( cliv ) 



protect the joints, but also strengthen the body and render 

 the insect slippery, so that it can withstand pressure more 

 successfully and glide more easily through the fur. As *a rule 

 these bristles are of the usual tapering kind, but bristles of 

 other shapes also obtain. Bristles resembling the blade of a 

 straight sword are, for instance, found in some fleas (Xiphio- 

 psylla) and some Anoplura (Polyplax, Hoploplura) (text-figs. 

 22, 23). Short, thick, peculiarly dentate bristles occur in 

 Cimicidae and appear among Ectoparasites to be confined to 

 that family. 



The combs, which are a very conspicuous feature of many 

 Ectoparasites, have a similar function to the bristles. They 

 are strongly developed in most Siphonaptera, all Polyctenidae 

 (text-fig. 15), in Nycteribiidae, and Platypsyllidae, and similar 

 structures may often be observed among Mallophaga. 

 These combs in most groups consist of peculiarly modified 

 bristles, but in the fleas they usually appear to be exaggerated 

 serrations of the edges of segments. Most of the Nycteri- 

 biidae have one or two combs on the underside of the abdo- 

 men and a semi-circular one dorsally on each side of the 

 thorax. In the Polyctenidae combs are found on the head, 

 thorax, and the elytra, or on one of them, and sometimes 

 also on the antennae. Platypsyllus has a comb on the head, 

 while in fleas, combs are found on head, thorax and abdo- 

 men, some fleas, however, being without combs altogether. 

 Some bat-fleas and the large mole-flea may serve as examples 

 of many-combed species, while the " bubonic plague " flea, 

 Xenopsylla cheopis, belongs to a combless genus. The human 

 flea is allied to the rabbit-flea and the hedgehog-flea. The 

 two latter species have a comb on the genal portion of the 

 head and pronotum, while in the human flea the pronotal 

 comb is entirely lost and the genal one at most represented 

 by one tooth. 



The question why the combs are lost in many species will 

 be difficult to answer. I may mention, however, one instance 

 which throws a little light on points of this kind. Bat-fleas 

 appear originally to have had a comb on the pronotum, 

 metanotum and abdominal tergites 1-6, i. e. 8 dorsal combs. 

 These are preserved in a number of species. In others some 

 of the combs are lost or reduced to a few very short teeth. 



