Anatomy of an Insect

by Michael Holmes

What makes an insect an insect?

First, to be considered an insect you must have a segmented body surrounded by an exoskeleton (an external casing that supports and protects the body). The insect body is composed of three main parts: a head, a thorax, and an abdomen. On the head is a pair of sensory antennae, as well as two large compound eyes and one to three small light-sensitive simple eyes, or "ocelli". Attached to the thorax are six segmented legs (three pairs) and two or four wings.

A few insects are wingless, such as silverfish (Lepisma saccharina). Other insects, like ants, are wingless for most of their lives, but may grow wings when it is time to mate and start a new colony.

Insects breathe through pairs of holes called spiracles located along their abdomen. Air passes through these holes into their trachea, a system of tubes that carries oxygen through the body. Unlike in the bodies of you and me, the oxygen breathed by insects does not mix with their blood. Insects have a closed respiratory system and an open circulatory system, meaning that oxygen is delivered directly to body tissues, not carried by the blood, while the blood, called hemolymph, moves freely within the body cavity.

Some aquatic insects and insect larvae breathe through a special tube called a siphon. Acting like a snorkel, this tube allows the insect to breathe air from the surface. Mosquito larvae breathe through the siphons at their backend, which is why they rest "upside-down" at the surface of stagnant water.

The mouthparts of insects vary widely. Typically, insect mouthparts can be described as either "sucking" or "chewing". The strong mandibles of grasshoppers and dragonflies tear and chew their food. Others, like butterflies and moths, have a long straw-like proboscis that they uncurl to suck up nectar deep inside flowers. The unique spongy mouthparts of flies allow them to absorb liquids. Bees seem to have a combination of sucking and chewing mouthparts, suited for drinking nectar, molding wax, and other bee-related activities. However an insect fuels its body, the little mouths of these little beings have evolved to facilitate food ingestion in the most sophisticated manner.

Come winter (it's coming!), many adult insects die from the cold and the frost. But some spend the winter in a state of hibernation called diapause. In spring, as the snow melts away, these insects will re-awaken to feed and begin producing offspring for the new season.

Arachnids, though not insects, possess many similar physical characteristics such as an exoskeleton. A distinguishing feature is the arachnid's eight legs, rather than six. Spiders, scorpions, mites and ticks all have eight legs and are all arachnids. Their bodies are divided into two main parts, the cephalothorax (including the head) and the abdomen. Whether spinning silk or brandishing giant claws, arachnids can be both elegant and formidable creatures.

To describe all the varied differences and unique characteristics within the insect world is much too great a task for this column. Diversity among insects, even within a single order, is astounding. Though people have studied insects for hundreds of years on every continent on the planet, there is still much we do not know and entire species yet to be discovered.

It is tempting to think of insects as simple and primitive creatures, yet the closer we look at them the more we find them to be among the most diverse, successful, and fascinating animals on Earth.