THE hard coat with which insects are covered obviously necessitates the provision of special organs of touch to connect the outside world with the nervous system so that the insect may react in the way best calculated to meet external stimuli.
The insect body is covered with hairs which rest on sensory cells, and when these hairs are touched, the stimulus is passed on to the nervous system by the sensory cells.
Touch to insects probably means something more than touch to man, for they often show in their reactions discrimination between types of touch other than degrees of pressure. For example, a caterpillar passing over a leaf may have its sensory hairs depressed by the leaf hairs and it shows no visible reaction; on the other hand, should it be touched by something new to its experience, it exhibits a definite response. As in other animals, heat and cold are distinguished by the sense of touch. There are also other hairs capable of detecting chemical stimuli. The sense of touch is very important to social insects like ants which spend much of their time in the dark.
The insect smells with its antenna?. In the bee, the smelling or olfactory organ is composed of an oval plate overlying sensory tissue, the whole being borne by the antenna. If the organ is cut away, the bee exhibits no power of smell. The social insects recognise members of their own colony by nest-smell. Since we have been speaking of the olfactory sense, this is the place to draw the reader’s attention to the fact that many insects are able to produce scents. These may be protective and are then used to deter enemies, or they may perform the function of attracting opposite sexes—male or female. Male moths are often attracted to a female in this way from quite a considerable distance.