Insects – physical characteristics

Insects can be large – like the 130 mm wingspan of the kapokapowai/bush giant dragonfly – or tiny – like the 2 mm namu/West Coast blackfly. Insects can be beneficial like the honey bee or a pest like the clover root weevil. They can be daytime fliers like the kahukura/red admiral butterfly or nocturnal fliers like the pūriri moth.

Regardless of their size, behaviour or habitat, insects worldwide have common physical characteristics.

The exoskeleton

Insects have an outer skeleton called an exoskeleton. This hard covering protects and supports the body. It is made up of two layers. The outer layer is thin, waxy and water resistant. The inner layer is much thicker and is made of chitin. Depending on the insect, the exoskeleton can be quite hard and rigid – like that of a cockroach – or less so when the insect is in its larval (caterpillar) stage or if it has a soft body – like a blue blowfly.

The exoskeleton is non-living so it cannot grow with the insect when it is in its larval form or as an adult. Insects shed their old exoskeletons, expand to a larger size and then wait for the new exoskeleton to harden. This is called moulting, and it happens many times during an insect’s lifespan. One of the most common signs of moulting comes in the spring when we find cicada shells (exoskeletons) on trees and fences.

The head

The head is the top section of an insect’s body. Many of the insect’s sense organs are located here. A pair of antennae allow insects to smell, feel the surface of an object, sense hot and cold, listen to sounds or detect movement. Insects have a pair of compound eyes – each made up of thousands of lenses. Compound eyes provide a wider field of vision, which is really useful for flight or when hunting prey. Insects may also have simple eyes called ocelli, which consist of single lenses. They allow insects to sense light and dark. You can see three ocelli on the head of the wasp in this image – they look like black bumps.

The insect’s mouthparts are also on the head. Insects have evolved mouthparts for a variety of purposes. Insects such as beetles and grasshoppers have mouthparts that allow them to chew. Butterflies and moths have a proboscis that allows them suck liquids. Insects like mosquitoes have mouthparts that pierce their food, while houseflies sponge their food, dissolving it in saliva before sucking it up.

The thorax

The thorax is an insect’s middle section. It consists of three segments. Each segment has a pair of jointed legs. An insect’s legs are adapted to suit their lifestyle and habitat. They include:

• long narrow legs made for running and fast movement (beetles and cockroaches)
• muscular hind legs made for jumping (grasshoppers and fleas)
• hunting forelegs made for grabbing and holding prey (praying mantises)
• swimming legs made for easy movement through water (water boatmen)
• broad, flat forelegs made for digging burrows (mole crickets).

If the insect has wings, a pair of forewings and a pair of hind wings are attached to the thorax. Like our native birds, Aotearoa New Zealand has a significant number of wingless or flightless insects. For example, the honi/mole cricket is the only wingless mole cricket in the world!

The abdomen

The abdomen is the rear or final section of the insect. This is where the digestive, excretory and reproductive organs are located. The abdomen has 9–11 segments. Each segment has a pair of spiracles or openings in the exoskeleton. Insects open the spiracles to allow air in but close them to prevent water loss. Aquatic insects have similar methods to prevent water from entering the spiracles.