Our Window on Nature

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Insect Self-Defense

Filed under: Bugs — Lowell Christie -- July 20, 2007 @ 1:27 pm

Moth on BookJust about everybody likes to eat insects – mammals, birds, amphibians, reptiles, spiders, other insects, and even some humans. Over in Great Britain, a scientist with time on his hands calculated that the country’s spiders consume enough insects each year to total the combined weight of the entire human population. And that’s just the spiders. Why, then, are there so many insects left over?

“Bug-ologists” point to several reasons why insects are nature’s most successful creatures. For one thing, insects breed in astronomical numbers. They can afford to lose a high percentage of their population to predation. And for another, insects have highly sophisticated strategies for self-defense. It’s these defense strategies that we’ll be investigating here – house building, chemical warfare, warning coloration, mimicry, and camouflage – because they are strategies that you’ll be able to view first-hand as you travel. First the architects.

The next time you go trout fishing in a mountain stream, take a break and look for the house-building larvae of the caddis fly. These little creatures live on the bottoms of ponds and streams, and each individual spins a silk case that it uses as a retreat. What makes this insect so remarkable is that the larvae camouflage their silken houses by covering the exteriors with bits of twigs, leaves, grains of sand, or tiny shells and seeds. Each caddis species uses a particular kind of building material. You don’t usually see the insect, though. All that you notice are those tiny one-inch castles stuck to the rocks.

Other insects defend themselves by practicing chemical warfare. In deference to the stinging wallop of bees, ants, and wasps, we generally prefer to do our insect research well outside of their territories. But we can watch other insects make war without endangering our hides.

Darkling beetles, for example, are common insects in many regions. On a morning hike in White Sands National Monument, we followed the zipperlike tracks of a darkling beetle marching up and over the dunes – shiny black beetle on stark white sand. With only the slightest encouragement, the beetle stuck its rear up into the air in a comic-aggressive posture that made it look as though it were standing on its head. Had we harassed it too much, we’d have understood the reason for that posture. The irate insect gives off an odor that’s strong enough to repel just about anyone, predator or naturalist.

By its behavior, the darkling beetle advertises its unpleasant odor. Other insects use different methods to advertise their unpleasant taste or sting. If an insect is brightly colored and very conspicuous in its normal surroundings, you can be reasonably sure that it is also unfit to eat. Bees, wasps, monarch butterflies, ladybugs, and tiger moths are good illustrations of this principle. These insects not only advertise their presence, but they fly at a languid pace, making no attempt to avoid predators. They don’t need to scurry or to hide, because anyone who takes a bite of such an insect won’t relish its brother.

That leads us to another interesting self-defense strategy – mimicry. We’ve already established that those insects that sting or that taste bad have little trouble with predators. This can turn into a benefit for insects that taste good if they have evolved to resemble the no-nos in appearance. Thus we have the delicious tasting viceroy butterfly (if you are into eating the scaly winged creatures), closely resembling the black-and-orange wing pattern of the evil-tasting monarch. [There’s new research showing that the viceroy doesn’t taste very good either.] We have harmless flies that are look-alikes of wasps. We also have confused predators.

There’s another form of subterfuge common among insects – camouflage. Using a strategy that is quite the opposite of advertising one’s presence, some insects have evolved an appearance that completely matches their surroundings. They simply disappear.

A case in point. Many night-flying moths spend the daylight hours clinging to tree bark or telephone booths. Now, if all goes as it should – and usually it does – the tree-trunk-clinging moths will be those with mottled gray and black wings. About the only time you notice one is if you lean against the bark and a moth jerks away. Pure white species of moths spend their resting hours against pure white backgrounds – such as the white plastic cover over the light inside a telephone booth. But one day we found all the exceptions to this rule.

While camped along the Blue Ridge Parkway, we walked up to the entry station to make a phone call. That particular telephone booth was covered with moths. Presumably, the night-flying insects had been attracted by the light the previous evening, and as dawn came they carelessly settled wherever they happened to land. White moths dotted raw gray wood. The same gray-and-black moths that would have been invisible on the tree trunk only a few feet away were instead clinging to the bulletin board inside the booth. And a huge green luna moth that should have been pretending to be a leaf on a shrub or a tree was instead lying exposed on the sidewalk. Three species completely out of habitat; three species beautifully designed to remain hidden during the day lying exposed to daytime predation.

What happened? Just about what you would expect. ‘Twas the season for flycatchers, and the birds inhabiting that territory never had such an easy meal. Snatch up a moth – fly to a perch – dine on succulent morsels of moth. Before long those birds’ bellies were ready to burst. Camouflage coloration doesn’t help much if you forget to land where your coloration matches your surroundings. Even the best designed self-defense strategy works only for those insects that keep their guards up.


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