previous episodes. While stinging shock and awe are the defensive syndrome employed by these caterpillars, adult moths use a different strategy. When resting on vegetation or on the ground, the brownish dappled forewings of the moth help it blend with background vegetation, effectively camouflaging the moth. If a predator draws too near, the moth spreads its forewings revealing large vertebrate eyespots on the hindwings. This ruse is thought to startle and frighten would-be predators, allowing the moth to escape or break-off the attack. False eyespots have evolved many times in the insect world and are found on caterpillars including swallowtail caterpillars, Promethea moths, and owl butterflies we met in previous episodes.

A cousin of Automeris metzli called the Io moth is relatively common in much of North America. including the DMV. This beautiful species of moth was once very abundant from New England to the Gulf States and west to the Great Plains. However, in New England and throughout much of its range, Io moths have declined. In addition to loss of natural habitat, the introduction of the parasitic fly Compsilura concinnata to control gypsy moth caterpillars has been implicated in local and regional declines in populations of Io moths and several other moth species attacked and killed by the fly. Habitat destruction and invasive species are but two threats to these charismatic and beautiful creatures throughout their ranges.

Acknowledgements

Bug of the Week thanks Hugh and Bridgette for discovering the gorgeous caterpillar, Costa Rica Vacations, and the intrepid guides Mono and Kenneth at Rafiki Lodge, for providing the inspiration for this episode.  “Automeris metzli Sallé (Lepidoptera: Saturniidae) in Trinidad, West Indies” by Matthew J.W. Cock, “Moth decline in the Northeastern United States” by David Wagner, and “Insect Defenses” by T. Eisner, M. Eisner, and M. Siegler, were used as references for this episode.

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On hot days with brilliant sunshine a dragonfly may point its abdomen upward to reduce the surface area exposed to the sun.

This week we continue our adventures in the aquatic realm where we recently visited whirligig beetles and water striders. Now let’s meet some tiny dragons. Dragons are mythological beings common to many cultures. These fanciful and fearsome predators are often chimeras of several creatures with wings of a bat, head of a reptile, scales of a fish, feet of an eagle, or tail of a serpent. With enormous compound eyes, reticulate wings, long legs, and a snaky tail, dragon seems a fitting name for the insects we call dragonflies.

Dragonflies belong to a group of flying insects called the Anisoptera. The name Anisoptera comes from the Latin roots aniso- (meaning unequal) and –ptera (referring to wing). Anisoptera is a reference to the fact that the forewings of dragonflies are not nearly as wide as the hindwings. Dragonflies are an ancient clan. Those patrolling the skies 250 million years ago had wingspans of two and a half feet. Modern dragonflies are smaller but no less magnificent. They are active predators hunting and capturing prey while on the wing. Spiny legs held beneath the body in a basket-like arrangement trap victims. Small insects such as mosquitoes or crane flies are common snacks, but larger insects including bees, butterflies, and other dragonflies or damselflies may be captured. There are reports of large dragonflies catching hummingbirds.

On a chilly day in February it’s nice to think about a warm summer day when a dragonfly might choose a dragonfly ornament to perch upon amongst zinnias in my flowerbed. But dragonflies are fierce predators. Watch as a dragonfly devours a damsel in extreme distress – damselfly that is.

An empty shed skin attached to a stem is all that remains of a dragonfly’s life underwater.

Male dragonflies often patrol specific territories along ponds or streams on the lookout for food, potential mates, or other males. Other males entering a territory are pursued and driven away. Females are wooed and mating pairs of dragonflies are often seen flying in tandem. The male sometimes guards his mate while she deposits eggs in the water or on aquatic vegetation. Eggs hatch into fascinating submariners called nymphs. These beautiful swimmers live in ponds or streams obtaining oxygen from the water through gills found inside their anus. Muscular contractions allow the nymph to pump water in and out of its rear-end to breath. How curious!

