OF WASHINGTON. 423 



modern systematists to lessen their taxonomic significance and 

 not to separate the parasitic forms from the larger groups from 

 which they have evidently sprung is, in my judgment, fully 

 justified. 



The modifications due to parasitic life are very noticeable in the 

 parasites of my first category. Changes in the form of the body 

 are an almost invariable result, and particularly flattening, which 

 permits of ready concealment or movement amid the dermal 

 covering of the host, or in some cases an extreme elongation, for 

 the same purposes. In most species, and notably in Hippoboscidas, 

 Pediculidae, Mallophaga, Ixodidas,*and Acaridae, this flattening 

 or depression is very marked. In the Pulicidae, on the other hand, 

 the flattening is by lateral compression. 



The direction of modification, so far as form is concerned, 

 would seem, however, often to be fortuitous. In Pediculidas we 

 have the elongated species of Haamatopinus occurring on the 

 squirrels, and the widened Phthirius infesting man, while in the 

 Mallophaga we have in the genus Lipeuris the extremely slender 

 Lipeurus baculus of pigeons and the broad Lipeurus taurus of 

 the albatross. 



The head in the Pulicida3, Nycteribiidse, and Polyctenidae 

 bears peculiar comb-like rows of teeth on the posterior margin, 

 very prominent in Nycteribia and Polyctenes, both, though 

 widely separated, systematically, occurring on bats. Similar 

 comb-like spines are developed on other parts of the body, as in 

 Platypsyllus, and they doubtless facilitate retention amid the hair 

 or fur of the host. 



The effect on the antenna is to reduce the number of joints, and 

 in many cases to develop specialized sensory and clasping or 

 gans. In Melophagus and other Hippoboscida? the antennas are 

 shortened and contained in cavities. In the Pediculidag they are 

 reduced in number of joints, usually to five ; but in some cases 

 (Pedicinus) to four and in Haematopinoides to three. In the 

 Polyctenidae they are four-jointed. In Mallophaga the joints are 

 almost always five in number, but in Trichodectes they are re 

 duced to three. They often present a number of sensory pits, or 

 one that is well developed at the end of the terminal joint. In 

 males of Lipeurus the third joint is elongated and apically pro 

 longed so that the fourth joint appears to spring from near the 



