10 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



the mandible is distinguished as the hypostomal sulcus (fig. 2 A, hs). 

 Posteriorly these grooves become continuous with the postoccipital 

 sulcus (pos) that surrounds the occipital foramen. In the mosquito 

 larva the lower ends of the postoccipital sulcus have extended forward 

 in the postgenal region carrying with them the minute rudiments of 

 the posterior tentorial arms, the position of which is marked externally 

 by a pair of pits (fig. 3 E,F, pt). The anterior tentorial arms are 

 extremely slender bars arising from the cranial margins mesad of 



Lrn- 



Md 



£W & /} 



,„,,„. Ant 



WlL \H 



Fig. 3. — The larval head, undersurface. 



A, Olbiogaster sp., Rhyphidae (from Anthon, 1943b). 

 Anopheles quadrimaculahis. D, Toxorhynchites rutilus. 

 Aedes aegypti. 



r, r, grooves in lower head wall. Other lettering as on figure 2 



B, Tipula sp. C, 

 E, Culex sp. F, 



the antennae that extend back to the posterior arms. Possibly it is 

 the great lengthening of the postgenal regions of the head that has 

 brought the posterior arms to their forward position. 



The postgenal bridge is known also as the hypostomal bridge be- 

 cause it is the hypostomal margins of the postgenae that come together 

 to form it. Lawson (1951) contends that the sclerotized ventral wall 

 of the head behind the mouth parts cannot be derived from the post- 

 genae because, he says, "the hypostomal sutures form the ventral 



