Q.Describe the nervous system of the cockroach with the help of a labelled diagram.

[!TLDR] The nervous system is a ganglionated ventral nerve cord: a fused three-part brain, a sub-oesophageal ganglion, three thoracic and six abdominal ganglia (the last enlarged), plus a visceral system.

The brain (protocerebrum + deutocerebrum + tritocerebrum, serving eyes, antennae and labrum respectively) connects via circum-oesophageal connectives to the sub-oesophageal ganglion (mouthparts/salivary glands), and then to the ventral chain of three thoracic and six abdominal ganglia (the last enlarged by fusion, supplying the sensory cerci and mediating the escape reflex), alongside a separate sympathetic system controlling gut/heart/spiracle muscl …

Overall plan

The cockroach's nervous system follows the typical insect design: a paired, ganglionated ventral nerve cord, historically ladder-like in the embryo (two parallel cords with paired ganglia per segment, joined by transverse commissures), fused in the adult into single median ganglia joined by paired longitudinal connectives, lying in the perineural sinus close to the ventral body wall.

Brain (supra-oesophageal ganglion)

At the front of the head, dorsal to the oesophagus, lies the brain, correctly the supra-oesophageal ganglion, formed from the fusion of three primitive ganglia: the protocerebrum (the largest, associated with the compound eyes and fenestrae, and acting as the principal association/coordinating centre), the deutocerebrum (associated with the antennae), and the tritocerebrum (associated with the labrum, and linked onward to the visceral nervous system). From the brain, a pair of stout circum-oesophageal connectives passes down around either side of the oesophagus.

Sub-oesophageal ganglion and the ventral chain

These connectives lead to the sub-oesophageal ganglion, itself a fusion of three segmental ganglia, which supplies the mandibles, maxillae, labium and salivary glands. From here the ventral nerve cord runs backward, bearing three thoracic ganglia (one per thoracic segment, each supplying that segment's legs, and wings where present) and six abdominal ganglia; the last abdominal ganglion is distinctly enlarged, representing the fusion of several posterior segmental ganglia, and it supplies the terminal abdominal segments and the sensory anal cerci — the pathway that mediates the cockroach's famously fast escape reflex to a puff of air or vibration.

Local reflex control and the sympathetic system …

• Treating the brain as a single, undifferentiated ganglion rather than a fusion of the protocerebrum, deutocerebrum and tritocerebrum. …