Desert Food Chain - Part 12
To humans, “insects are the destroyers,” said Carl Olsen in Insects of the Southwest, which he co-authored with Floyd Werner, “but to Nature, they are the recyclers, the reworkers and the designers,” terms that, in effect, serve as definitions of roles they play in the food chain. “They’ve been at this game of survival much longer than human beings, and they’ve surely succeeded.”
So, What is an Insect, Anyway?
As in the rest of the world, the insects of our deserts belong to the invertebrate division of the animal Kingdom. They range from near-microscopic in size to several inches in length. Unlike a vertebrate, which has an articulated internal skeleton, an insect has an articulated external skeleton that encases its “eyes, mouthparts, antennae, body, legs, and fore and hind sections of the digestive tract, and some respiratory surfaces,” according to Barbara Terkanian’s “A Vertebrate Looks at Arthropods,” published in the Arizona Sonoran Desert Museum book A Natural History of the Sonoran Desert. The insect has a distinct head, a thorax and an abdomen. It comes equipped with three pairs of legs and two pairs of wings, with one pair of wings sometimes modified through evolution to serve a purpose other than flight. Insects, the only invertebrates that can fly, “must have extensive respiratory systems to supply quantities of oxygen to flight muscles [and] sophisticated nervous systems to coordinate rapid flight responses,” according to Terkanian. Fossilized evidence of their antiquity lies embedded in stones hundreds of millions of years old.
Our Insect Population
In our Chihuahuan, Sonoran and Mojave Deserts, as in the other lands of the world, the insect population accounts for more species than all the other animal groups combined. In our deserts, the insects comprise well over 10,000 species that have been described, likely with as many more awaiting description. Worldwide, millions of species may still await description.
During the summer, we may be “overwhelmed by their abundance,” said S. W. Frost in his Insect Life and Insect Natural History. Some insects “cut leaves, roots, or stems into shreds, some attack meat, some feed on filth, some fly through the air with a buzzing sound, some sing monotonous songs, and some even press their beaks relentlessly into our tender skin.”
Insects have the capacity to reproduce prodigously. For instance, a single queen termite, among the most prolific of insects, may lay millions of eggs, said Frost.
One scientist, according to Frost, “figured that a pair of flies beginning in April might be the progenitors, if all were to live, of 191,010,000,000,000,000,000 individuals by August. Allowing 1/8 cubic inch per fly, there would be enough of them to cover the earth 47 feet deep.” Fortunately, there “are many adverse conditions, parasitic and predacious enemies that prevent such things from happening and thus these insects are held in check.”
Theoretically, a single pair of romantically inclined Pomace Flies, acting as the Adam and Eve of their species, could yield enough descendants within a year to “form a ball 96 million miles in diameter!” according to Donald J. Borror and Richard E. White in their book A Field Guide to the Insects of America North of Mexico. That is based on the assumption that you would pack 1000 flies into every cubic inch within the ball. (The mean distance from Earth to the Sun, incidentally, is 93 million miles, three million miles less than the diameter of the ball.) Fortunately for the real Adam and Eve and their descendants, Pomace Flies, like other insects, face many “enemies, adverse environmental conditions, and the like” that keep them in check.
By any measure, the insects play a powerful role, through the food chains, in shaping our deserts.
A Few Examples of Desert Insects
The insects have evolved as herbivores, carnivores and omnivores, with some species highly specialized and others quite generalized in their choices of foods. “Small animals,” said Terkanian, “can exploit habitats more fully than large ones. A single plant may be a meal to a vertebrate, but to arthropods [which include the insects] it can be a universe. One species might complete larval development in a flower bud, while another species spends its entire life feeding on the woody stems. A large plant like the saguaro can support an entire community…” A small sampling of the diverse insects common to our deserts include beetles, grasshoppers, ants, bees and butterflies.
“Beetles,” said Arthur V. Evans in his article on those insects in A Natural History of the Sonoran Desert, “comprise the largest group of insects on earth, representing one-quarter of all living organisms and one-third of all animals…” Beetles “are almost everywhere and feed on all sorts of plant and animal materials,” said Borror and White. “They are abundant on vegetation; they occur under bark, stones, and other objects; many are found on or in the ground, in fungi, rotting vegetation, dung, and carrion. Some are aquatic. A few are parasites of other animals.”
The beetle’s exoskeleton, which may be covered with spines or with a waxy coating, protects the insect’s head, thorax and abdomen. Its leathery forward wings, which nearly always meet in a straight line down the center of the insect’s back, said Borror and White, serve to protect the membranous hind, or flight, wings. In our deserts, the beetles range in size from a small fraction of an inch to several inches in length.
