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 insertion 

 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 pink except in the 

 strongly chitinized regions, where the primarj^ 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, EngHsh trans., 1758:32) saw seven pairs of stigmata with their 

 tracheae. He described their structure and their numerous branches 



