CHAPTER II. 
Jointed Animals,— continued. 
Insects, — continued. 
The Flies and Fleas, —Order Diptera, 
As implied by their scientific name, the typical members of the order now claiming 
attention are distinguished from all other insects by the possession of but a single 
pair of wings. In this case one pair of these organs has disappeared, and examina¬ 
tion will reveal the fact that it is the front pair that is retained in full functional 
importance, while the hinder pair has become reduced to a couple of short slender 
club-like organs, known as halteres or balancers. From their small size it might 
be supposed that these balancers were organs of but little physiological importance, 
but the experiment of removing them will show that this is not the case; for an 
insect thus mutilated is thereby entirely deprived of the power of maintaining its 
equilibrium and of directing its course in the air. Hence the name balancers that has 
been assigned to these rudimentary wings. The mouth-parts, instead of being of 
the primitive mandibulate type, are formed for purposes of piercing or sucking. In 
the former kind of structure, as represented for instance in Pangonia longirostris, 
one of the horse-flies ( Tabanidcc ), these organs are composed of seven pieces, which 
have been interpreted by Mr. Waterhouse as follows. The uppermost is a long 
pointed instrument, the labrum. Immediately below this, and more or less 
concealed by it, is an almost equally long and slender piece, which is probably the 
hypopharynx. The mandibles are modified into a pair of sharp lancets, and below 
them are two extremely slender instruments, which, from the presence of palpi, are 
recognisable as parts of the maxilla. All these pieces lie concealed in the basal half 
of the proboscis, which, for part of its length, is gutter-shaped, but afterwards 
assumes the form of a tube, and is believed to be comparable to the labium. In 
the gnats the mouth is formed upon the same plan, but the lancets are all more 
slender. In piercing the skin the lancets only are used, the labium or proboscis 
serving merely as a guide. In the flies that use the mouth for sucking—as for 
instance in the blow-flies and drone-flies—the jaws are still more modified, so that 
the identity of the separate pieces is difficult to establish. The most prominent part 
is the proboscis, the expanding terminal lobes of which are the paraglossce of the 
labium. The maxillae are represented by two scales or .short stylets closely 
adherent to the sides of the proboscis, and of two club-like palpi; but the 
mandibles seem to have disappeared. 
The only character that need be specially noticed in the wings is that they are 
usually naked,—being but rarely furnished with short hairs,—and that the veins 