The desert beetles have developed a range of adaptations and behaviors for surviving heat, drought and predators. Their thick exoskeletons, for instance, help minimize water loss. They have beneath their forewings, according to Evans, cavities for trapping moisture that would otherwise be lost through respiration. Some bury themselves deep in cooling sand during hot summer days. Confronted by an enemy, some species exude a foul liquid from their abdomens. Others merely mimic species that exude a foul liquid. Some find refuge and protection in the thorny embrace of cacti.
“Beetles,” as the San Diego Zoo says on it Internet site, “eat almost everything: plants, other insects, carcasses, and dung. Some beetles living in water eat fish and tadpoles; [one species] eats snails. Most beetles have a very good sense of smell to help them find food. A beetle’s front jaws, called mandibles, vary in size and shape depending on the species. Predatory beetles have extended mandibles that can seize, cut, or crush prey. Specialized nectar feeders have tube-like mouthparts.”
The showy large metallic green Fig Beetle “plays an active role in the decomposition of plant materials,” according to Evans. The adults feed on soft fruits and sap-oozing mesquite tree wounds. The larvae feed on organic material in compost and dung.
Adults and larvae of various species of the orange and black-spotted ladybird beetles, smaller than a pea, prey on the destructive plant-feeders such as aphids and scale insects. Charming as they may be in the literature of our children, the ladybird beetles actually smell bad and taste worse, characteristics that discourage predators.
Adult tiger beetles, metallic-colored miniature Sherman tanks of insects with powerful sickle-like jaws, move swiftly and athletically to prey ferociously on other insects and on spiders. Even their larva ambush prey at the mouths of the beetles’ burrows. Conversely, they fall prey to various carnivorous insects as well as birds and mammals.
Grasshoppers, said Goggy Davidowitz in A Natural History of the Sonoran Desert, “can leap well over 20 times their body length… Most of the kinetic energy to do this comes not from the muscles, but from the semilunar crescent located in the knee of the hind leg. This crescent-shaped organ is made of elastic fibers that store energy in preparation for a jump; they release this energy explosively, propelling the grasshopper forward many times its body length.”
In addition to its powerful hind legs, the grasshopper is especially distinguished by its hind wings. It (along with its closest insect relatives) has, according to William Atherton DuPuy, Our Insect Friends and Foes, “a hard shell-like wing on top, of little use in flying, and underneath it the really effective wing all folded up in straight lines for all the world like my lady’s fan. These are the only insects in all the world that fold their wings this way.”
The species generally called “Desert Grasshoppers” have developed a unusual adaptation to the desert environment. “On cool mornings,” said Werner and Olson, “they can be seen sitting perpendicular to the sun, obviously catching as many rays as they can. By mid-day their orientation is parallel to the sun, so that they get minimum heating.”
Davidowitz says that most “…grasshoppers are generalist feeders, eating plants from an extremely broad range of families.” Some species may eat their fill of a single plant host then switch to another host. Other species may nibble on one plant then switch quickly to another. In one exception, the green, perfectly camouflaged Creosotebush Grasshopper feeds only on the plant that gives it its name. This is the only example of more than 8000 grasshopper species worldwide that eats but a single plant species. Another exceptional desert grasshopper is the omnivorous Plains Lubber, which eats not only a variety of plants but also eats other grasshoppers and other insects.
Grasshoppers also hold, for insects, an unusual distinction in the food chain because they have served as an important food source for the human species, including not only some Indians of our deserts, but also some peoples of North Africa, South Africa, the Red Sea, Iraq, China, the Philippines, Australia and the West Indies, according to DuPuy.
In some areas of the world, grasshoppers reach plague proportions. In its Internet site, the Kansas State Historical Society tells of the historic plague of grasshoppers in 1874: “In late July they came without warning in swarms so large they blocked out the sun and sounded like a rainstorm. When a swarm landed, the omnivorous pests brought near total destruction. Crops were eaten out of the ground, as well as the wool from live sheep and clothing off people’s backs. Paper, tree bark and even wooden tool handles were devoured.”
We sometimes describe a gregarious person as a “social butterfly,” but it is the humble ant, a close relative of the wasp, that might make a better comparison. It ranks near the top of the hierarchy of social insects. An established ant colony, ensconced in elaborate subterranean catacombs, functions like a highly organized and structured community of thousands of fastidious, strictly law-abiding, six-legged, citizens.
A queen, which may live for more than two decades according to Diana E. Wheeler and Steven W. Rissing, A Natural History of the Sonoran Desert, reigns over her ant kingdom, tended by courtiers and retainers. She lays the eggs that give rise to new generations, which emerge and take flight once a year to establish new colonies. The flights usually occur just after a rainfall. Females the workers in the world of ants construct specialized chambers, tend larvae, keep house, defend the colony and collect and even raise food. Male ants small and spoiled weaklings compared with the queen and the worker ants appear only during the period of flight. They mate with prospective new queens, delivering a lifetime supply of ant sperm, then having fulfilled their biological role, they die. Only a small fraction of one percent of the would-be queens survive to found new colonies. “If ten percent of those that start out founded colonies,” DuPuy said, “the world would soon be overrun with ants.”