Dragonfly nymphs capture many kinds of insects such as the larvae of aquatic beetles, midges, and mosquitoes. Yes, dragonfly nymphs are important predators that help hold mosquitoes at bay. Crustaceans, worms, tadpoles, and even small fish are fair game for these stealthy hunters. Dragonfly nymphs sit and wait for a potential meal to come near. When in range, a remarkable, hinged mouthpart snaps forward like a bullfrog’s tongue to ensnare the victim. Nymphs usually move about by crawling slowly. However, when startled or under attack, they expel a blast of water from their rear ends and are propelled forward like a jet. After molting several times and completing development, a nymph will climb out of the water and attach itself to a plant or stone. The nymphal skin splits and the adult dragonfly emerges. Once the exoskeleton has hardened, the daring aerialist will take wing to hunt, mate, and entertain lucky humans on a warm summer day.

The insect on the left with gills on its tail is a damselfly nymph. The insect more centered is a dragonfly nymph. Watch mosquito larvae disappear into the maw of the dragonfly nymph, captured by the lightning strike of the nymph’s hook-bearing mouth-parts. Clips at normal speed have been slowed by 85% to observe prey captures.

Acknowledgements  

We thank Bill Lamp for providing the gorgeous dragonfly nymphs featured in this bug of the Week. Information was gleaned from “An Introduction to the Study of Insects” by D.J. Borer, D.M. De Long, and C.A. Triplehorn, “Beginner’s Guide to Dragonflies” by B. Nikula, J. Sones, D. Stokes and L. Stokes, and “Eco-Friendly Control of Mosquito Larvae by Brachytron pratense Nymph” by S.N. Chatterjee, A. Ghosh, and G. Chandra. 

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What forces and clever adaptations enable water striders to literally walk on water?

Last week we visited whirligig beetles and learned their secrets of living at the intersection of water and air. This week we return to the water to visit another member of the neuston, the interesting community of organisms that spend their lives on or near the surface of water. Gerrids go by a number of colorful common names including water striders, pond skaters, and Jesus bugs owing to their remarkable ability to walk just a few millimeters above the water’s surface. One fine autumn afternoon along a gentle stream in the Blue Ridge Mountains I happened upon a nice collection of water striders scooting across the surface of a small pool. Water striders are predatory members of the true bug clan that includes terrestrial predators such as wheel bugs and spined soldier bugs we met in previous episodes. These aquatic predators dine on small insects and other arthropods, either living or dead, whose fortunes deposit them on the water’s surface. Powerful jaws are used to penetrate the exoskeleton of a victim while needle-like stylets inject proteolytic enzymes to
liquefy internal structures of the prey. A pump in the head of the water strider sucks the nutrient-rich liquid into the predator’s digestive tract. Yum.

The majority of water striders are denizens of fresh water but a few live in brackish waters or truly saline waters of the ocean. As a group, they have evolved remarkably clever strategies for dealing with the uncertainties of aquatic life. Those utilizing large permanent water sources like lakes may lack wings entirely and forgo the ability to fly, putting their bodily resources into reproduction rather than mobility. Others found in temporary water sources often have winged individuals capable of escaping vanishing pools and colonizing new water-filled ones.

Small dimples in the water caused by cohesion of water molecules beneath each of six widely spread
legs distribute the strider’s weight, enabling it to stand and zoom across the surface without sinking.

Perhaps the most interesting aspect of water strider life is the unique ability to walk on the water’s surface. Here’s how they do it. The body of the water strider is covered with thousands upon thousands of fine hairs. The larger of these dense hairs covering the body repel water, keeping the strider from becoming water-logged and sinking when splashed or submerged by a tiny wave or pelted by raindrops in a downpour. The real magic comes by way of tiny microhairs found at the tips of the water striders legs. These can number more than a thousand hairs per square millimeter. By distributing its weight across six legs, each with water repellent hairs, the water strider takes advantage of the cohesive force of water molecules. Water molecules
simply stick together due to the attraction of one charged molecule to another. This cohesion causes the water to form small depressions beneath each leg as it bears the weight of the insect. To move forward, the water strider shifts minute amounts of weight to one of the middle legs and then pushes against the back wall of the depression thereby propelling itself forward. By alternating movements of the middle legs, steering with the hind legs, and carefully distributing its weight among all legs, the strider walks on water. One can only imagine what fun could be had if we had skates or shoes to exploit the cohesive force of water molecules.   

