442 MEDICAL ENTOMOLOGY 



The maxillae, on account of their size and position, can only enter 

 the superficial part of the wound if they enter it at all, and are not 

 provided with a musculature which would enable them to assist in the 

 cutting operation ; possibly they assist in fixing the flea in position 

 while the blood is being sucked up. The labial palps, or labella, do 

 not enter the wound, but are bent to right and left as the mandibles 

 enter the skin. Whether the bending of these organs is entirely a 

 passive process, or whether there are intrinsic muscles to bring about 

 the action as in the Diptera, it is impossible to say. It is noted by 

 Jordan and Rothschild that they are less chitinized in the Sarcop- 

 syllidae, which fix themselves to the skin of the host after the manner of 

 ticks, than in fleas of ordinary habit. 



: The three segments which compose the thorax are quite distinct from 



one another, and the joints between them permit of a certain amount 



of movement. The head is attached to the thorax 



Tit A frlftAMOV 



(Plate LIU fi* 1) w ith ut the intervention of a neck, but the joint between 

 it and the prothorax is more freely moveable than are 

 the others. Each segment consists of two plates, a tergite and a sternite, 

 the former being undivided and simple, while the latter is fused more or 

 less closely with' certain sclerites representing the pleural plates as 

 described in the Diptera. 



- The tergites are simple broad arched plates, extending over the dorsum 

 and the dorsal half of the lateral aspect of the thorax. The pronotum 

 bears on its hind edge a row of stout spines forming a comb like that on 

 the genal edge of the head. This pronotal comb is absent in many fleas 

 (Plate LVI, figs. 3 and 6). Both this and the genal comb are of Service to 

 the insect in facilitating forward progression among the hairs of the host. 



The ventral plate of the prothorax is large and undivided, and extends 

 as far forward as the level of the posterior margin of the eye, being only 

 separated from the posterior portion of the genal edge by a narrow 

 interval. In this and many other species it bears no hairs. The two 

 prosternites are continuous with one another in the middle line of the 

 ventral surface, where the chitin is thinner than elsewhere. The ventral 

 plate of the mesothorax is divided: by ridges into portions which have 

 been homologized by Jordan and Rothschild with the sternum, epister- 

 nnm, and epimerum ; these ridges mark the lines along which the discrete 

 plates were separated in a more primitive condition. It is divided into 

 two approximately equal portions by a ridge running from the insertion 

 of the coxa of the middle leg upwards and a little forwards, the upper- 

 extremity of the ridge being situated just posterior to the lower and 



