CHAPTER XIII. 
THE DIPTERA — FLIES. 
THERE are only two large and well developed wings, which 
are membranous and without scales, in the Diptera ; the second 
pair are quite rudimentary, and form small projections of wing- 
membrane, which are called balancers or poisers. In the common 
gnat these poisers are like simple foot-stalks surmounted by a 
round knob, and are attached one on each side to that segment 
of the body which ought to sustain a large hind wing. This 
is their form in the house fly. They are generally more or 
less concealed beneath the winglets, which are little appendages 
to the fore wings. The excessive size of that segment of the 
body which bears the great proper wings of the Diptera , the front 
pair, appears to render a diminution in the size of the segment 
which ought to maintain the hind wings inevitable. 
The Diptera are sucking insects, but their suckers are formed 
differently to those of the Hemiptera or the Aphaniptera. More- 
over these important organs of the mouth differ greatly in many 
genera of the flies themselves. These insects undergo complete 
metamorphoses ; the larvae are ordinarily vermiform and legless, 
are known as maggots, and usually possess considerable activity 
and can crawl backwards and forwards. They do not require 
maternal assistance, and are usually placed so that their food is 
close at hand. The nymphs of the Diptera differ greatly; some 
of them are active, although they take no nourishment ; some are 
inactive, and quite as much so as the chrysalides of the Lepi- 
doptera or the nymphs of Coleoptera , and others which are per- 
fectly immobile, do not lose the skin belonging to the larvae ; these 
