190 ZOOLOGY. 



never leaving the wasp. The larvae 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 th^r feet and become larvae ot" a different form, presenting an example 

 of a retrograde metamorphosis. 



Order 11. Diptera {pi. Ill, 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, maxillae, labrum, and tongue are present. The two palpi of the 

 Diptera are supposed to correspond to those of the maxillae 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 antennae are sometimes composed of a 

 succession of simple articulations, as in other orders ; but in general 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 pupae 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 Culicidce the pupae are active. 



The larvae are cylindric and without feet, the head corneous or fleshy, 

 and the mouth is generally provided with a pair of hooks. The aquatic 

 larvae 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 larvae occur under various circumstances, as in carrion, fungi, in 

 galls, like those of Cijnips ; 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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