The main or central nervous system consists of a large ganglion, the "brain," situated in the head above the œsophagus, which sends nerves to the antennæ and eyes, a ganglion in the head below the œsophagus connected with the brain by a short commissure on each side of the œsophagus, and sending nerves to the mouth-parts; and a ventral nerve-chain composed of a pair of longitudinal commissures lying close together and running from the head to the next to the last abdominal segment, which bears a series of segmentally disposed ganglia, each ganglion being composed of two ganglia more or less nearly completely fused. There is, in addition, a lesser system called the sympathetic system, which comprises a few small ganglia and certain nerves which run from them to the viscera. The function of the nervous system of insects reaches a very high development among the so-called "intelligent insects" and certain extraordinarily complex and interesting instincts are possessed by many forms. The social or communal habits of the ants, bees, and wasps and the habits connected with the deposition of the eggs and the care of the young exhibited by the digger wasps and other insects are of extreme specialization. The organs of special sense are highly specialized, the sense of smell (fig. 48) reaching in particular a high degree of perfection. One of the compound eyes (figs. 49 and 50) may contain as many as 30,000 distinct eye-elements or ommatidia, but the sight is probably in no insect very sharp or clear. Among insects there are organs of hearing of two principal kinds. In one kind the organ for taking up the sound-waves is a group of vibratile hairs usually situated on the antennæ, as is the case with the mosquito; in the other kind, it is a stretched membrane or tympanum such as is found in the fore leg of a cricket or katydid or on the first abdominal segment of the locust (fig. 51). [Illustration: FIG. 50.--Part of cornea, showing facets, of the compound eye of a horse-fly (_Therioplectes_ sp.). (Photo-micrograph by Geo. O. Mitchell.)] [Illustration: FIG. 51.--The auditory organ of a locust (_Melanoplus_ sp.). The large clear part in centre of the figure is the thin tympanum, with the auditory vesicle (small black pear-shaped spot) and auditory ganglion (at left of vesicle and connected with it by a nerve) on its inner surface. (Photo-micrograph by Geo. O. Mitchell.)]