_Nervous System._--The nervous system in the Hexapoda is built up on
the typical arthropodan plan of a double ventral nerve-cord with a
pair of ganglia in each segment, the cords passing on either side of
the gullet and connecting with an anterior nerve-centre or brain (fig.
7) in the head. The brain innervates the eyes and feelers, and must be
regarded as a "syncerebrum" representing the ganglia of the three
foremost limb-bearing somites united with the primitive cephalic
lobes. Behind the gullet lies the sub-oesophageal nerve-centre (fig.
7, sb), composed of the ganglia of the four hinder head-somites and
sending nerves to the jaws. A pair of ganglia in each thoracic segment
is usual (fig. 8), and as many as eight distinct pairs of abdominal
ganglia may often be distinguished, the hindmost of which represents
the fused ganglia of the last four segments. But in many highly
organized insects a remarkable concentration of the trunk-ganglia
takes place, all the nerve-centres of the thorax and abdomen in the
chafers and in the Hemiptera, for instance, being represented by a
single mass situated in the thorax. The legs, wings and other organs
of the trunk receive their nerves from the thoracic and abdominal
ganglia, and the fusion of several pairs of these ganglia may be
regarded as corresponding to a centralization of individuality. A
special "sympathetic" system arises by paired nerves from the
oesophageal connectives; these nerves unite, and send back a median
recurrent nerve associated with ganglia on the gullet and crop, whence
proceed cords to various parts of the digestive system.
In connexion with the central nervous system there are usually
numerous organs of special sense. Most insects possess a pair of
compound eyes, and many have, in addition, three simple eyes or ocelli
on the vertex. The nature of these organs is described in the article
ARTHROPODA. The surface of a compound eye is seen to be covered with a
large number of hexagonal corneal facets, each of which overlies an
ommatidium or series of cell elements (fig. 9, A, B). There are over
25,000 ommatidia in the eye of a hawk moth.
[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.