DIPTERA. 313 



sition, and frequently in their appendages ; but in the following and 

 greater number of families they consist of but two or three joints, 

 the last 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 

 nutritive substances are contained in particular vessels, with per- 

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

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

 sure, 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. This sheath appears 

 to represent the inferior lip of the triturating Insects, just as the ap- 

 pendages of the sucker, at least in those genera where it is most 

 complete, seem to be analogous to the other parts of the mouth, such 

 as the labrum, mandibles, and maxilloe *. The base of the proboscis 

 frequently bears two filiform or clavate palpi, composed, in some, of 

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

 are simply veined, and most frequently horizontal f . 



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 cuilleruns) 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 alulse is in an 

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

 short, and frequently we can merely discover its lateral jjortions. In 

 some, such as the Scenopini, certain Culices, and Psychodae, they are 

 prominent and tuberculous. The greater part of the trunk or 

 thorax is composed of the mesothorax. Before, on each side, or 

 behind the prothorax are tw'o stigmata; two others may be observed 

 near the origin of tlie halteres; those of the mesothorax, as in the 

 Hymenoptera, are concealed or obliterated. 



The abdomen is frequently attached to the thoiax 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; 



sen 



* This anterior part of the head, called clypeus (uiy epistoiua), is here repre- 

 sented by that superior portion of the proboscis that precedes the sucker and palpi. 



t These organs, like those of the Hymenoptera, furnish good, secondary, divi- 

 sional characters. I was the first who employed thcni. See the works of F&llen, 

 Kirby, Meigen, Macquart, &c. 



vol,. IV. V 



