540 MEDICAL ENTOMOLOGY 



set with elongate and pointed spines, between the tibia and the tarsus ,* 



Enderlein distinguishes this as thepretarsal scJerite. (Plate LXVI, fig. 2) 



The abdomen is flattened in the dorso- ventral direction, and has 



usually the shape of an elongated ovoid ; the contour is more rounded 



when the ovaries are mature or when the mid-gut is 

 The abdomen ,. . , , , . _ . ... 



distended with blood. It consists of nine segments, 



each of which has a tergite and sternite except the first, in which, as in 

 the flea, the sternite is wanting. The tergites and sternites are only 

 thinly chitinized as a rule, while the pleural plates which connect them 

 together may be very strongly developed and heavily pigmented ; in 

 Pediculus and in many species of Haematopinus the pleurae form a 

 conspicuous festooned border on each side of the abdomen, the divisions 

 between the segments appearing as notches. The shape of these pleural 

 plates, and the number and disposition of the hairs or spines with 

 which they are adorned, are used to a large extent in the differentiation 

 of species, especially in the genus Haematopinus. The ninth segment 

 is modified for sexual purposes, and differs in the two sexes, serving as a 

 ready means of distinguishing them. In the male, as, for example, in 

 Pediculus (Plate LXVI, fig. 6), the tip of the abdomen is produced to 

 a rounded point, at the apex of which the penis projects, while in the 

 female (fig. 5) the two sides are distinctly separated from one another, 

 leaving a notch in the middle line. 



The stigmata are situated on the pleural thickenings of the abdomen, 

 and appear as clear spaces in preparations. They are usually present on 

 the third to the eighth abdominal segments. 



The external genitalia of the male are of a comparatively simple type. 

 The penis consists of two pairs of chitinous rods, lying one in front of 



_ . . ., .. the other in the terminal segment of the abdomen, and 

 External genitalia . 



articulated by a moveable joint. The anterior pair 

 are deeply pigmented, and are conspicuous structures in cleared prepara- 

 tions ; the posterior pair converge towards one another at the distal end 

 of the abdomen, and often project a little beyond the last segment ; they 

 are in close contact with one another, and convey the ejaculatory duct to 

 the exterior between their inner surfaces. The articulation between 

 these two sets of rods, the object of which is to enable the penis to be 

 thrust out and retained in position in the genital opening of the female 

 during copulation, is a complex one, and is acted upon by an intricate 

 arrangement of muscles. 



The external genitalia of the female (Plate LXVI, figs. 3 and 4) are 

 the modified sternites of the eighth and ninth segments. The last 



