All living creatures must have sensory organs that receive information about the outside world, and a nervous system, which can convey the data to be translated into appropriate behaviour. The senses on which insects rely are very much the same, as we know them from ourselves. This means sense of touch, hearing, vision, heat and cold and chemical senses such as smell and taste.
The sense of touch is tied to different kinds of hair. When hair is moved, a sensing cell is aroused and sends messages to the central nervous system. The sense of hearing can also be tied to hair that is so sensitive that they can be moved by acoustic waves, but there are also insects that have developed actual eardrums. Insect eyes and perception differs in many ways from ours. The insect eye is divided into many – often thousands – small facets. Each facet is a tiny lens that sits at the end of a tubular eye. Each of these eyes forms, on the retina, an image of the small part of the surroundings, which it is now targeting. With that construction, the insect eye cannot focus on a single object, and the insect image of the surroundings is very much like a mosaic. The insect eye is suited to perceive changes in the visual field, thing that moves, for example, a prey or a predator. In addition to the large compound eyes, many insects have small, dot-like eyes on top of their heads. Much suggests that they act as a kind of built-in light meter, which somehow regulates the light sensitivity of the big eyes. Insect eyes are not equipped with an iris diaphragm, such as our own eyes are.
Many insects can distinguish between different colours. For example, experiments have shown that house flies prefer dark red and black colours, and that fact is used in several types of fly traps. We also know that the sensitivity of insect eyes is staggered compared to the frequency range that we see. Most insects do not react to long wave light, which is red to the human eye. However, most insect eyes perceive the short-wave ultraviolet light that we do not see. Many insects are sensitive to ultraviolet light and are attracted to it more than to other wavelengths of light. This is used in certain light traps. Insects seek towards light because light usually means a way out. For this same reason it is common to find adult drugstore beetles in windows.
We have not yet been able to locate the cold and heat senses to special sense organs, so it is probably through a process in the insect brain that they are kept informed of the temperature.
The chemical senses are extremely well developed in many insects. The main biological significance of the sense of smell is that it makes it possible for the insects to distinguish between hazardous and favourable things at a distance. The sense of smell is tied to tiny hairs that sit on their antennae. These hairs are designed like a sieve with tiny holes that allow the fragrance substance molecules to penetrate. Inside hollow hairs are the fragrances associated with sensory cells that convert the chemical signal into electrical impulses. The sense of taste is also tied to hair, which in principle work like the hairs tied to smell. They are particularly located around the insect mouth, but can also be part of the feet which for example makes a fly able to taste whatever it is standing on.