Insect

I

Introduction

Insect, small, air-breathing animal characterized by a segmented body with three main parts—head, thorax, and abdomen. In their adult forms, insects typically have three pairs of legs, one pair of antennae, and in most instances, two pairs of wings.

For sheer variety and abundance, insects rank among the most successful animals on Earth. About one million species of insects have been identified so far, which is about half of all the animals known to science. Insects live in almost every habitat on land. For example, distant relatives of crickets called rock crawlers survive in the peaks of the Himalayas by producing a kind of antifreeze that prevents their body fluids from freezing solid. At the other extreme are worker ants that forage for food in the Sahara Desert at temperatures above 50° C (122° F). Insects consume an enormous variety of food. In the wild, many eat leaves, wood, nectar, or other small animals, but indoors some survive on a diet of wool clothes, glue, and even soap. As a group, insects have only one important limitation: although many species live in fresh water—particularly when they are young—only a few can survive in the salty water of the oceans.

Insects are often regarded as pests because some bite, sting, spread diseases, or compete with humans for crop plants. Nevertheless, without insects to pollinate flowers, the human race would soon run out of food because many of the crop plants that we rely on would not be able to reproduce. Insects themselves are valued as food in most of the world, except among Western societies (see Entomophagy). They help to recycle organic matter by feeding on wastes and on dead plants and animals. In addition, insects are of aesthetic importance—some insects, such as dragonflies, beetles, and butterflies, are widely thought to be among the most beautiful of all animals.

Insects are invertebrates, animals without backbones. They belong to a category of invertebrates called arthropods, which all have jointed legs, segmented bodies, and a hard outer covering called an exoskeleton. Two other well-known groups of arthropods are crustaceans, which include crayfish and crabs, and arachnids, which include spiders, ticks, mites, and scorpions. Many types of arthropods are commonly called bugs, but not every “bug” is an insect. Spiders, for example, are not insects, because they have eight legs and only two main body segments.

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II

Body Plan

Insects range in length from the feathery-winged dwarf beetle, which is barely visible to the naked eye at 0.25 mm (0.01 in), to the walkingstick of Southeast Asia, which measures up to 50 cm (20 in) with its legs stretched out. The vast majority of insects fall into the size range of 6 to 25 mm (0.25 to 1 in). The heaviest member of the insect world is the African goliath beetle, which weighs about 85 g (3 oz)—more than the weight of some birds.

Regardless of their size, all adult insects have a similar body plan, which includes an exoskeleton, a head, a thorax, and an abdomen. The exoskeleton protects the insect, gives the body its form, and anchors its muscles. The head holds most of an insect’s sensory organs, as well as its brain and mouth. The thorax, the body segment to which wings and legs are attached, is the insect’s center of locomotion. An insect’s large, elongated abdomen is where food is processed and where the reproductive organs are located.

A

Exoskeleton

Like other arthropods, an insect’s external skeleton, or exoskeleton, is made of semirigid plates and tubes. In insects, these plates are made of a plasticlike material called chitin along with a tough protein. A waterproof wax covers the plates and prevents the insect’s internal tissues from drying out.

Insect exoskeletons are highly effective as a body framework, but they have two drawbacks: they cannot grow once they have formed, and like a suit of armor, they become too heavy to move when they reach a certain size. Insects overcome the first problem by periodically molting their exoskeleton and growing a larger one in its place. Insects have not evolved ways to solve the problem of increasing weight, and this is one of the reasons why insects are relatively small.

B

Head

An insect obtains crucial information about its surroundings by means of its antennae, which extend from the front of the head, usually between and slightly above the insect’s eyes. Although antennae are sometimes called feelers, their primary role is to provide insects with a sensitive sense of smell. Antennae are lined with numerous olfactory nerves, which insects rely on to smell food and detect the pheromones, or odor-carrying molecules, released by potential mates. For example, some insects, such as ants and honey bees, touch antennae to differentiate nest mates from intruders and to share information about food sources and danger. The antennae of mosquitoes can detect sounds as well as odors.

Antennae are composed of three segments, called the scape, pedicel, and flagellum. They may have a simple, threadlike structure, but they are often highly ornate. Some male giant silkworm moths, for example, have large, finely branched antennae that are capable of detecting pheromones given off by a female several miles away.

An insect’s head is typically dominated by two bulging eyes, which are called compound eyes because they are divided into many six-sided compartments called ommatidia. All of an insect’s ommatidia contribute to the formation of images in the brain. Insect eyes provide a less detailed view of the world than human eyes, but they are far more sensitive to movement. Insects with poor vision, such as some worker ants, often have just a few dozen ommatidia in each eye, but dragonflies, with more than 20,000 ommatidia, have very keen vision—an essential adaptation for insects that catch their prey in midair.

Most flying insects also have three much simpler eyes, called ocelli, arranged in a triangle on top of the head. The ocelli can perceive light, but they cannot form images. Clues provided by the ocelli about the intensity of light influence an insect’s level of activity. For example, a house fly whose ocelli have been blackened will remain motionless, even in daylight.

The head also carries the mouthparts, which have evolved into a variety of shapes that correspond to an insect’s diet. Grasshoppers and other plant-eating insects have sharp-edged jaws called mandibles that move from side to side rather than up and down. Most butterflies and moths, which feed mainly on liquid nectar from flowers, do not have jaws. Instead, they sip their food through a tubular tongue, or proboscis, which coils up when not in use. Female mosquitoes have a piercing mouthpart called a stylet. House flies have a spongy pad called a labellum that dribbles saliva onto their food. The saliva contains enzymes that break down the food, and once some of the food has dissolved, the fly sucks it up, stows away the pad, and moves on.

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