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 Yegetation (illustrated edition), Boston, 1862. 



Parasites belonging to the Diptera. 

 The Fleas. 



PULICI D^. 



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 

 irritant) ^ which so far as I have observed, is comparatively 

 rare, though in some localities it may be sufiiciently 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. Tlie 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. The head is small ; the eyes 



