Art. 19 



LICE OF THE GENUS PEDICULUS EWING 



influence of hybridization. Where such specimens have been ex- 

 amined it has nearly always been possible for the writer to divide 

 those varieties occurring on man into two groups : One in which the 

 antennae are relatively much longer, as in figure 1, C, and in which 

 each segment is relatively much longer in proportion to its width, 

 than in those of the opposite type as shown in figure 1, A and B. 



The variation in the size, and to a certain extent in shape, of the 

 thoracic spiracles and the spiracular bulbs (fig. 2) is a character 

 of much importance in separating both species and varieties. In 

 the head louse of Europe these spiracles are almost twice as large as 

 they are in the head louse of American Indian mummies (see fig. 2, 



R cKaplm R h. ni^r itarum. 



Rconsobrlnus RK.amet^lcan\js Ph.humauus 



Fig. 2. — Drawings of the right thoracic spiracle of a female louse of five dif- 

 ferent KINDS OF PEDICULI. ALL ENLARGED, X 100. TlIE LINE ABOVE DRAWING OF 

 EACH SPIRACLE REPRESENTS THE LENGTH OF THE LOUSE, X 10 



P. humanus humanus and P. humanus americanus) ^ yet the Eu- 

 ropean head louse is a smaller louse on the average than that of pre- 

 historic American Indians. 



Dorsally, the thorax bears six large setae (macrochaetae), and 

 the relative size of these setae is of great value in certain species. 

 The writer finds that these setae are always present as suggested 

 by Nuttall (1920) and never absent as claimed by Fahrenholz (1916) 

 for one species. However, the variations in the size and positions 

 of these setae are considerable. 



Various characters have been used by different investigators and 

 are used to a certain extent in this paper, taking into consideration 

 the range of individual variation and the possible effect of hybridi- 

 zation. 



