190 ZOOLOGY. 
never leaving the wasp. The larve resemble minute parasites; they have 
six feet, and are active, running about to find an insect upon which they 
may become parasitic. When they have penetrated to a proper place they 
lose their feet and become larve of a different form, presenting an example 
of a retrograde metamorphosis. . 
Orver 11. Diprera (pl. 77, figs. 87-129). In this order the body and 
integument are rather soft, the head generally free, and attached by a very 
short thin neck. The labium forms a kind of soft extensile rostrum 
adapted for suction; its sides are turned up to form a canal, and it incloses 
a varying number of sharp slender organs, sometimes adapted for piercing. 
These are well developed in the bloodsucking genera, in which the 
mandibles, maxille, labrum, and tongue are present. The two palpi of the 
Diptera are supposed to correspond to those of the maxilla of the other 
orders, although the question is not settled. In some cases the mouth is 
obsolete. There are two or three stemmata; the eyes are large, being in 
some males larger than in the females, and in some cases they occupy 
nearly the entire head. The antenne are sometimes composed of a 
succession of simple articulations, as in other orders; but in generai they 
are short, composed of few articulations, the last of which bears a bristle 
(arista) on its upper surface. The prothorax is reduced to a narrow collar, 
the metathorax is also much reduced, having no wings, and bearing their 
representatives, the small knobbed organs named halteres, so that the 
thorax is made up chiefly of the mesothorax, which bears the single pair of 
wings, constituting the distinguishing character of the Diptera. The wings 
are absent in some cases, but the halteres are nearly always present. 
The precise use of the halteres is not known. They are vibrated in flight, 
and if they are removed an insect is prevented from flying. Many Diptera 
have a pair of single or double membranes (calypta) in connexion with the 
halteres, and varying in size in inverse proportion with them. The tarsi 
are pentamerous, and the abdomen has from four to seven segments apparent. 
The pupe of the Diptera are of two kinds: in one the integument of the 
larva is not cast, but contracts into the form of a cocoon, from the inside 
of which the pupa becomes disengaged ; in the other the larva skin is cast, 
-and the pupa takes the incomplete form (in which the limbs are visible) 
without a cocoon. In the Culicide the pupe are active. 
The larve are cylindric and without feet, the head corneous or fleshy, 
and the mouth is generally provided with a pair of hooks. The aquatic 
larvee have jaws and palpi, and respiration is sometimes effected by means 
of tubes which are held at the surface of the water, and they swim with 
the aid of appendages at the posterior extremity. 
The larvee occur under various circumstances, as in carrion, fungi, in 
galls, like those of Cynips; or in living caterpillars, like Ichneumon. 
Some are to be found in vessels of vegetables pickled with vinegar, and 
others in the acrid brine of salted fish, or in the brine vats of salt works. 
The greater part are produced from eggs laid by the female; some are 
excluded alive ; and in the Pupipara the young are not excluded until they 
have reached the pupa state. 
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