How Not to Be Eaten. Dr. Gilbert Waldbauer
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a twig they look like a short branching spur. “When a prey animal touches [its] posterior end,” Montgomery wrote, “the caterpillar suddenly loops backwards to seize the prey with its thoracic legs…and quickly returns to a straightened, elevated posture to feed. The entire strike takes about
Unlike the many web-spinning spiders, only a few terrestrial insects fashion traps to catch their prey. But several years ago, Alain Dejean and his coworkers published an account of traps built by a tree-dwelling ant (Allomerus decemarticulatus) of French Guiana. As do quite a few other ants, Allomerus has a mutually beneficial association with a particular species of plant. The plant provides leaf pouches in which the ants nest, and the ants reciprocate by destroying insects that feed on the plant. Dejean and his coauthors reported that the ant “uses hair from the host plant's stem, which it cuts and binds together with purpose-grown fungal mycelium [long threadlike strands], to build a spongy ‘galleried’ platform for trapping much larger insects. Ants beneath the platform reach through…holes and immobilize the prey, which is then…transported and carved up by a swarm of nestmates.”
The most famous of the trap-making insects are the ant lions of the order Neuroptera, relatives of the aphid-eating green lacewings. Harold Bastin described how an ant lion larva digs its pitfall trap in the soil:
The larva is a strange-looking insect, thick-set and somewhat oval in contour, with a flat head armed with formidable, curved mandibles. It has an ingrained habit of walking backwards, and uses its convex abdomen as a plough. When constructing the pitfall for which it is famous, it usually begins by making a circular groove to correspond with the margin of the proposed excavation. It then ploughs round and round in diminishing circles, constantly jerking out the sand with its shovel-like head. The final result is a funnel-shaped hollow, in the bottom of which the maker lies buried with only its ugly jaws exposed to view. Any small insect which chances to run over the edge of the pit slides downward on the yielding sand, its descent being hastened by the ant-lion, which casts up jets of sand upon its victim.
After its fanglike mandibles inject the victim with venom and digestive enzymes that liquefy its internal tissues, the ant lion sucks up its predigested meal—much like a robber fly or an ambush bug.
It is a wonder of evolution that a fly (genus Vermileo) maggot, or larva, of the snipe fly family (Vermileonidae) has independently invented this trap. In Demons of the Dust, William Morton Wheeler described how this larva, the worm lion, digs its pitfall:
The procedure is very simple compared with the usual circuitous performance of the ant-lion, because the worm-lion merely curls its anterior end after thrusting it in the sand and then suddenly straightens it, thus tossing the sand out onto the surface. At the same time it rotates more or less on its long axis, so that the direction in which the sand is thrown differs somewhat with each discharge. In this manner a small conical pit, with the larva at its apex, is soon formed…when the pit is completed the larva awaits its prey…. Usually…it lies horizontally on its back with its posterior half buried in the sand and its thoracic and first abdominal segments crossing the floor of the pit like a bar and covered with a very thin layer of sand.
When an insect falls into the pit, the worm lion usually strikes “at the prey violently and repeatedly till it [can] fix its mandibles in some portion of its body.” Next “it pump[s] venom into its victim and then commence[s] imbibing its juices.”
The delightful robin-sized burrowing owls of the Great Plains and southern Florida are very unusual. Unlike most owls, they are partly diurnal, unusually long-legged, live and nest in burrows that they dig to a depth of as much as 8 feet, and spend much of the day surveying their surroundings from the top of the large mound of excavated soil. If alarmed, perhaps by a birdwatcher or an approaching predator, they give a loud chattering call as they agitatedly—and amusingly—bob and bow.
One of the more remarkable things about these little owls is that they scatter chunks of horse or cow dung (probably bison dung earlier in the species' history) around the entrances of their burrows. The dung is evidently important to them, because if it is removed, they will usually replace it. What purpose the dung serves was a mystery until Douglas Levy and his coworkers showed that it is bait to attract dung beetles, relatives of the scarabs, that the owls eat. The researchers' first step was to remove all dung, beetle scraps, and regurgitated owl pellets (which contain the indigestible parts of beetles and other prey) from around the entrances to twenty occupied burrows. Then they put a quantity of cow dung “typical of the amount [usually] found at a burrow entrance” around the entrances to half of these burrows and none around the entrances to the other half. After four days, they collected the dung beetle scraps and regurgitated pellets from around the entrances to all the burrows. Then they repeated the experiment, switching the bait from one group of burrows to the other. Examination of the beetle remains found on the ground and in the pellets showed that “when dung was present at the burrows, owls consumed ten times more individual dung beetles of six times as many species than when dung was not present.” The inescapable conclusion is that burrowing owls use dung as bait. The use of bait to catch insects is, indeed, very unusual for a bird, but not unique. On several occasions a green heron was seen catching fish attracted to bits of bread that it had dropped on the water at the edge of a pond in a park.
Birds have evolved many other truly remarkable anatomical, physiological, and behavioral adaptations for exploiting insects as food. Roger Tory Peterson's words put the birds and insects in an ecological context, neatly setting the stage for a look at birds as predators of insects:
The insects, which have invaded nearly every terrestrial environment on earth, are unable to evade the birds that probe the soil, turn over the leaf litter, search the bark, dig into the trunks of trees, scrutinize every twig and living leaf. The water is no safe refuge, nor is the air, nor the dark of night. There is a bird of some sort to hunt nearly every insect. Warblers and vireos methodically work the leaves while swallows, swifts and other hunters of flying-insect prey spend most of their waking hours on the wing, ranging hundreds of miles daily in their aerial forays.
This story began about 155 million years ago with the first known bird, the famous Archaeopteryx, represented by beautiful, complete fossils from a limestone quarry in Bavaria. Archaeopteryx combined avian and reptilian characteristics, which shows—as do more recently discovered feather-bearing dinosaur fossils from China—that the birds are direct descendants of the dinosaurs. During the next 120 million years many species of birds evolved, but relatively few of them were insect eaters. During the Miocene epoch, beginning about 30 million years ago, the rapid evolution of the flowering plants and the hundreds of thousands of insects that exploit them resulted, as Frank Gill noted, in an explosive evolutionary radiation of insectivorous birds, mainly songbirds (order Passeriformes), which today constitute close to six thousand of the almost ten thousand known species of birds.
Most birds include insects in their diet. With few exceptions, most notably pigeons and doves, even the most dedicated vegetarian birds, fruit and seed eaters such as finches, buntings, grosbeaks, and cardinals, feed their nestlings a high-protein diet of animal matter, mainly insects. The behavior of a cardinal observed by Josselyn van Tyne is illustrative: “At noon on May 24 the adult male, on his way back to the nest territory, stopped at my feeding shelf with his beak full of small green worms [caterpillars] such as I had often fed to the young. He immediately put the worms down on the shelf and began cracking and eating sunflower seeds…. He then picked up the worms, flew across the street, and (presumably) fed the young.”
Insect eaters avoid competition by sharing the environment, specializing in where and how they hunt. Ground feeders such as towhees and fox sparrows search the litter on the forest floor for both insects and seeds; warblers, chickadees, and other leaf gleaners search foliage for caterpillars and other insects; nuthatches and brown creepers are among the bark gleaners; wood and bark probers, such as woodpeckers, bore into trees to find grubs and other burrowing insects; flycatchers and, on occasion, many other birds are air salliers that dash from a perch to snatch insects from the air; and finally, gleaners of aerial plankton, among them swallows, swifts,