How Not to Be Eaten. Dr. Gilbert Waldbauer
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to the female parts of another. Most flowers have coevolved with bees, butterflies, or other pollinators. Their colors and scents attract insects and reward them with nectar and pollen, which many insects eat and which are virtually the only foods consumed by the thousands of species of bees (at least 3,500 in North America alone). No one knows how many of the flowering plant species are pollinated by insects, but Stephen Buchmann and Gary Nabhan have reported that of the 94 major crop plants on earth, the wind pollinates 18 percent, insects 80 percent, and birds 2 percent.
Insects have many other functions in the web of life, only a few of which I will mention here. Plant-feeding insects help to keep plant populations from increasing to a size that would disrupt a stable ecosystem. For example, when the European Klamath weed, also known as St. John's wort or locoweed, reached California, its population exploded because it had no natural enemies there; it choked out grasses in pastures to the extent that they were useless for grazing cattle. After a European leaf beetle that eats Klamath weed was introduced into California, the weed became scarce, and grew mainly in shady places, where it was less likely to be attacked by the leaf beetle. An agricultural entomologist remarked that insects are their own worst enemies. And indeed they are. Thousands of insects, probably more than 300,000 of the known species, eat other insects. As Peter Price noted, insects, mostly ants, are the “world's premier soil turners,” more so than earthworms, which are generally given credit for this. Without the scarabs and other dung-feeding insects, we might, to use a bit of hyperbole, be knee-deep in excrement. Furthermore, ants and other insects disperse the seeds of some plants.
In the next chapter we will meet a few of the many animals—spiders, scorpions, toads, birds, bats, mice, and even bears—that eat insects. The threat to the insects from these insectivores is enormous, but as we will see in following chapters, insects have evolved many, often amazing ways to avoid being eaten. But keep in mind that neither insects nor other organisms are completely immune to predation. If they were, their populations would probably explode, causing ecological havoc.
TWO
The Eaters of Insects
In the middle of the night, a little bolas spider hangs from a plant by a few threads of silk. When a flying moth comes close enough, the hungry spider flicks at it a length of silk thread with a droplet of very sticky glue at the end. This is its bolas. With a bit of luck, the glue catches the moth, which the spider reels in and makes a meal of. The bolas spider's weapon is unique among animals—except for humans, who have invented similar weapons, the lasso and the Argentine gaucho's bolas, for which the spider is named. The spider doesn't just wait for an insect to wander along. It lures in its prey, which are always moths, by means of a false signal, a counterfeit version of the odorous sex-attractant pheromone released by the female moth to attract a mate. The clue that led to the discovery of this surprising chemical subterfuge is that these spiders catch only male moths of one species.
The bolas spider is just one among hundreds of thousands of animals that eat insects and is by no means the only one that has evolved an ingenious way of capturing its prey. The most abundant of the insectivores are themselves insects, at least three hundred thousand species. The other insectivores are far fewer, but they run the gamut of the animal kingdom: spiders, scorpions, centipedes, fish, frogs, toads, salamanders, turtles, young alligators, lizards, snakes, birds such as woodpeckers, nuthatches, swifts, swallows, and warblers, and mammals such as anteaters, armadillos, skunks, shrews, mice, bats, and even bears.
Figure 2. A pair of burrowing owls have placed clumps of cow manure around the entrance to their burrow, a bait that attracts dung beetles, which the owls will eat.
The eaters of insects have evolved innumerable tactics and strategies for finding, capturing, and consuming their share of this great multitude of creatures. Dragonflies course over ponds, capturing mosquitoes in a basket formed by their spiny legs. Well-camouflaged praying mantises sit motionless as they patiently wait to grab passing insects with their raptorial front legs. Among the birds, colorful warblers flit from twig to twig, rapidly scanning leaves for caterpillars; flycatchers dart from their perches to snap up flying insects; and blue jays sometimes scan tree trunks for camouflaged moths that blend in with the bark on which they rest during the day. Bats employ sonar—echolocation—to find flying insects at night. An armadillo uses its long, sticky tongue to capture the ants, beetles, and other insects it uncovers as its rather long snout furrows through leaf litter and loose soil. We usually think of squirrels as vegetarians, but William Burt watched a thirteen-lined ground squirrel dig in the soil for white grubs (the larvae of a June beetle). The bolas spider's tactics are unique, but other spiders use a variety of quite different hunting strategies. Among them are spiders that spin the familiar webs of sticky silk that snare flying insects, wolf spiders that chase their prey, and spiders that lurk in tunnels covered by trapdoors and dash out to grab and envenom passing insects that stumble into their trip wire, a single strand of silk.
The familiar flat, circular webs spun by the orb-weaving spiders are sticky lacework nets designed to catch unwary insects that blunder into them. Although they are marvels of engineering, they are also very beautiful, poems in symmetry—especially in early morning, when they glitter with little drops of dew. Long, straight strands of silk, which are not sticky, radiate from the hub of the web like the spokes of a wheel. Interspersed among them are long strands of exceedingly sticky silk that form a sequence of closely spaced loops spiraling from the hub of the web to its outer edge.
When the spider, waiting motionless on the hub, feels the vibrations caused by a struggling insect stuck to the web, it plucks the radial threads one by one, “apparently,” wrote Rainer Foelix, an expert on spider biology, “to probe the load on each radius. In other words, it tries to find the exact position of the prey.” It then “will rush out of the hub using exactly that [nonsticky] radial thread which leads to the prey.” Only after it has wrapped the prey in silk does it administer its venomous bite. The spider “then cuts the neatly wrapped ‘package’ from the web and carries it to the hub. There it is attached by a short thread before it is eaten.”
Few insects other than moths, as Thomas Eisner and his coworkers discovered, manage to escape from the webs of orb-weaving spiders. Moths are sometimes saved by the tiny, easily detached scales that cover their wings and bodies. (The colored “powder” that clings to your fingers when you handle a moth or a butterfly consists of these scales.) Moths that blundered into a web, Eisner noted, “were detained only momentarily, and usually flew off seemingly unaffected by the encounter. However, they invariably left behind, stuck to the particular viscid threads…that bore the impact, some of the scales that ordinarily cover their wings and bodies…. Coated with scales, the threads are no longer adhesive, and the moth is free to escape.”
On a sunny June day in northern Michigan, I spotted an interesting-looking insect on a large white blossom of a Canada anemone. (It turned out to be a fly that mimics a yellow jacket wasp. You will read a lot more about mimicry in later chapters.) This insect, oddly enough, wasn't moving at all, and its posture seemed unnatural. Looking more closely, I saw that it was clutched by a white crab spider that was all but invisible on the flower. Many crab spiders, like this one, are ambushers that lurk motionless in a blossom as they wait for their prey to land—a fly, bee, or other insect. Some can, in the course of about a week, change from white to yellow or yellow to white to match the color of the blossom on which they are lurking. “Fortified with extremely potent venom in compensation for weak Chelicerae [pincers], the small crab spiders,” Foelix commented, “are formidable creatures that attack insects and other spiders much larger than themselves.”
The fact that these spiders evolved the ability to change their color suggests that at least some insects are wary and will not land on a flower occupied by something that might be a predator. In The World of Spiders, W. S. Bristowe described a simple experiment which showed that this seems to be so. On half of sixteen yellow dandelion blossoms that he had placed on a lawn, he put a black pebble about the size of a crab spider; on the other half he placed a pebble that was about the same size but matched the dandelion blossom's color. As Bristowe