What is an insect?

Overview

We live in the "age of insects." Humans have walked on Earth for only a mere fraction of the 350 million years that insects have crawled, burrowed, jumped, bored, or flown on the planet. Insects are the largest group of animals on Earth, with over 1.5 million species known to science up to now, and represent nearly one-half of all plants and animals. Although scientists do not know how many insect species there are and probably will never know, some researchers believe the number of species may reach 10 to 30 million. Even a "typical"

backyard may contain several thousand species of insects, and these populations may number into the millions. It is estimated that there are 200 million insects for every human alive today. Just the total biomass of ants on Earth, representing some 9,000 species, would outweigh that of humans twelve times over. Insect habitats are disappearing faster than we can catalog and classify the insects, and there are not enough

trained specialists to identify all the insect specimens housed in the world's museums.

The reproductive prowess of insects is well known. Developing quickly under ideal laboratory conditions, the fruit fly (Drosophila melanogaster) can complete its entire life cycle in about two weeks, producing 25 generations annually. Just two flies would produce 100 flies in the next generation—50 males and 50 females. If these all survived to reproduce, the resulting progeny would number 5,000 flies! Carried out to the 25th generation, there would be 1.192 × 10⁴¹ flies, or a ball of flies (1,000 per cubic inch) with a diameter of 96,372,988 mi (155, 097, 290 km), the distance from Earth to the Sun. Fortunately this population explosion is held in check by many factors. Most insects fail to reproduce, suffering the ravages of hungry predators, succumbing to disease and parasites, or starving from lack of suitable food.

Physical characteristics

Insects are at once entirely familiar, yet completely alien. Their jaws work from side to side, not up and down. Insect eyes, if present, are each unblinking and composed of dozens, hundreds, or even thousands of individual lenses. Insects feel, taste, and smell the world through incredibly sensitive receptors borne on long and elaborate antennae, earlike structures on their legs, or on incredibly responsive feet. Although they lack nostrils or lungs, insects still breathe, thanks to small holes located on the sides of their bodies behind their heads, connected to an internal network of finely branched tubes.

Like other members of the phylum Arthropoda (which includes arachnids, horseshoe crabs, millipedes, centipedes, and crustaceans), insects have ventral nerve cords and tough skeletons on the outside of their bodies. This external skeleton is quite pliable and consists of a series of body divisions and plates joined with flexible hinges that allow for considerable movement.

As our knowledge of insects has increased, their classification has inevitably become more complex. They are now classified in the subphylum Hexapoda, and are characterized by having three body regions (head, thorax, and abdomen) and a three-segmented thorax bearing six legs. The orders Protura, Collembola, and Diplura, formerly considered insects, now make up the class Entognatha. Entognaths have mouthparts recessed into the head capsule, reduced Malpighian tubules (excretory tubes), and reduced or absent compound eyes.

The remaining orders treated in this volume are in the class Insecta. Insects have external mouthparts that are exposed from the head capsule, lack muscles in the antennae beyond the first segment, have tarsi that are subdivided into tarsomeres, and females are equipped with ovipositors. The word "insect" is derived from the Latin word insectum, meaning notched, and refers to their body segmentation. The second and third segments of the adult thorax often bear wings, which may obscure its subdivisions.

Insects are one of only four classes of animals (with pterosaurs, birds, and bats) to have achieved true flight, and were the first to take to the air. The evolution of insect wings was altogether different from that of the wings of other flying creatures, which developed from modified forelimbs. Instead, insect wings evolved from structures present in addition to their legs, not unlike Pegasus, the winged horse of Greek mythology. Long extinct dragonflies winged their way through Carboniferous forests some 220 million years ago and had wings measuring 27.6 in (700 mm) or more across. Today the record for wing width for an insect belongs to a noctuid moth from Brazil whose wings stretch 11 in (280 mm) from tip to tip. Insects are limited in size by their external skeletons and their mode of breathing. While most species range in length from 0.04 to 0.4 in (1 to 10 mm), a few are smaller than the largest Protozoa. The parasitic wasps that attack the eggs of other insects are less than 0.008 in (0.2 mm) long, smaller than the period at the end of this sentence. Some giant tropical insects, measuring 6.7 in (17 cm), are considerably larger than the smallest mammals.

Behavior

The small size of insects has allowed them to colonize and exploit innumerable habitats not available to larger animals. Most species live among the canopies of lush tropical forests. Some species are permanent residents of towering peaks some 19,685 ft (6,000 m) above sea level. Others live in eternal darkness within the deep recesses of subterranean caves. Some occupy extreme habitats such as the fringes of boiling hot springs, briny salt lakes, sun-baked deserts, and even thick pools of petroleum. The polar regions support a few insects that manage to cling to life on surrounding islands or as parasites on Arctic and Antarctic vertebrates. Fewer still have conquered the oceans, skating along the swelling surface. No insects have managed to penetrate and conquer the depths of freshwater lakes and oceans.

The feeding ecologies of insects are extremely varied, and insects often dominate food webs in terms of both population size and species richness. Equipped with chewing, piercing/sucking mouthparts, or combinations thereof, insects cut, tear, or imbibe a wide range of foodstuffs, including most plant and animal tissues and their fluids. Plant-feeding insects attack all vegetative and reproductive structures, while scavengers plumb the soil and leaf litter for organic matter. Some species collect plant and animal materials—not to eat, but to feed to their young or use as mulch to grow fungus as food. Many ants

"keep" caterpillars or aphids as if they were dairy cattle, milking them for fluids rich in carbohydrates. Predatory species generally kill their prey outright; parasites and parasitoids feed internally or externally on their hosts over a period of time or make brief visits to acquire their blood meals.