THE WORLD OF LIFE with Douglas Drenkow

Over three-quarters of a million: Insects are by far the most diverse group of life on Earth, with more species than all the other animals put together! Furthermore, insects are probably the most numerous group of animals on Earth, their individuals perhaps outnumbering us human beings by about a billion-to-one! Relying primarily on intelligence, we human beings are the most successful single species on the planet; whereas insects, relying primarily on instinct, are the most successful group of species: This is indeed the age of insect and human beings!

DESCRIPTION

Insects are usually 1/4" long or less (which is advantageous for hiding), although some species (such as certain walkingsticks) grow to over a foot in length.

Colors range from pale or drab to dark or colorful, often with markings (as for camouflage, recognition of mates, or warding off predators).

The body of an insect is composed of three main divisions: The head, the thorax, and the abdomen.

The head of an insect typically bears three light-sensing "simple eyes", one pair of image-resolving "compound eyes", one pair of antennae, one "labrum" (upper lip), one pair of jaw-like "mandibles", and two pairs of "maxillae" -- the front pair jaw- and finger-like and the rear pair fused into a "labium" (lower lip). The mouthparts of various insects are variously modified, such as the chewing mouthparts of a grasshopper (the basic design), the coiled nectar-siphoning mouthparts of a butterfly, the piercing/sucking mouthparts of a sap-sucking aphid or a blood-sucking mosquito, or the sponging mouthparts of a house fly.

On the thorax, three pairs of jointed legs are almost always present (As arguably the most advanced arthropods, insects have no more legs than are absolutely necessary for stability: In walking, half the legs are off the ground at any given time, while the other "three points determine a plane"). One or, usually, two pairs of wings are also usually present on the thorax (of the adults). The wings are usually marked by "veins" (structural thickenings, useful in identifying families of insects). The front pair of wings are modified as hard wing covers in beetles (the most common insects), and the rear pair of wings are simply a pair of "gyroscopic" knobs in true flies (providing stability in flight).

Many insects can fly -- insects were the first animals to fly and still are the only invertebrates to fly (although some small spiders "balloon" through the air on "gossamer" strands of their silk). The ability to fly is helpful for getting to foods, mates, and new territories. Insect flight probably evolved as an escape from hunting spiders, on the ground; and web spiders probably evolved in response to flying insects. Successful defensive behaviors of insects include not only flight but also fright (Various weevils "play possum" when disturbed) and fight -- as with physical weapons (such as the formidable spines or jaws of various large beetles) or chemical weapons (as of stink bugs, bombardier beetles, or stinging wasps).

The abdomen of an insect occasionally bears long or finger-like, typically sensory or reproductive appendages at the tail-end.

METAMORPHOSIS

The "ovipositor" (egg-laying organ) at the tail-end of a female insect is typically used to place the eggs in locations favorable for the development of the hatchlings. Some wasp-like insects can insert their long, fine ovipositor through solid wood, into the burrow of a wood-boring grub, the "host" of their parasitic larvae. The ovipositor of ants, wasps, and bees is typically modified into a sting.

The more primitive orders of insects develop by means of a "simple "metamorphosis". Typically, from the egg hatches a "nymph" (called a "naiad" in aquatic species), which looks like a small adult (although virtually never with wings). After feeding, growing, and molting, the nymph (which may have developed "wing pads") finally molts into the adult, which typically bears fully developed wings, with which it can fly away to mate and disseminate the species.

More advanced insects undergo a "complex metamorphosis". Typically, from the egg hatches a "larva" (such as a grub, caterpillar, or maggot), which looks more like a segmented worm than the adult. After feeding, growing, and molting, the larva (which never develops any sorts of wings) finally molts into a "pupa" (as in the cocoon of a moth). Although this is typically an apparently resting stage, by the time the adult finally emerges from the pupal skin, it has transformed into a winged form, much different from any of the young, not only in appearance but also in habits: Complete metamorphosis typically allows the larvae to exploit different food sources and other environmental "niches" than the adults do, thus minimizing competition between members of the same species.

In addition, some insects are "social", with a division of labor between specialized "castes": Please see Termites and Wasps, Ants, & Bees.

HABITATS

Insects are found in the water (typically in freshwater, but sometimes in saltwater marshes), on the land (especially on or in soil, plants, or other animals), or in the air.

Some insects live in shelters of their own construction, such as caddisworms in their silk-and-sand tubes or wasps in the pulped-paper, perfectly hexagonal cells of their nest.

FOODS

Virtually anything edible is eaten by at least one species of insect.

Various insects, as young or adults, are scavenging, filter-feeding, herbivorous, carnivorous, "omnivorous" (eating almost anything), or parasitic (Unlike typical parasites, however, insect parasites of other invertebrates don't just make their "hosts" sick: They typically kill them -- a useful habit in the "biological control" of insect pests).

DAMAGES/BENEFITS

Many insects are typically well-known pests, as of buildings, crops, animals, and human beings: These species do billions of dollars' worth of damage every year and cost many human lives (especially by spreading disease).

However, many, often less well-known insects are beneficial, to us and to the environment in general: These species include insects that improve the soil, honey bees and other insects that pollinate plants (the biggest direct monetary benefit from insects), insects that are predators or parasites of pests (an often overlooked benefit), silkworms and other insects that produce valuable products, and insects that are food for wildlife.

Overall, beneficial insects do more good -- in terms of dollars returned, human lives supported, and ecological upkeep -- than all the pests do harm: Insects in general are beneficial to us and absolutely vital to nature.

Representatives

Arthropods (Arthropoda)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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