An animal’s existence depends upon its ability to avoid being eaten by predators, to find food and to be able to find other members of its species. The animal must, therefore, have sense organs which can receive information about what is happening around it, and it must have a nervous system which can transmit this information to organs capable of giving an appropriate response. With their hard exoskeleton, insects and other arthropods appear somewhat stiff, and one might wonder how much they actually perceive. In fact they receive a great amount of information about what is going on around them and in general they have the same kinds of senses as man and the other vertebrates: touch, hearing, vision, sense of heat and cold, and the chemical senses of taste and smell. It would be wrong, however, to think that they perceive the outside world in the same way as we do.
In the arthropods, the sense of touch is associated with various types of hair. When a hair is touched it stimulates a sensory cell which then sends a message to the central nervous system.
Hearing may be associated with hairs which are moved by sound waves, but there are also some insects which have developed true tympanic organs, as for example in the crickets.
Such organs resemble part of a vertebrate ear in having a thin membrane which is made to vibrate by sound waves. Vision in the insects is served by the large lateral compound eyes, which are often made up of thousands of individual eyelets or ommatidia.
Taste and smell are extremely well developed in many insects. They are associated with certain specialised sensory hairs, which are situated on many parts of the body but particularly on the mouth- parts and antennae.
The perception of heat and cold is not localized in special sense organs, but arthropods are probably made aware of the temperature in the first place by its effect on their internal body processes.
The arthropods are therefore very well provided with senses, which are often extremely sensitive. Bluebottles, for example, can smell meat at a distance of several kilometres and find their way to it with unfailing accuracy.
The behaviour of invertebrates often appears to be extremely rational but there is no evidence that they can actually think about what they are doing.
If their normal activity appears to be rational, this is because they have innate behaviour patterns which are in certain situations released by a given set of stimuli. Animals, such as insects and other arthropods, which live such a short time, are in fact better served by effective, inbuilt reactions than by having a well-developed capacity to learn, for they simply do not have the time to gain experience and to learn from it.