Note: This page describes insects, but not spiders. Spiders belong to a different class, Arachnida. You can read about them here.
There are millions of species of insects in the world and their appearance can vary hugely from each other (think of the difference between a dragonfly and an ant.). But all insects have these three things in common, regardless of their individual size, coloration, life style, behavior or habitat:
1. An exoskeleton
2. Three main body parts: head, thorax, abdomen
3. Six jointed legs
The exoskeleton is an insect’s entire outer body. Made of chitin, it protects the muscles and organs. Chitin is a chemical mixture made of natural elements and can be firm or yielding, depending on its thickness. So, an insect’s legs and face have thin layers to maintain pliability, while the rest of the exoskeleton is thicker and more protective. The exoskeleton is sometimes called the cuticle.
An insect’s head is all about food intake and sensing. The mouthparts are designed for chewing, sucking, piercing or sponging, depending on the insect’s method for ingesting food. For instance, grasshoppers chew their food, while adult butterflies suck their food up. Mouthparts include the labrum and labium (upper and lower “lips”), the maxillae (max-SIL-ee) and the mandibles, which are sets of jaws designed to bite. There are some insects that don’t eat while in the adult stage and don’t have mouthparts.
The head is fitted with a pair of antennae (plural: an-TEN-ee; sing.: an-TEN-uh). They serve as primary sense organs for smell, touch and taste. They may be long, short or stubby, feathery, bristly, straight or hair-like. Some even look like a string of beads.
Most insects have two large compound eyes. These are formed by hundreds or even thousands of lenses called ommatidia (singular: ommatidium). Each lens is fixed in place and faces a direction that is slightly different from the direction faced by all the other lenses. And, each is equipped with an optic nerve through which it feeds its one tiny visual angle to the brain. The brain integrates them all together to form a complete picture.
Compound eyes enable insects to detect motion and light changes quickly — even the slightest movement. Most insects have good eyesight only within a few feet, however. Some insects also have as many as three “simple eyes,” called ocelli (o-CELL-eye), which are cells sensitive to light and sometimes motion.
Insects have a brain and a nerve bundle. The brain processes information and nerve centers elsewhere in the body do, too. Insects don’t have ears. Instead they detect sound as vibrations in the air. Some insects use antennae to pick up the vibrations, but mostly it’s the hairs on their body that do it.
The thorax is the middle part of the body and is comprised of three segments, with some parts of them hardened. In adults, one pair of legs is attached to each segment (many larvae are caterpillar or worm-like and don’t have legs). Spiracles, which are openings for air intake, are often present in the segments. Wings are usually present, but not always.
The legs are jointed and each has a femur and tibia. The feet are called tarsi (TAR-sigh; sing. tarsus) and the “toes” are claws. Besides being used for walking, legs may be used for jumping, catching prey, burrowing or swimming. Also, some insects make sounds by rubbing their hind legs together. When walking, insects usually move three legs at a time — the front and back legs on one side and the middle leg on the opposite side. This helps the insect maintain balance.
Different species of insects use their legs for different purposes. For instance, insects who swim have legs that look like paddles. Insects who dig have strong front legs shaped something like shovels, while insects who jump have, as you would imagine, powerful back legs.
Usually two pair of wings is attached to the thorax of adult insects, one to the second segment and the other to the third. Some insects, like worker ants and fleas, don’t have wings. Wings are usually thin and are given support by many veins running through them. A very muscular hinge joins them to the thorax and provides the range of motion and strength needed for flight.
Flies only have one set of wings, a front pair. The hind wings are modified into halteres (HALL-tirs), little nubs that act as flight stabilizers. Some insects have hardened forewings that are used to protect membranous hind wings, which do the flying. Some insects have wings that are transparent, but many are cloaked in hairs or scales.
The abdomen is usually the largest part of the body. Within it are the heart and aorta, trachea, digestive tract and reproductive organs. The abdomen is made up of six to 11 segments shaped like rings, making it very flexible. Along the outside are spiracles and at the tip are the external sex organs.
The heart is long and tube-like. It doesn’t transport oxygen, like in humans, but it does beat regularly and pump nutrient-rich haemolymph (insect blood) to the cells and removes waste products.
The heart doesn’t have arteries. Instead the blood pumps from the heart to the front of the body and into the head, then back through open areas of the body, where it washes all the organs with nutrients and takes up waste. From there it flows to the rear end and back to the heart. Insect blood isn’t red; it’s greenish, yellowish or clear. (Human blood is red because it contains oxygen-grabbing hemoglobin, which insects don’t need.)
Insects need oxygen to survive. They get it through tiny openings in the sides of their bodies called spiracles. Air flows through the spiracles and into the trachea. The trachea is tube-like and breaks down into smaller and smaller tubes, feeding air to every cell in the body. Some aquatic insects breathe by way of gills filled with air tubes and get their oxygen from the water, just like fish. Others go to the surface and grab an air bubble, which they carry under their wings or abdomen.
The digestive system is usually a long tube that stretches from the mouth to the anus. It consists of several sections (guts), including a stomach. Nutrients are extracted from food and passed through semi-permeable membranes into the body cavity. The hindmost section is the “intestines” and ends with a rectum that compresses waste products. From there the waste passes out of the body through the anus.
*Top image: Piotr Jaworski / Wiki; cc by-sa 3.0