PARASITES. OF ANIMALS. 11 
white ants and Lepisma, are beneficial by destroying noxious 
insects. 
To those who desire more information upon the struc- 
ture, habits, and classification of insects generally, we would 
especially recommend Packard’s Guide to the Study of In- 
sects, Salem, Mass., 1869, and Harris’ Treatise on Insects 
Injurious to Vegetation (illustrated edition), Boston, 1862. 
PARASITES BELONGING TO THE DIPTERA. 
The Fleas. 
PULICIDA. 
It is somewhat singular that parasites are almost always 
degraded species of the orders and families to which they 
belong. This is very evident in the case of parasitic insects, 
for: they are mostly destitute of wings, and are imperfectly 
organized in other respects when contrasted with their self- - - 
supporting allies. The fleas furnish us with examples of 
degraded diptera, and the sheep-tick is even more degraded 
than the fleas. Of fleas there are several species that attack 
man and domestic animals; but the species do not appear to 
. be so strictly limited to particular animals as are the lice and 
some other parasites ; for the common dog-flea and cat-flea 
will attack man without hesitation, and in New England ap- 
pear to be far more troublesome to the human race than is 
the species commonly regarded as the real human flea (Pulex 
irritans), which so far as I have observed, is comparatively 
rare, though in some localities it may be sufficiently common, 
as it is in parts of Europe. 
In the genus Pulex, which includes the common species of 
fleas, the body is compressed and the integument is very firm, 
so that they are well adapted for gliding among hairs or 
feathers, and are not liable to be injured by any ordinary 
pressure. The wings are represented only by two pairs of 
short, stiff scales, which have little or no power of motion. 
The posterior legs are large and powerful, enabling them to 
leap with remarkable,agility, ,,,Rhg,diead is small; the eyes 
