52 Trans. Acad. Sci. of St. Louis. 



and several other groups ; or they may be developed upon 

 the palpi as in the case of the parasitic Cheyletidae (PL 

 III, Fig. 10), or the larval parasites of the families, 

 Trombidiidae, Hydrachnidae or Halacaridae (PL V, 

 Figs. 18 and 20, and PL VII, Fig. 28) ; or again they 

 may be developed upon some other structure of the 

 mouth-parts as the hypostoma in the case of the ticks. 



The possession of stout, backwardly directed spines 

 or bristles is a characteristic almost universally found 

 in external parasites inhabiting fur or feather bearing 

 animals. The fleas, Mallophaga and Pediculidae have 

 them, and in the Acarina we find them well developed 

 in members belonging to the Dermanyssidae, Cheyletidae 

 (PL III, Fig. 10), Analgesidae (PL IV, Figs. 15 and 16, 

 and PL VI, Figs. 24 and 25), Sarcoptidae (PL II, Fig. 

 6, and PL III, Fig. 11), and Listrophoridae (PL V, 

 Fig. 19). 



In order to establish the significance of the possession 

 of these backwardly directed spines, I have made a num- 

 ber of observations and experiments and from these have 

 decided that they fulfill chiefly two functions. First, be- 

 cause they are very stiif and are directed toward the 

 rear, they aid the possessor of them very materially in 

 its forward movements. Frequently I have watched fleas 

 advance themselves very successfully through a mass of 

 hairs simply by a backward and forward movement of 

 the body and legs, the spines permitting a free forward 

 movement of the parts, but blocking a backward thrust 

 of the legs and thus compelling an advance of the body. 

 Another advantage of these spines and bristles is in re- 

 tarding any effort which is made by the host to rid it- 

 self of its tormentors. Dogs, cats, chickens, and almost 

 all other infested animals, being much irritated by the 

 parasites, will make repeated efforts to get free from 

 them. They do this by scratching themselves with beak 

 or claw, or by rubbing against some object, or wallowing 

 in the dust. In these efforts frequently the parasites are 

 dislodged either from their hold on the skin or the hairs 