Acknowledgements

Two wonderful references “Aquatic InsectEcology” by J.V. Ward and “Insect Ecology” by P.W. Price, R.F. Denno, M.D.Eubanks, D.L. Finke, and I. Kaplan were used to prepare this episode.  

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Viewed from the side you can see compound eyes of the whirligig beetle, one looking up, another looking down. Two on the right and two on the left make four. But the question is why?

Having crossed the optical threshold of unaided vision many years ago, I find having four eyes – that is, wearing glasses – to be incredibly useful, especially when trying to observe the antics of very small creatures like insects. In this week’s episode we meet a fascinating beetle with four eyes. No, it doesn’t wear glasses, although that would be pretty funny, but it really does have four eyes. Beetles in the family Gyrinidae, commonly known as whirligig beetles, live the life aquatic at the interface between the world of air and sunlight and the world of swirling water. The name whirligig stems from their habit of swimming rapidly and changing direction frequently, often in circular patterns.  Some observers comment that these beetles seem to gyrate on the water’s surface.

Life at the interface of two worlds, one of air and the other of water, presents interesting challenges. One challenge of course is the very different way light passes through water compared to its passage through air, and how this difference affects vision. Many of us have had the interesting experience of observing aquatic creatures underwater at an aquarium or through the lens of a dive mask. You’ve seen that objects appear significantly larger underwater than when viewed in air. This phenomenon results from water refracting or bending light as it moves through water, making objects appear about one third larger than they actually are. Imagine the dilemma of a surface dwelling insect faced with the prospects of attempting to keep an eye open for predatory fish lurking below while simultaneously monitoring for hungry birds ready to pounce from above. Whirligig beetle have a unique solution to this visual dilemma. They have evolved two unique sets of eyes, one pair gazing upward above the water’s surface, one downward-looking pair immersed in the water below.  

Sneaking up on a raft of whirligigs plying the gentle currents of the Gunpowder River, the beetles appear orderly and intent on holding their place in the current. But with a quick wave of my hand, whirligigs quickly shift into the swirling, dizzying, pandemonium from which they gain their name and possibly avoid the jaws of predators.

Upward-looking eyes are covered with a maze of incredibly tiny features called nanostructures that enhance their ability to sense wavelengths of light in the visual range. Downward-facing eyes that peer into the relative gloom below the surface lack these nanostructures. Separate nerve centers in the beetle’s tiny brain receive visual information from both pairs of eyes, integrate this information, and use it to hunt prey, find mates, and avoid predators. It is difficult for a binocular animal like me to even imagine what all of this must look like. Amazing!

The rapid, swirling movements of whirligigs are thought to help confound visually hunting predators. Whirligigs have yet another defense against their aquatic enemies, this one a nasty tasting chemical called gyrinidal produced by glands in their abdomen. When offered whirligig beetles as a snack, largemouth bass rejected them. Crazy swirly swimming, noxious chemical defenses, and four compound eyes, these are the special powers needed for whirligig beetles to survive at the interface of air and water.

Acknowledgements

Bug of the Week enjoyed “Secret Weapons” by Thomas Eisner, Maria Eisner, and Melody Siegler, and “Under- and over-water halves of Gyrinidae beetle eyes harbor different corneal nanocoatings providing adaptation to the water and air environments” by Artem Blagodatski, Michail Kryuchkov, Anton Sergeev, Andrey A. Klimov, Maxim R. Shcherbakov, Gennadiy A. Enin and Vladimir L. Katanaev, that provided the fascinating information for this episode.

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