NO. 8 ANATOMICAL LIFE OF THE MOSQUITO SNODGRASS 9 



this lobe are the united labium and hypopharynx (fig. 7 A, Lb, Hphy). 

 We may, therefore, following Anthon (1943a), Hennig (1948, 1950), 

 and Lawson (1951), appropriately call this lobe the hypostomium 

 (Hstm), as it is termed also by Chiswell (1955) in the tipulid larva. 

 Though Schremmer (1949) called it "mentum" in the mosquito larva, 

 he later (1950) expressed doubt of the correctness of this designa- 

 tion, concluding that the lobe is rather a part of the cranial wall. More 

 recently, Gouin (1959) has termed the dentate lobe the hypochilum 

 (underlip). 



From the base of the hypostomium there arises in some species a 

 thin fold bearing a fringe of pectinate hairs or blunt teeth (fig. 15 A, 

 Aul). The fold is the aulaeum (curtain) of Cook (1944a), but it has 

 been variously named. Shalaby calls it the "glossa" on the assumption 

 that it is formed by the union of a pair of labial glossae, a highly im- 

 probable interpretation since the hypostomium itself is no part of the 

 labium. However, Shalaby has given detailed illustrations of the 

 pectinate hairs of the lobe in Aedes aegypti (1957a) and Cidex 

 quinquefasciatus (1957b), and its armature of eight blunt teeth in 

 Anopheles qiiadrimaculatus (1956). In Psorophora ciliata (1957c) he 

 says the fold is absent. 



In most mosquito larvae two dark lines in the ventral wall of the 

 head diverge posteriorly from the basal angles of the hypostomium. 

 In some species the lines are short (fig. 3 C,D, r), in others (E,F) they 

 extend back to the posterior tentorial pits (pt) ; in Chironomus 

 (fig. 2 F) they are absent. These lines when present are the external 

 marks of internal ridges ; their variable development suggests that the 

 ridges are secondarily formed to strengthen the head wall. The surface 

 area between the lines, however, has commonly been regarded as the 

 basal part of the labium, probably because the structural pattern they 

 produce resembles that of the head shown at B of figure 2. It has been 

 suggested even that the median suture is the line where the two origi- 

 nal labial appendages have united! Cook (1944a), for some obscure 

 theoretical reason, calls the area in question the "maxillary segment," 

 though the maxillae have no relation to it. That the ventral closure 

 of the head results entirely from the union of the lateral cranial walls 

 is clearly indicated in illustrations by Hennig (1948, figs. 31-37) of 

 larval heads of Sciophilidae, in which are shown various degrees of 

 approximation and union of the postgenal margins. 



In most adult insects the lower edges of the cranium are reinforced 

 by submarginal internal ridges formed by external grooves known as 

 the subgenal sulci. The part of each groove on the postgena behind 



