Mosquitoes and Parasite Problems. 151 



for one or two years after being deprived of their 

 proper aliment ; houses shut up for a year or 

 longer are sometimes found infested with them ; 

 possibly in the absence of " vegetable juices " they 

 flourish on dust. I have never detected them 

 hopping on the ground in uninhabited places, 

 although I once found them in Patagonia, in a 

 hamlet which had been attacked and depopulated 

 by the Indians about twenty months before my 

 visit. On entering one of the deserted huts I found 

 the floor literally swarming with fleas, and in less 

 than ten seconds my legs, to the height of my knees, 

 were almost black with their numbers. This proves 

 that they are able to increase greatly for a period with- 

 out blood'; but I doubt that they can go on existing 

 and increasing for an indefinite time ; perhaps their 

 true position, with regard to the parasitical habit, is 

 midway between that of the strict parasite which 

 never leaves the body, and that of independent 

 parasites like the Calex and the Ixodes, and all 

 those which are able to exist free for ever, and are 

 parasitical only when the opportunity offers. 



Entomologists regard the flea as a degraded fly. 

 Certainly it is very much more degraded than the 

 bird-borne Ornithomyia, with its subtle motions 

 and instinct, its power of flight and social pastimes. 

 The poor pulex has lost every trace of wings ; 

 nevertheless, in its fallen condition it has developed 

 some remarkable qualities and saltatory powers, 

 which give it a lower kind of glory ; and, compared 

 with another parasite with which it shares the 

 human species, it is almost a noble insect. Darwin 

 has some remarks about the smallness of the brain 



