Nervous System

Overview

The cockroach nervous system is built on the typical annelid/arthropod plan — a paired, ganglionated, ventral nerve cord, sometimes called a ladder-type nervous system because in the embryo it consists of two parallel nerve cords with a pair of ganglia in every segment, joined like the rungs of a ladder by transverse commissures; in the adult the two cords and paired ganglia of each segment have fused together into single median ganglia joined front-to-back by paired longitudinal connectives. The whole ventral cord lies in the perineural sinus, close to the body wall on the ventral side, well clear of the gut above it.

Brain (supra-oesophageal ganglion)

At the front of the head, dorsal to the oesophagus, lies the brain, correctly termed the supra-oesophageal ganglion. It is formed by the fusion of three primitive ganglion pairs — the protocerebrum (associated with the compound eyes and ocelli/fenestrae, the principal association centre), the deutocerebrum (associated with the antennae), and the tritocerebrum (associated with the labrum and connected to the visceral/sympathetic nervous system). From the brain, a pair of stout connectives — the circum-oesophageal connectives — passes downward on either side of the oesophagus to reach the next ganglionic mass below the gut.

Sub-oesophageal ganglion

Just beneath the oesophagus lies the sub-oesophageal ganglion, itself a fusion of three segmental ganglia belonging to the mandibular, maxillary and labial segments. It supplies nerves to the principal mouthparts (mandibles, maxillae, labium) and to the salivary glands, and also sends nerve fibres backward to coordinate with the rest of the ventral chain.

Ventral nerve cord

From the sub-oesophageal ganglion, a pair of connectives runs backward through the thorax and abdomen as the ventral nerve cord, bearing a chain of segmental ganglia:

• Three thoracic ganglia, one in each of the prothorax, mesothorax and metathorax, each supplying its own segment's legs and (where present) wings.
• Six abdominal ganglia, distributed along the abdomen; the last of these is noticeably larger than the rest because it represents the fusion of the ganglia of several posterior abdominal segments, and it supplies the terminal segments, including the cerci — which is why the cercal sensory pathway to this last ganglion mediates the cockroach's very fast escape reflex. …