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 (Tabanidai), 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 



