179 



Many species that ordinarily make primary, if not exclusive, use 

 of the anal respiratory organs while in the larval stage make exclusive 

 use of the prothoraeic spiracles in the pupal stage. This is noticeably 

 so in species that live under aquatic or semiaquatic conditions. 



Keys to Suborders 



LARVAE 



1. Head complete, or the posterior portion with deep longitudinal in- 



cisions ; mandibles moving horizontally 



ORTHORRHAPHA-NEMATOCERA. 



— Head incomplete, without a strongly developed upper arcuate plate ; 



mandibles moving vertically 2 



2. Maxillae well developed, their palpi distinct; mandibles normally 



sickle-shaped, not protruded much beyond the apices of the maxil- 

 lae, often extending less than half-way to their apices ; antennae 

 well developed, situated upon the upper surface of a slightly 



arcuate chitinized dorsal plate 



ORTHORRHAPHA-BRACHYCERA. 



— Maxillae poorly developed, their palpi visible only in a few groups ; 



mandibles short and hook-like, usually capable of protrusion much 

 beyond apices of maxillae if these are present; antennae poorly 

 developed or absent, when present situated upon a membranous 

 surface CYCLORRHAPHA. 



PUPAE 



1. Pupa not enclosed within the indurated last larval skin, or if so the 

 head is distinct as in the larva, or the puparium is slightly flat- 

 tened dorso-ventrally, its texture leathery, not chitinous, and the 

 anterior respiratory organs not distinguishable; imago, or pupa, 

 emerges through a rectangular split on dorsum of larval skin. . . 

 ORTHORRHAPHA 



— Pupa enclosed within the indurated last larval skin; head always 



retracted, the chitinous portion occupying a position on the inner 

 side of the ventral surface of the puparium ; anterior respiratory 

 organs distinct, either protruded from the antero-lateral angles 

 of the cephalic extremity or from dorsum of base of abdomen; 

 imago usually emerges by forcing off the rounded anterior ex- 

 tremity of the puparium in cap-like form, or the dorsal half of 

 the thoracic portion — the lines of cleavage being along the lateral 

 margins to a point at base of abdomen; rarely emergence is 

 through rectangular splitting of the dorsum of the puparium . . . 

 CYCLORRHAPHA. 



