DIPTERA. 403 



bly fastened to it. They commonly have several small thorns on 

 each end of the body, and a row of smaller prickles across each 

 of the rings of the back. By the help of these thorns and prickles 

 they work their way out of the places wherein they had previous- 

 ly lived, just before they burst open their pupa-skins to come 

 forth in the perfected or winged state. The pupae of rnosquitos 

 are not prickly, but they possess the povVer of swimming or tum- 

 bling about in the water, by the help of two little fins on their 

 tails.* The larvse of the Dipterous insects in general do not 

 make cocoons ; those of some gnats (My cetophilce) , wh\ch live in 

 tree mushrooms, or boleti, not only cover themselves with a sil- 

 ken web, under which they live, but also spin cocoons, wherein 

 they undergo their transformations. The larvse of the other flies 

 are not so variable in their forms as the foregoing. They are 

 commonly plump, whitish maggots, obtuse behind, and tapering 

 before, with a small and soft head, that can be drawn within the 

 forepart of the body. They take their food almost entirely by 

 suction, for their jaws are merely two little hooks, that enable 

 them to fasten themselves upon the substances which serve for 

 their nourishment. They increase rapidly in size, and when they 

 are fully grown, they change their forms, without casting off their 

 skins at all, merely by the gradual shortening of their bodies, 

 which take an oblong oval shape, and turn hard and brown on the 

 outside. The hardened skin of the larva thus becomes a shell 

 or kind of cocoon, within which the insect is afterwards changed 

 to a pupa, having its imperfect limbs folded on its breast, and from 

 which, in due time, it comes forth in the form of a fly, by forcing 

 off one end of the shell. f 



In the introductory chapter,^ a short account has already been 

 given of the habits of the various kinds of gnats and flies, be- 

 longing to the principal families of this order. Besides the kw 

 species that are injurious to vegetation, and are to be more fully 

 described hereafter, there still remain some of our native flies, 

 that deserve a passing notice, on account of their size, or of pe- 

 culiarities in their forms, structure, and habits, although they arc 

 not to be included among the insects which are hurtful to plants. 



* See pages 5 and 6. t See page 6. t Page 15. 