Like the other insects, the ants’ bodies have three parts: a distinct head, a thorax and an abdomen (or gaster). They have wings at the time they take flight, but they shed them once they establish a new colony. Depending on the species, the ant may sting or bite, with various species able to inflict a painful wound.
The ants take refuge from the desert’s surface in their cooler and more humid underground chambers, a practice that also brings important benefits to the desert with its frequent shallow caliche subsurface. Indeed, ants serve as “the primary soil-workers in the Southwest…” according to Werner and Olson. “Their nest-making…aerates the soil, helps retain soil moisture or drainage, and fertilizes and distributes many native plants.” Meanwhile, ants link desert food chains together in diverse and, sometimes, strange ways.
Seed harvester ants, which include one species that can deliver what Wheeler and Rissing call an “unforgettable sting,” clear the ground immediately surrounding the opening to their colony, and they wear permanent trails that radiate away from the clearing. They harvest the seeds primarily of grasses and annual, or ephemeral, plants, storing them within their nests, in chambers “toward the top…where dry conditions discourage germination.” The harvester ants also eat other insects, said Werner and Olson.
Leafcutter Ants, which have spiny bodies, excavate large, radiating nests made conspicuous on the surface by “very symmetrical mounds of particles of fresh soil, more or less like volcanic cinder cones…” said Werner and Olson. These ants take cut leaves from many different plant species. They carry the snippings to their nests. They chew the leaf material into a mulch, which they use, like farmers, to raise a fungus that serves as food for the colony.
The ferocious Army ants, nomadic raiders, do not make nests. Rather, they commandeer temporary residences in rodent burrows or tree roots, say Wheeler and Rissing. Hunting by night, they prey on other ants and on termites, plundering nests like the Vikings once looted the settlements of Britain. Army ants also feed on other insects and even on spiders and scorpions.
If ants are the quintessential social animals, most desert bees are resolute loners, although they often take up residence in bee-friendly neighborhoods. Typically, according to Stephen L. Buchmann, A Natural History of the Sonoran Desert, a female bee excavates a personal burrow a few inches to six feet deep in desert sand. She lines her burrow with cells, which she waterproofs with a waxy secretion from abdominal glands. She provisions each cell with pollen and nectar. She lays an egg in each cell. Satisfied that her work is complete, she then abandons the burrow, leaving her progeny to shift for themselves for the rest of their lives.
Other desert bees, rather than digging burrows, move into abandoned tunnels left in dead trees by wood-boring beetles, and they fashion their cells from leaf clippings, small pebbles and mud. A few bees, like their relatives, the ants, follow a social life style. There are also native parasitic bees, which, according to Walter Ebeling, University of California Riverside’s Urban Entomology Internet site, co-opt the provisioned nests of harder working bee species, and they lay eggs that hatch early and feed on the ready supply of pollen and nectar and even on the larvae of the unwilling host.
Bees, like the other insects, have three body parts: head, thorax and abdomen. They “resemble some wasps, except that bees are usually hairier and more robust,” said Buchmann, “and they possess specialized structures for carrying pollen back to their nests.” As Borror and White said, “Pollen sticks to the bee’s body hairs when the bee visits a flower and is periodically combed off and placed on the hind tibiae [a segment of the hind leg modified for the purpose].” Some species have long tongues, others, short tongues, reflecting the depth of blooms which they visit in their quests for pollen and nectar. Female bees come equipped with a stinger, actually a modified egg-laying tube, says Buchmann. The desert bees range in size from a fraction of an inch to more than an inch in length. The Sonoran Desert’s Perdita minima bee, at eight hundredths of an inch in length, is the smallest bee in the world.
Bees hold a special place in the desert food chains. Largely dependent on flowers, bees visit blooms like connoisseurs attending wine tastings. According to the Earth-Life Web Production Internet site, some bees deign to visit only a single species of flower. Others visit but a few species. Some bees prefer flowers of one color over another, perhaps like some people prefer red wine over white. Some bees mark flowers with a short-lived repellent, a reminder not to try that particular bloom again. Still others, presumably the bee lowbrows, visit many species indiscriminately.
Buchmann said, “By moving pollen around from flower to flower and plant to plant, bees perform vital and often unappreciated roles as the most important group of pollinating animals on earth…”
For example, “Of the approximately 640 flowering plant taxa growing in the Tucson Mountains near the Desert Museum, approximately 80 percent of these species have flowers adapted for and pollinated by bees…
“Without the pollination services bees provide, many plants would not produce seed-laden fruits from which the next generation of plants would grow. Without bees, there would be few or no fleshy berries or fruits to sustain birds, mammals, and other wildlife. The tunneling activity of bees aerates the soil and allows water from infrequent rains to quickly penetrate and reach plant roots; and bees’ nitrogen-rich feces fertilize the soil. The bees themselves often provide food for lizards, mammals, birds, insects, spiders, and other arachnids.”
