Spoken and especially written language, moreover, enable man to exploit his brain to a wonderful extent. This leads us to underestimate animals. Both in animals and man the true value of the brain is falsified by training, i. e., artificially heightened. We overestimate the powers of the educated negro and the trained dog and underestimate the powers of the illiterate individual and the wild animal.
I beg your indulgence for this lengthy introduction to my subject, but it seemed necessary that we should come to some understanding concerning the validity of comparative psychology. My further task now consists in demonstrating to you what manner of psychical faculties may be detected in insects. Of course, I shall select in the first place the ants as the insects with which I am most familiar. Let us first examine the brain of these animals.
In order to determine the psychical value of a central nervous system it is necessary, first, to eliminate all the nerve-centers which subserve the lower functions, above the immediate innervation of the muscles and sense-organs as first centers. The volume of such neuron-complexes does not depend on the intricacy of mental work but on the number of muscle-fibres concerned in it, the sensory surfaces, and the reflex apparatus, hence above all things on the size of the animals. Complex instincts already require the intervention of much more plastic work and for this purpose such nerve-centers alone would be inadequate.
A beautiful example of the fact that complex mental combinations require a large nerve-center dominating the sensory and muscular centers is furnished by the brain of the ant. The ant-colony commonly consists of three kinds of individuals: the queen, or female (largest), the workers which are smaller, and the males which are usually larger than the workers. The workers excel in complex instincts and in clearly demonstrable mental powers (memory, plasticity, etc.). These are much less developed in the queens. The males are incredibly stupid, unable to distinguish friends from enemies and incapable of finding their way back to their nest. Nevertheless the latter have very highly developed eyes and antennae, i. e., the two sense-organs which alone are connected with the brain, or supra-oesophageal ganglion and enable them to possess themselves of the females during the nuptial flight. No muscles are innervated by the supra-oesophageal ganglion. These conditions greatly facilitate the comparison of the perceptive organs, i. e., of the brain (corpora pedunculata) in the three sexes. This is very large in the worker, much smaller in the female, and almost vestigial in the male, whereas the optic and olfactory lobes are very large in the latter. The cortical portion of the large worker brain is, moreover, extremely rich in cellular elements. In this connection I would request you to glance at the figures and their explanation.
Very recently, to be sure, it has come to be the fashion to underestimate the importance of brain-morphology in psychology and even in nerve-physiology. But fashions, especially such absurd ones as this, should have no influence on true investigation. Of course, we should not expect anatomy to say what it was never intended to say.
In ants, injury to the cerebrum leads to the same results as injury to the brain of the pigeon.
In this place I would refer you for a fuller account of the details of sensation and the psychic peculiarities of insects to my more extended work above mentioned: Sensations des Insectes.
It can be demonstrated that insects possess the senses of sight, smell, taste, and touch. The auditory sense is doubtful. Perhaps a sense of touch modified for the perception of delicate vibrations may bear a deceptive resemblance to hearing. A sixth sense has nowhere been shown to occur. A photodermatic sense, modified for light-sensation, must be regarded as a form of the tactile sense. It occurs in many insects. This sense is in no respect of an optic nature. In aquatic insects the olfactory and gustatory senses perhaps grade over into each other somewhat (Nagel), since both perceive chemical substances dissolved in the water.
The visual sense of the facetted eyes is especially adapted for seeing movements, i. e., for perceiving relative changes of position in the retinal image. In flight it is able to localise large spatial areas admirably, but must show less definite contours of the objects than our eyes. The compound eye yields only a single upright image (Exner), the clearness of which increases with the number of facets and the convexity of the eye. Exner succeeded in photographing this image in the fire-fly (Lampyris). As the eyes are immovable the sight of resting objects soon disappears so far as the resting insect is concerned. For this reason resting insects are easily captured when very slowly approached. In flight insects orient themselves in space by means of their compound eyes. Odor, when perceived, merely draws these animals in a particular direction. When the compound eyes are covered, all powers of orientation in the air are lost. Many insects can adapt their eyes for the day or night by a shifting of the pigment. Ants see the ultra-violet with their eyes. Honey-bees and humble-bees can distinguish colors, but obviously in other tones than we do, since they cannot be deceived by artificial flowers of the most skilful workmanship. This may be due, to admixtures of the ultra-violet rays which are invisible to our eyes.
