ultimate goal of any organism is, of course, to reproduce itself, to pass its genes on to future generations, and to accomplish this it must survive long enough to attain sexual maturity. As the great English naturalist Henry Bates wrote in 1862 in “Contributions to an insect fauna of the Amazon Valley, Lepidoptera: Heliconidae”:
Every species in nature may be looked upon as maintaining its existence by virtue of some endowment enabling it to withstand the host of adverse circumstances by which it is surrounded. The means are of endless diversity. Some are provided with special organs of offence, others have passive means of holding their own in the battle of life. Great fecundity is generally of much avail…. A great number have means of concealment from their enemies, of one sort or another. Many are enabled to escape extermination or obtain subsistence, by disguises of various kinds: amongst these must be reckoned the adaptive resemblance of an otherwise defenceless species to one whose flourishing race shows that it enjoys peculiar advantages.
The last sentence refers to the fascinating subject of the last chapter of this book, harmless insects, and a few other harmless animals, that foil predators by bluffing, mimicking the appearance and even the behavior of other insects or other animals that sting, are unpalatable, or are avoided by predators for other reasons.
Besides reproducing themselves, insects perform indispensable ecological services. As discussed above, they are the most important link between plants and animals that don't eat plants, and they have other important roles in virtually all terrestrial and freshwater ecosystems. One of their major functions, which we have all heard about, is to pollinate plants. Most of the green plants are flowering plants, called angiosperms (Greek for “a seed encased by an ovary”), and except for hummingbirds, bats, and just a few other animals, it is the insects that transport the sperm-containing pollen from the male parts of one flower 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
