Mosquitoes and Parastte Problems, 15 
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 toincrease 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 Culex 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 
