Unlike our blood insect blood rarely has any special substances, such as haemoglobin, for absorbing oxygen, and the blood plays only a minor part in the breathing process. Oxygen in carried directly to the tissues through branching tubes called trachea, which are found in all but the most primitive of insects, and some very specialised internal parasites.
A trachea is a flexible tube which branches into smaller tubes called tracheoles (see the drawing of a flea showing the trachea and spiracles below). The tracheal opening to the exterior is called a spiracle. These usually run down the sides of the insect body, and are most easily seen in a large caterpillar.
In small insects diffusion through the tracheoles is sufficient to supply their needs. In larger insects abdominal pumping is necessary. This can be seen by watching a stationary insect, especially a bumblebee, its abdomen will pulsate.
The spiracles act as valves to the outside, and most can be partly or completely closed. This also helps to reduce water loss from the body. Carbon dioxide escapes through the exoskeleton. Some insect living in water have a siphon, or breathing tube, (Water stick insect, Water scorpion), others have gills, (damsel fly nymphs, may fly nymphs), and others hold a bubble of water, (water beetles).
The Insect Mouth
Although insect mouthparts vary enormously (see left and below), they all fall into two main types; biting for solid food, and sucking for liquid food. At the sides of the mouth insects often have a pair of palps which help in detecting and tasting food. The type of mouthparts are a good guide to the kind of food the insect consumes, and include:
piercing and sucking stylets of insects that must penetrate the tough external
covering of skin or plant cells, e.g. mosquitoes and aphids
The long proboscis required to reach the nectar of flowers with long
corollas, e.g. in butterflies and moths
The sponging or
lapping mouthparts found in houseflies that are efficient at mopping up
easily reached liquids
The crushing, chewing mouthparts of predatory beetles.
However some adult insects do not feed at all, and have no, or reduced mouthparts, e. g. some may flies. Also the feeding habits of the adult insect may be very different to the larva or nymph, e. g. wasp larvae are carnivorous and eat insects, whilst adults are vegetarian and eat only sugary fluids such as nectar.
Left is the mouthparts of the whirligig beetle. The mandibles are for biting/chewing, and are the equivalent of our jaws. The maxillae are accessory jaws; the maxillary palps are sensory for testing food, and the labium and labial palps are also sensory. The labium is the equivalent of our lower lip.
Some insects such as crickets, grasshoppers and cicadas have special hearing organs called tympana. Other insects have sensory hairs scattered about their body which vibrate in response to sounds.
Complete metamorphosis involves a resting stage, e. g. a chrysalis, cocoon or pupa. During this time the body of the insect larva breaks down into a kind of mush with only a few cells remaining intact. It is these few cells that will go on to develop the eyes, wings, etc. of the adult insect by feeding on the mush and dividing.
So really to call this a resting stage does not seem right. There may not be much going on if looked at from the outside, but inside it is like a factor at full production transforming a maggoty eating machine into a gloopy soup, and then into the mating machine that is the adult insect.
The insect eyes and vision
Insect sight varies largely according to its habitat and feeding requirements. In general insects that live in the soil, inside structures, or are parasitic tend to have no or smaller, flatter eyes with fewer ommatidia, e. g. some termites. Insects which have to hunt for their food tend to have larger, convex eyes with more ommatidia, e. g. flies and dragonflies. Also male insects that have to seek out females tend to have larger eyes than the females of the same species.
Insects have 2 types of eyes - compound eyes (right and below left) composed of many hexagonal ommatidia, and ocelli (below right), also called simple eyes.
The compound eye is made up of a number of hexagonal facets or ommatidia. Each facet has a thin outer cuticular layer, slightly convex, which acts as a simple, rigid lens (see the bumblebee eye on the right). But unlike the human eye, there is no focusing muscle or any means of altering the focal length.
Light passes through this to a crystalline layer, then to the retinal cells at the base, which leads to the optic nerve fibres and then the brain (see the diagram below left). So each omatidium is a separate unit, and picks up only one small section of what human eyes see.
The size of the omatidia vary in some insects, and can have different functions, e. g. small omatidia may be useful in prey or mate recognition, whereas large omatidia may interpret light intensity.
The number of ommatidia in the compound eye varies greatly in the different types of insect
Simple eyes (ocelli) are usually round, convex and shiny black. They are usually arranged in a triangle behind the antennae - see the photograph on the right of the head of a male bumblebee. And appear simply to detect different light intensities.
Most insect larvae do not have compound eyes, but do have 3 ocelli arranged in a triangular pattern.
Insects tend to be able to see colours differently to humans - basically they can see yellow, green, blue and ultra violet, but not red. In pollinating insects, those that pollinate what we see as red flowers, when these flowers are viewed under UV light they usually have a pattern of lines and blotches leading to the nectary. These patterns are called honey guides.
Many insects can use polarised light to detect the sun, and use this as a navigational aid, e. g. honeybees. And some nocturnal moths can use the moon and stars in the same way.
Frequently asked questions about insects
Does every insect go through metamorphosis?
No. Some such as the Thysanura (bristletails and silverfish) look the same all their life after hatching from the egg. Others are termed hemimetabolous, this means they go through a partial metamorphosis, e.g. the Orthoptera (crickets, grasshoppers and locusts). The young do resemble the adults, but often the adults will have wings, and these wings will develop more at each moult, so the metamorphosis is gradual. Others go through a complete metamorphosis and are termed holmetabolous, e.g. Lepidoptera (butterflies and moths). These have a resting stage (chrysalis, pupa, etc.) where the body changes completely from the young larval form to the adult form.
Does every bug have compound eyes?
No. Some have no eyes at all, e. g. some species of termite, some ants.
Can an insect get too fat?
This question has been asked many times about bumblebees, so you can read the answer here.