Thousands of species of bees populate our deserts of the Southwest, with the area around Tucson having perhaps the most diverse community of these insects in the world.
Our deserts’ butterfly population, winged lyrics with hundreds of species, brings distinctive dimensions of mystery, color, daintiness and wanderlust to the world of the insects. It is the butterfly that enters the pupa, or chrysalis, stage of its life as a modest caterpillar and emerges as colorful, winged fairy-like creature. One species, the Painted Lady, has established itself throughout much of the world, including North America, Africa, Asia and Europe. In one of nature’s true spectacles, another species, the Monarch, migrates annually by the tens of millions for thousands of miles, with some wintering along California’s central and southern coastlines and others wintering in central Mexico’s mountain forestlands.
While the butterfly, like all the insects, has a head, thorax and abdomen, it (like a moth) is most distinguished by it wings, “which are covered with thousands of tiny overlapping scales, much like tiles on a roof,” said Richard A. Bailowitz and Mark P. Sitter in A Natural History of the Sonoran Desert. Each scale is one color, but collectively a butterfly’s color pattern is produced by a complex mixture of differently colored scales.” (You can usually differentiate between a butterfly and a moth because a resting butterfly holds its wings vertically and a resting moth holds its wings horizontally.)
Compared with many other insects, the butterfly poses little threat to economically important plants. “In short,” said Bailowitz and Sitter, “butterflies are benign, aesthetically pleasing faunal members.” They are nevertheless important components of the food chain. The caterpillars usually feed on the plants where they hatched from eggs. Painted Lady caterpillars, for instance, eat the leaves of thistles. Monarch caterpillars eat the leaves of milkweeds. The adult butterfly serves as an important pollinator as it flies from bloom to bloom, seeking nectar. Caterpillars and adults serve as prey for other insects as well as for birds and small mammals.
One of the most spectacular of the butterflies in the Southwest is th yellow and brown Giant Swallowtail, which has a wingspan of some four inches, according to Werner and Olson. “…it is the largest insect most people will ever see in the wild.”
The slightly smaller Pipevine Swallowtail, with iridescent blue wings punctuated with orange spots, rates high among the most beautiful of the butterflies, but it can also be deadly. “These butterflies are highly poisonous and distasteful, even to birds,” said Werner and Olson. “The distinctive color identifies the butterfly as something not fit to eat for any bird that has tried one before and learned the hard way.” The insect takes its poison from the Pipevine plant, the food of the caterpillar.
According to Bailowitz and Sitter, some of the other common butterfly families of the desert include: Brush-footed Butterflies, typically orange and brown medium-size species that include the Monarch; Whites and Sulphurs, medium to small white, yellow or orange butterflies that appear in early spring; and Gossamer Wings, diminutive, often bronze-colored butterflies that look like as though they have been crafted by a master jeweler.
Insects and their relationships with other organisms have held a fascination for the human species for millennia.
In Book 11 of his monumental Natural History, the Roman author, Pliny the Elder (23 to 79 AD), said of the insects: “…in no one of her works has Nature more fully displayed her exhaustless ingenuity.”
In 1733, Johnathan Swift, in his On Poetry: A Rhapsody, wrote:
So Nat'ralists observe, a Flea
Hath smaller Fleas that on him prey,
And these have smaller Fleas to bite' em
And so proceed ad infinitum.
In the 19th century, an unknown author, possibly Oliver Wendell Holmes, parodying Swift, wrote:
Big bugs have little bugs
Upon their backs to bite them.
And little bugs have littler bugs,
And ad infinitum.
Before his death in 1992, naturalist Floyd Werner wrote, in Insects of the Southwest, “My background is as a naturalist, and my continuing interest is in the development and use of classification of insects. Diversity fascinates me. The Southwest has a concentration of diversity that is unbeatable in the United States. So this has become my chosen home.”
By Jay W. Sharp
Part 1 Desert Food chain - Introduction
Part 2 Desert Food chain - The Producers
Part 3 Desert Food chain - The Cacti: A Thorny Feast
Part 4 Desert Food chain - The Yuccas
Part 5 Desert Food chain - The Agave
Part 6 Desert Food chain - Desert Grasslands
Part 7 Desert Food chain - Desert Shrubs
Part 8 Desert Food chain - The annual forbs
Part 9 Desert Food chain - Mavericks of the Desert Plant
Part 10 Desert Food chain - Outlaw desert plants
Part 11 Desert Food chain - Animals: The Consumers
Part 12 Desest Food chain - The Insects
Part 13 Desest Food chain - The Ugly, the Uglier and the Ugliest
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