174 



abdominal, and occasionally also thoracic, segments sometimes with 

 locomotor spinules in transverse series on portions of their ventral 

 surface (Mycetophilidae, part) or with pseudopod-like elevations or 

 fusiform transverse locomotor areas; respiratory system consisting, 

 in its highest development, of i pair of prothoracic, 7 pairs of ab- 

 dominal, and 1 pair of anal spiracles, the abdominal pairs not func- 

 tional in some cases (Simuliidae, and Bibionidae and Scatopsidae ?), 

 while in other families they are absent entirely, respiration being car- 

 ried on either by means of the anal spiracles, or by these and the pro- 

 thoracic pair, or by means of blood-gills on the anal segment which 

 are sometimes retractile (Simuliidae, Ceratopogonidae) or perma- 

 nently exserted (Chironomus, part). 



The acephalous larvae differ very markedly from those of the 

 other group in the reduction of the head-capsule. In Cecidomyiidae 

 the mandibles are vestigial or absent and the posterior portion of the 

 head is poorly defined and membranous, but the larvae may be distin- 

 guished by the presence of 13 segments, in addition to the head; by 

 the lateral abdominal spiracles; and, usually, in the last instar, by the 

 presence, on the ventral surface of the first and second thoracic seg- 

 ments, of a chitinized plate, generally referred to as the "breast-bone", 

 which is differently shaped in different species and is used by the 

 mature larvae as a means of propulsion in making their leaps after 

 leaving their cells for pupation — in cases where this change is made 

 in the ground. In other larvae of this group there are at most but 

 12 distinct segments in addition to the head. 



The reduction of the head-capsule in Tipulidae and Limnobiidae 

 is in the form of a breaking up of the fusion of the component parts 

 posteriorly, the caudal portion of the head thus having the appearance 

 of several slightly diverging rods, the membrane of the prothorax be- 

 ing attached to the head just anterior to the point of divergence. These 

 larvae may be distinguished from those of the higher families of the 

 Brachycera by their opposed, instead of parallel, mandibles. Respira- 

 tion is carried on by means of prothoracic and anal spiracles — the 

 latter supplemented in the aquatic forms by retractile blood-gills on 

 the ventral surface of the last segment — or in some genera by the anal 

 spiracles only. 



Division Brachycera 



The larvae of Brachycera are usually readily distinguished by 

 the large maxillae, with their normally conspicuous palpi, between 

 which are the slender labrum and the vertically moving, knife- or 

 sickle-shaped mandibles on each side of it. The antennae are some- 

 times conspicuously elevated but occasionally very short, and the eyes 



