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 hears 
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 lias 
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 
