PROCEEDINGS OF THE NATIONAL. MUSEUM 



Vol.68 



of fundamental importance for the separation of the different species 

 of the genus. These plates are observed to best advantage from 

 the side, and such a view may be obtained by stripping off the 

 lateral portion of the abdominal wall with a very small and sharp 

 scalpel and mounting this strip separately on a microscope slide; 

 or if specimens are treated with strong caustic and then much of the 

 body contents worked out through a small hole made in the body 

 wall, the walls will collapse when the specimens are mounted, and in 

 doing so will bring the pleural plates into a horizontal position. 



In addition to the structures of th3 pleural plates the writer has 

 found that the chaetotaxy of the thumb of the first pair of legs of 



Fio. 1. — The kight axtenna of female from above of; A, pedicdlus humanus hdma- 

 Kus Linnaeus ; B, pediculls humanus americanus, new \ariety ; and, C, pediculus 



HUMANUS NIGRITAUIUM FaBKICIUS. (ALL FIGURES DRAWN TO THE, SAME SCALE.) 



the males is of much taxonomic value. The variation in the number 

 of teeth on the claws, particularly the first tarsal claws of the male, 

 has been used by many investigators, but I doubt that it has much 

 real value, particularly in separating the races of lice that are found 

 on man. This belief is in accord with Nuttall's statement (Xuttall, 

 1920). The number of teeth that can be observed and counted 

 depends largely upon tha degree of chitinization, and this in turn 

 depends both on the situation in which the louse has been and the 

 length of time that has elapsed since its last moult. 



The ratio which exists between the lengths of the different seg- 

 ments in the antennae and the ratio existing between the length and 

 breadth of each individual segment of the antennae is apparently 

 constant in material which is known to be free from the possible 



