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 maxille*. ‘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 cuillerons) alulee. 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 alulze is in an 
inverse ratio to that of the halteres. The prothorax is always very 
short, and frequently we can merely discover its lateral portions. In 
some, such as the Scenopini, certain Culices, and Psychede, 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 two stigmata; two others may be observed 
near the origin of the halteres; those of the mesothorax, as in the 
Hymenoptera, are concealed or obliterated. 
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; 
A ele nd 25 2 Le Ns Sa OR ee ey 
* This anterior part of the head, called clypeus (my epistoma), 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 them. See the works of Fallea, 
Kirby, Meigen, Macquart, &c. 
VOL. IV. x 
