480 INSECTA. 



of which is fusiform or shaped like a lenticular or prismatic palette, 

 furnished either with a little styliform appendage, or a thick hair or 

 seta, sometimes simple and sometimes hairy. Their mouth is only 

 adapted for extracting and transmitting fluids. When these nutri- 

 tive substances are contained in particular vessels with permeable 

 parietes, the appendages of the sucker act as lancets, pierce the 

 envelope, and open a passage to the fluid, which, by their pressure, 

 is forced to ascend the internal canal to the pharynx, situated at the 

 base of the sucker. The sheath of the latter, or the external part 

 of the proboscis, merely serves to maintain the lancets in situ, and 

 when they are to be employed it is bent back. The base of the 

 proboscis frequently bears two filiform or clavate palpi, composed, 

 in some, of five joints, but in the greater number of one or two. 

 The wings are simply veined, arid most frequently horizontal. 



The use of the halteres is not yet well known; the Insect moves 

 them very rapidly. In many species, those of the last families par- 

 ticularly, and above the halteres, are two membranous appendages 

 resembling the valves of a shell, and connected by one of their sides, 

 called (ailerons or cuillerons) alulae. One of these pieces is united 

 to the wing and participates in all its motions, but then the two parts 

 are nearly in the same plane. The size of these alulae is in an in- 

 verse ratio to that of the halteres. The prothorax is always very 

 short, and frequently we can merely discover its lateral portions. 



The abdomen is frequently attached to the thorax by a portion 

 only of its transversal diameter. It is composed of from five to nine 

 apparent annuli, and usually terminates in a point in the females; in 

 those where the number of annuli is less, the last ones frequently 

 form a sort of ovipositor presenting a succession of little tubes slid- 

 ing into each other like the joints of a spy-glass. Their usually 

 long and slender legs are terminated by a tarsus of five joints, the 

 last of which has two hooks, and very often two or three vesicular 

 or membranous pellets. 



Many of these Insects are noxious both by sucking our blood 

 and that of our domestic animals, by depositing their eggs on 

 their body in order that their larvae may feed on them, and by infect- 

 ing our preserved meats and cerealia. Others in return are highly 

 useful to us by devouring noxious Insects, and consuming dead bo- 

 dies and animal substances left on the surface of the earth, that poi- 

 son the air we breathe, and by accelerating the dissipation of 

 stagnant and putrid water. 



