NO. 8 ANATOMICAL LIFE OF THE MOSQUITO — SNODGRASS 7 



maxillae are articulated on a transverse margin between the bases of 

 the antennae. The long ventral cranial wall behind them is sclerotically 

 closed by the union of the postgenae (C, Pge) along an incomplete 

 median suture (C,D,E, ms). This same condition occurs in certain 

 other insects, and to understand how it has come about we shall have 

 to digress on some comparative studies. 



The hypognathous position of the insect head in which the mouth 

 parts are ventral (fig. 2 A) is clearly primitive, because the mouth 

 parts, being modified legs, thus hang down from the head in the posi- 

 tion of the thoracic legs. The prognathous condition has been attained 

 in some cases by a mere turning forward of the head on the neck, 

 involving a ventral elongation of the occipital foramen on the under- 

 side of the head (fig. 3 B). More commonly, however, the foramen 

 remains in the vertical plane, as in the mosquito larva (fig. 1 E), and 

 the underside of the head is lengthened by a ventral elongation of the 

 postgenae {Pge). 



With the elongation of the postgenae the entire labium, as in some 

 beetles (fig. 2 B), may be simply enclosed between them, with a gular 

 addition (Gu) to the submentum. This condition, however, does not 

 occur in the larval mosquito, though some writers have so interpreted 

 the mosquito head structure. More commonly, the postgenae come 

 together medially and displace the labium. A primary stage of this 

 transformation is seen at C, which might represent the head of a 

 caterpillar or an adult honey bee, in which lobes of the hypostomal 

 margins of the postgenae are intruded between the occipital foramen 

 and the base of the labium. In other cases the lobes become united 

 (D) , forming a bridge between the foramen and the labium. An elon- 

 gation of the bridge then produces the condition seen in the beetle 

 larva at E, in which the labium is still fully exposed. Finally, as in the 

 larvae of Chironomidae (F), the labium has become greatly reduced 

 and is hidden from below by a median hypostomal lobe (Hstm) of the 

 united postgenae. 



This same process of closure and elongation of the postgenae and 

 the reduction of the labium can be traced among nematocerous fly 

 larvae. For example, in the primitive rhyphid larva of Olbiogaster 

 (fig. 3 A) described by Anthon (1943b), a pair of small postgenal 

 lobes are approximated behind the submentum (Smt) of the labium. 

 In others, as in Trichocera and Philosepedon figured by Anthon 

 (1943a, figs. 7, 10) the postgenal lobes are united in a bridge; the 

 labium, though much reduced, is still mostly exposed. In the mosquito 

 larva (fig. 3 C) the united postgenae form the long underwall of the 



