March 15, 2012 2:34:54 PM
We have more insects, and more insect species, than any other animals. On land and in freshwater, in most places, if you weighed all the insects and all the other animals, the insects would win. Because insects are numerous and ubiquitous, they are also the largest and most reliable animal foodstuff, and though we in the west don't often snack on them (though we enjoy their cousins such as crabs), birds, spiders, scorpions, frogs, mammals and other creatures (including other insects) benefit from insects as a source of nutrition. And the insects aren't going to stand still for it. Well, some of them are, for standing still is a good protective tactic for some. It, and all the other means that insects use for protection, are detailed in How Not to Be Eaten: The Insects Fight Back (University of California Press) by entomologist Gilbert Waldbauer. Waldbauer has written plenty of scientific papers, but like his valuable What Good Are Bugs?, this volume is a guide to help everyone appreciate insects more. Each insect is a little miracle of evolution, and they have many surprising ways of avoiding becoming someone's meal. And then their predators have evolved further means to overcome those obstacles. Waldbauer says the purpose of the ten chapters here is to "elucidate the strategies and counterstrategies in the everlasting evolutionary arms race between predators and prey." His book is beautifully organized by specific tactics.
One of the tactics is not surprising at all to anyone who has turned on a kitchen light and found a cockroach on the floor. Instantaneously, the cockroach flees (and if it is in my kitchen, it usually gets away before I can stomp on it). Fleeing is a fine tactic, and that cockroach will duck into a dark crevice before you know it. One of the things that makes the cockroach so good at getting down into a hiding place is first, that it can scurry really fast; and second, it knows how deep into hiding it has to go because it has eyes in its tail. Well, not exactly; no animal has real eyes in its tail. But a cockroach does have a ganglion at the end of its abdomen that is sensitive to light. If the roach has gone deep enough into a crevice that even its tail isn't picking up light, then it knows it is pretty well hidden. Roaches that have had their eyes painted over can still tell light from dark.
Roaches run. Grasshoppers and others jump, propelled by strong muscles in the back legs. Click beetles straighten their bodies out with a click and fly into the air without using leg muscles. Like countless other insects, when they aren't pursued, they use the associated strategy of simply staying under cover. Larvae, for instance, often bore into plant tissues or burrow into the soil, where they can get nourishment and also remain hidden. The adults eventually have to get out into the scary world to eat, mate, and lay eggs. Many are hidden by camouflage that is nothing exotic, like the color green for a leaf dweller, or a mottled brown for one that stays on the ground or on tree trunks. Many exhibit countershading, having their upper regions dark compared to their lower ones; fish, snakes, birds, and rabbits do this, too. This means that the lower body, with less light falling on it, is closer to the color of the upper, so that the insect looks less three-dimensional, and seems to blend into the background better. Some caterpillars are just the opposite, with dark on their lower bodies and light on the upper; the reason is that they spend their time hanging upside-down.
The more elaborate deceptions of camouflage can be spectacular, even witty. A caterpillar in India looks the perfect double of a bird dropping, earning praise from an observer in 1892, who was struck by "the skill with which the colouring rendered the varying surfaces, the dried portion at the top, then the main portion, moist , viscid, soft, and the glistening globule at the end. A skilled artist working with all materials at his command could not have done it better." There are planthoppers that come in two colors, some pink and some green. When twenty or so of them gather and want to be overlooked, they arrange themselves to look like a spike of many small blossoms and buds, and observers say it looks so much like a flowering plant that it is startling, when the "plant" is disturbed, to see its component parts jumping off in all directions.
Sometimes looking drab is a good tactic, but sometimes it is good to look flashy. Some insects look drab until they are disturbed, and then unfold wings that are brightly colored. This would surprise a predator bird, for instance, giving the insect a little time to make a getaway. If the momentarily-startled bird pursues, it will be looking for the flash colors that made such an impression, and will not see them when the insect lands quietly and is still. Plenty of moths and other insects have eye spots; often the insect is camouflaged and unremarkable, but when disturbed will spread its wings to show eyespots, sometimes complete with a highlight on the "pupil." Flash colors and eyespots have evolved in many different kinds of insects, including butterflies, grasshoppers, mantises, and beetles, and so must be reliable tactics.
Eyespots represent a bluffing offense strategy, and insects do a lot of such bluffing. Most spectacular is a caterpillar in the Amazon which when disturbed turns itself over, puffs itself out, and exhibits fake eyes and even fake scales to look like a snake. It scares people and it scares birds. More subtle, but with good effectiveness, is to imitate the look of an insect that is poisonous or distasteful. It has been shown in the lab that birds and other insect eaters can reliably recognize insects that will taste bad after trying to eat just one of them; they learn the important lesson fast. They are helped in the learning if the insect is dramatically marked with warning colors, the same bright yellows, orange, and reds that we humans use for our warning signs on the roads. So mimics take advantage of such learning; they might be tasty, but if the predator thinks they are not, they are safe. Some jet-black darkling beetles found in the desert have that black color as a warning, since they stand out on desert sands; and the warning is a true one, for when they are threatened, they stand on their heads, put their abdomens in the air, and ready themselves to spray a repellant from glands at the tip of the abdomen. But other beetles in the desert have adopted not only the black warning color, but also the headstand posture; in their case, they are bluffing, because they have no repellant glands, but the bluff works.
That such tricks often fool humans, including experienced fool field collectors like Waldbauer himself, is a frequent theme here. Especially interesting is the mimicry of a drone-fly which looks like a honeybee, and thereby avoids being bothered by predators who do not wish to be stung, although it is stingless. It looks so much like a honeybee that it has fooled people for millennia, and has led to the confused belief that honeybees originate spontaneously from the carcasses of dead animals like oxen. Bees never infest carrion, but the flies do, leading to stories like that of Samson in Judges, who killed a lion and a few days later found the carcass was swarming with bees and packed with sweet honey he and his family were able to enjoy. Waldbauer jokes, "I wonder, do those who accept the words of the Bible as the literal truth ever search for honeycombs in rotting carcasses?"
The tactics of the insects are clever, and the tactics of researchers trying to understand them are clever, too. Waldbauer describes many field and laboratory experiments; I did not realize that one of the problems in insect labs is cockroach infestations: "Entomologists can't use insecticides in their laboratories; insecticides kill not only cockroaches but also the insects that are the subjects of our experiments." One experiment described here involves the entomologist using a casting rod to present lizards with tethered butterflies with and without warning colors, and tallying the degree to which the lizards responded to the butterflies cast to them. There are many such experiments reported here, each adding a little bit to our understanding of the complex behavior and ecological fit of these creatures. Waldbauer's curiosity and enthusiasm is present on every page, and although insects may annoy, impoverish, or sicken us, you won't be able to read the descriptions here without admiration for their superb adaptations.
4. A Stone's Throw: Beware COLUMNS