498 MODERN CLASSIFICATION OF INSECTS. 
The head in these insects is almost invariably distinct, and at- 
tached to the thorax by a very short and narrow neck; the eyes are 
large and lateral, those of the males, in many species, occupying 
nearly the whole of the head; the ocelli are generally present, and 
three in number ; only two, however, are found in some of the Tipulide. 
The antenne are generally inserted on the forehead, and approxi- 
mating at the base; they are of very variable construction: there 
appear, however, to be two distinct types of formation*; one ex- 
emplified in the Tipulida, in which these organs assimilate to those 
of many preceding groups, being composed of a series of distinct 
continuous articulations, often ornamented with whorls of hairs, 
bristles, &c.; the other occurring in the greater number of Diptera, in 
which the antenne, unlike those of any. other tribe, are very short and 
apparently composed of only two or three thick joints, the last of 
which is generally the largest, and is furnished with a bristle (arista) 
on its upper edge ; this joint, however, is, in fact, composed of several 
of the articulations observable in the other type soldered together, the 
bristle representing also two or three other joints, — it being quite 
easy, as we shall see in the sequel, to trace the gradation of structure 
through the various families. 
The mouth of these insects is formed only for imbibing fluid mat- 
ter; when, therefore, such fluid is enclosed in peculiar vessels, the 
internal pieces of the sucker are employed as lancets to pierce the 
envelope and afford a passage to the fluid, which ascends by power 
of suction, produced by the fleshy lips of the insect into the mouth. 
The outer case of the rostrum evidently represents the lower lip of 
the mandibulated orders, having the sides turned up, so as to form a 
canal, for the double purpose of containing the other parts of the 
mouth, and of forming a channel for the flowing up of liquids 
into the pharynx: these internal pieces of the mouth are variable 
in number in the different families, but are found in their greatest 
state of development in the blood-sucking gnats and breeze-flies, 
in which the upper lip, the two mandibles, the two maxilla, and the 
tongue of other insects are represented under the form of lancet-like 
organs: another pair of jointed organs is also attached to some part of 
the rostrum, or to that pair of lancets which represents the maxilla ; 
whence it is evident that they are the true analogues of the maxillary 
palpi, although the maxillary sete themselves are sometimes obsolete. 
* See MacLeay on the antenne of this order, in Taylor’s Philos. Magaz. 1827. 
