THe Hoa LousE 669 
description of the nervous system of the clothes louse, and he has called 
attention to the fact that, although in the mature louse the ganglia are 
concentrated in the thorax, in the embryo figured by Cholodkovsky (1903: 
124) they extend some distance’ into the abdomen. 
The central nervous system of the hog louse consists of five ganglia, 
their connectives, and commissures, and its approximate length from the 
anterior border of the brain to the posterior border of the metathoracic 
ganglion is 0.93 millimeter (Plate LIX, 8). The supra-esophageal ganglion 
lies in the posterior half of the head behind the level of the insertion of the 
antennae. It is a large, compact ganglion, deeply grooved anteriorly 
and surrounding the dorsal and lateral surfaces of the esophagus like a 
collar; its position is somewhat oblique, and the three segments of which 
it is composed are very closely fused. Its anterior lobes are Joined on 
the ventral surface by the esophageal commissures, which can be easily 
seen in sections but are invariably broken in the process of gross dissection. 
These commissures were seen also by Miller (1915:34) in the clothes 
louse, and he suggested that they be named the ‘‘ Commissura cerebri 
subpharyngealis.” From the tritocerebron a pair of nerves pass out 
anteriorly and soon divide, one branch of each going to the frontal ganglion 
and the other to the labrum, where each subdivides into at least four 
branches terminating in large multinuclear sensory cells from which 
slender processes pass to the anterior wall of the head on either side of the 
haustellum. The ventral anterior part of the deutocerebron forms the 
olfactory lobes. In gross dissection these could not be distinguished, 
but they were found in series of longitudinal sections through the head, 
and from each a large nerve passes to the antennae. These nerves lie 
dorsad and somewhat laterad of the nerves from the tritocerebron. The 
optic lobes, also indistinguishable from the mass of the brain, send nerves 
out to the eyes, which are situated on prominences behind the antennae, 
are poorly developed, and are without pigment. The sub-esophageal 
ganglion is concealed anteriorly by the protocerebral lobes of the brain, 
and the esophageal connectives are so short as to be invisible unless the 
brain be raised. It is a heart-shaped ganglion, broadest anteriorly, and 
having a small indentation in which the esophagus rests. In sections, 
three pairs of nerves can be seen passing from it to the mouth parts. 
From the apex of the sub-esophageal ganglion two closely apposed 
connectives pass backward along the median line to the prothoracic 
