Central Nervous System
This consists of the brain and a double series of ganglia joined by longitudinal connectives.
Brain. The brain lies dorsal to the oesophagus between the supporting arms of the tentorium and consists of three parts—the protocerebrum, deutocerebrum and tritocerebrum respectively The protocerebrum innervates the compound eyes and ocelli and is divisible into two, the protocerebral lobes (Fig. 87, Pl) and the optic lobes (Fig. 88, Ol). The protocerebral lobes send off three ocellar nerves each terminating in an ocellar ganglion Only the median ocellus remains as functional in M. filifer with a well developed nerve (Fig. 87, Mon) and ganglion. The two lateral ocelli have disappeared externally, however, proof that they did once exist is given by the vestigial pair of lateral ocellar nerves (Fig. 87, Lon) with their poorly developed ganglia The optic lobes give rise to the two optic nerves (Fig. 87, On) These swell into the moderately developed optic ganglia (Fig. 87, Opg) which are connected to the compound eyes by a layer of post-retinal fibres The deutocerebrum is composed of the large paired antennary, or olfactory lobes (Figs. 87, 88, Dl), which give rise to the stout antennary nerves (Fig. 87, An) innervating the long antennae. The tritocerebrum is well developed in Macropathus in contrast to its poor development in both Hemideina and Periplaneta It consists of two widely separated pear-shaped lobes (Fig. 87, Tl) joined together by the post-oesophageal commissure (Fig. 90, Poc) which passes immediately behind the oesophagus. From the tritocerebrum arise the paired circumoesophageal connectives (Figs. 87, 88, 90, Coc) which pass ventrally, on either side of the oesophagus, to the suboesophageal ganglion (Fig. 90, Sog) Each lobe also gives rise to the labrofrontal nerve (Figs. 87, 88, Lfn) which divides into two, a short distance from its origin, the outer nerve passing to the labrum (Fig. 87, Ln) and the inner one curving inwards to join the frontal ganglion (Fig. 87. Fg), which also sends a branch to the labrum (Figs. 87, 88, Ln).
The suboesophageal ganglion, which is connected to the brain by the circumoesophageal connectives, gives off paired nerves to supply the labrum (Fig. 90, Lbn), maxillae (Fig. 90, Mxn) and mandibles (Fig. 90, Mn), and
Text-Fig. 18—Nervous System (Scale in cm) Fig. Sg—Central nervous system Fig. 87—Frontal view of brain Fig. 88—Posterior view of brain showing sympathetic system Fig. 89—Ventral view oesophageal ganglia showing hypocerebral ganglion. Fig. 90—Head opened up to show lateral view of brain and suboesophageal ganglion with nerves to mouth parts
A1, A2, A3 A4, A5, abdominal ganglia, An, [ unclear: ] nerve, Ca, corpus [ unclear: ] Cd connective to deutocerebrum, Coc, circumoesopliageal connective. Dl, deutocerebral lobe, Fg, frontal ganglion, Hg, hypocerebral ganglion, Hp, hypopharynx, L, labrum; Li, Lii, Liii, leg 1, 2, 3. Lb, labium, Lbn, labial nerve Lc, longitudinal connective Ltn labro-frontal nerve Ln, labial nerve, Lon, lateral ocellar nerve, Mn, mandible. Mnn mandibular nerve, Mon, median [ unclear: ] nerve. Mx, maxilla. Mxn, maxillary nerve. Ncr, nerve to [ unclear: ] and reproductive organs, Nm, nerves to muscles, Og oesophageal ganglion. Ol, optic lobe, Opg, optic ganglion. Pl, protocerebral lobe; Poc, post-oesophageal commissure; Rn, recurrent nerve Sn stomatogastric nerve Snc. sympathetic nerve cord, Sog, suboesophageal ganglion. T 1, T 2. T 3, thoracic ganglia 1, 2, 3, Tl. tritocerebral lobe, Vnc, ventral nerve cord
also a pair of long connectives (Fig. 90, Vnc) which pass back under the tentorium, through the occipital foramen and neck to the first thoracic ganglion.
Ventral Nerve Cord Fig. 86. It consists of three thoracic and six abdominal ganglia connected by paired longitudinal connectives (Lc) into a long chain extending the whole length of the body. From each thoracic ganglion arise several pairs of nerves, which supply the muscles of the corresponding segment. Each ganglion gives rise to a large pair of nerves, which innervate the legs (Li, Lii, Liii). In the abdominal ganglia, a gradual reduction in number from six to four can be seen taking place. The first abdominal ganglion (A1) has moved forward into the thorax and lies just anterior to the metafurca, so that it has nearly fused with the third thoracic ganglion (T3). The second (A2), third (A3), and fourth (A4) abdominal ganglia are equidistant from each other, but the fifth ganglion (A5) has moved posteriorly so that it is nearing fusion with the sixth abdominal ganglion (A6). In the various specimens of M. filifer examined it was noted that the third thoracic and first abdominal ganglia, and the fifth and sixth abdominal ganglia were closer together in some wetas than in others, showing that the process of reduction is actively taking place. The second, third, fourth and fifth abdominal ganglia are approximately the same size, but the first ganglion is slightly larger than these others. From the first and second abdominal ganglia arise three pairs of nerves, and from the third, fourth and fifth ganglia two pairs of nerves are given off. These nerves pass to the abdominal muscles in each segment. This differs from Hemideina where there are two pairs of nerves from the first abdominal ganglion and one pair from the second, third, fourth and fifth abdominal ganglia; and from Periplaneta where there are two nerves from each abdominal ganglion. The sixth ganglion, which is the largest, is composite in structure, being composed of the fused ganglia of the posterior segments. It gives off several nerves to the muscles and cerci; and to the ovipositor, ovaries and spermatheca in the female, and the vesiculae seminales in the male (Ncr).