DIPTERA. 449 



head covered with a horny shell. Larvuc of this kind, when 

 fully grown, cast off their skins to become pupae or chrysalids. 

 These pupae are usually of a brown color, and somewhat 

 resemble the chrysalids of certain moths, or more nearly those 

 of Hymenopterous insects; for their short and imperfect legs 

 and wings, though folded on the breast, are not immovably 

 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 previously lived, just before they burst open their 

 pupa-skins to come forth in the perfected or winged state. 

 The pupaj of mosquitos are not prickly, but they possess the 

 power of swimming or tumbling about in the water, by the 

 help of two little fins on their tails.* The larvae of the Dipte- 

 rous insects in general do not make cocoons; those of some 

 gnats {Mycetophila), which live in tree mushrooms, or boleti, 

 not only cover themselves with a silken web, under which they 

 live, but also spin cocoons, wherein they undergo their trans- 

 formations. Some of the Cecidomyians also make silken 

 cocoons. The larvae 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 fore part 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 nourish- 

 ment. 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 fold.^d 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 



* See pages 4 and 5. t See page 5. 



57 



