24 ON THE ORIGIN AND [CHAP. 



quit the egg generally in the form of fat, fleshy, 

 legless grubs. They feed principally on decaying 

 animal or vegetable matter, and are no doubt usefu^ 

 as scavengers. Other species, as the gadflies, deposit 

 their eggs on the bodies of animals, within which the 

 grubs feed, when hatched. The mouth is generally 

 furnished with two hooks which serve instead of jaws. 

 The pupae of Diptera are of two kinds. In the true 

 flies, the outer skin of the full-grown larva is not shed, 

 but contracts and hardens, thus assuming the appear- 

 ance of an oval brownish shell or case, within which 

 the insect changes into a chrysalis. The pupae of the 

 gnats, on the contrary, have the limbs distinct and 

 enclosed in sheaths. They are generally inactive, but 

 some of the aquatic species continue to swim about. 



One group of Flies, which is parasitic on horses, 

 sheep, bats, and other animals, has been called the 

 Pupipara, because it was supposed that they were 

 not born until they had arrived at the condition of 

 pupae. They come into the world in the form of 

 smooth, ovate bodies, much resembling ordinary dip- 

 terous pupae, but as Leuckart has shown, 1 they are 

 true, though abnormal, larvae. 



The next order, that of the Aphaniptera, is very 

 small in number, containing only the different species 

 of Flea. The larva is long, cylindrical, and legless ; 

 the chrysalis is motionless, and the perfect insect is 

 too well known, at least, as regards its habits, to need 

 any description. 



The Heteroptera, unlike the preceding orders of 

 insects, quit the egg in a form differing from that of 



1 Die Fortpflanzung tind Entwickelung der Pnpiparen. Von Dr. 

 R. Leuckart. Halle. 1848. 



