324 SYSTEMATIC ARRANGEMENT 



nine. In the females it frequently terminates in a 

 tubular ovipositor, the joints of which are retractile 

 within each other. The legs are generally long and 

 slender, the articulations of the tarsi always five in 

 number. The terminal joint, or that which bears 

 the claws, is often provided with two or three mem- 

 branous lobes, by the aid of which the fly is enabled 

 to walk on glass and other smooth surfaces against 

 gravity. This it was long supposed to do by the 

 pressure of the atmosphere, the lobes in question 

 acting as suckers and forming a vacuum. It has 

 been recently conjectured, however, in opposition to 

 this view, that it is accomplished by means of a glu- 

 tinous secretion. 



The larvae of dipterous insects are in some respects 

 even more peculiar than the mature fly. They are 

 generally of a conical shape, the head being the nar- 

 rowest part, and in all cases destitute of feet. The 

 head is small, retractile, and variable in form even 

 in the same individual, that is to say, it is composed 

 of a comparative soft fleshy substance which the in- 

 sect can modify in shape at pleasure, to answer its 

 various purposes. The colour is generally pale, but 

 sometimes it is dark, and even bright red. The stig- 

 mata, in the species not aquatic, are most commonly 

 placed in a cavity in the hinder segment of the body, 

 which is capable of closing over them, so as to pre- 

 serve them from being closed up by the fluid and putrid 

 substances among which the larvae often live. The 

 breathing apparatus of the aquatic larvae is often 

 very singular, consisting of appendages of various 



