656 LaurA FLORENCE 
thorax at various angles after impregnation of the tissue with silver 
chromate proved conclusively that the structure has no connection with 
the tracheae and that the canals are unmodified ingrowths of the body 
wall. They are composed of chitinous cuticula covered with a layer of 
small hypodermal cells, and form a rigid internal frame, analogous to 
the skeleton of higher animals, for the partial support of the muscles of 
the first and second pairs of legs and a transverse muscle of the thorax. 
No communication between these and a canal extending cephalad, as 
described by Stevenson, has been found. They are to be regarded as a 
paired apodeme of the prothorax and the prosternum. 
In the region of the metathorax on the median line there is a marked 
ingrowth of the cuticula, which forms the center of a ridge-like thickening 
on the inner surface of the segment. This ridge serves for the insertioh 
of the muscles of the neck, the legs, and the dorsal abdominal plate, and 
may be named the metathoracic apodeme. In the abdomen the 
segmentation is clearer on the dorsal than on the ventral surface. Segments 
1 and 2 are small and have the appearance of belonging to the thorax. 
As already said, these tergites are clearly defined in both sexes. Segments 
3 to 8 have strongly chitinized plates on the pleurites and moderate 
chitinization of the tergites, while the sternites are almost colorless. 
The primary cuticula is very thin and can be dissected off with ease from 
the secondary cuticula, which is of a leathery consistency and in sections 
has a striated appearance as if deposited in layers. When stained with 
hematoxylin and eosin the secondary cuticula stains pmk except in the 
strongly chitinized regions, where the primary and secondary cuticulae 
both retain their yellow color. 
The hypodermis underlying the cuticula is made up of uniform cells 
which become longer and more slender on either side of the trichogen 
cells. The latter are considerably larger than the hypodermal cells 
and their basal part is subcircular, and in some cases multinuclear sensory 
cells lie alongside them sending a prolongation into the hair. 
THE RESPIRATORY SYSTEM 
Hooke (1665) saw numerous tracheae intimately connected with the 
fat cells of the louse, but did not recognize their true function. Swammer- 
dam (1682, English trans., 1758:32) saw seven pairs of stigmata with their 
tracheae. He described their structure and their numerous branches 
