Nervous System

In insects, as with other animals, the nervous tissue is composed of nerve cells (neurons), which are grouped into linear nerves and gangliar masses to form a central nervous system (Treherne 1974, Miller 1979), an autonomic (or stomato-gastric) system, and a peripheral or sensory nerve system. The first is ventral, lying in the floor of the hemocoel, and is characterized by a succession of ganglia interspersed along a paired, ventral nerve cord. The nerve cell bodies are located peripherally in the ganglia, the center of which are occupied by a complex of nerve fibers (the neuropile) that connect the ganglia as the nerve cord.

The largest and most complex ganglion is the anteriormost. It is dorsal, above the pharynx, in the head. This is the brain (Howse 1970), which may actually be composed of two or more fused primary ganglia. It is the overriding center of neural integration to which the other ventral ganglia are ultimately subjugated, although each of the latter may have some degree of autonomy. A beheaded insect may continue to live and exhibit locomotory and sensory activity for some time before it eventually dies from such injury.

The major sensory organs of the head, the eyes, antennae, and palpi, are connected by large nerves directly to the brain. The brain also contains neurosecretory cells and functions partly as an endocrine organ as explained above.

The first ventral ganglion is also located in the head region and is associated with ingestive processes. There follows a varying number of segmented ganglia, primitively, one per segment, up to eleven in insects, and many more in myriapods, but the number is often less, due to fusion of segments, especially in the thorax.

The autonomic system is closely associated with the digestive tract and consists of a small number of small ganglia and short fine nerves. Its function is to control visceral activity. It is also involved with parts of the endocrine system.

Efferent nerves run from the central nervous system to the muscles in all parts of the body. Afferent nerves lead from the sensory system, mainly the integumentary sense organs, to the central nervous system. The cell bodies of sensory neurons are located near the sensilla themselves, and their axons connect them directly to the ganglia without intervening synapses.

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