The Hog 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 Mtiller (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 

 olfactorjr 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 ej^es, 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 l)e 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 



