MOSQUITOS OF THE PALAEARCTIC REGION. 293 



also be defined on the structure of the mesosome of the aedoeagus. In the group 

 which possesses claspettes the mesosome is a simple tube, rather lightly chitinised 

 below and at the sides, membranous above. In the other group the mesosome is a 

 paired structure, the halves of which are more or less crenulate, spiny or brnsh-like. 

 Both these distinctions are so well marked that one would expect to lind corresponding 

 distinctions in the body characters or in the larvae, but all efforts in this direction 

 have so far been fruitless. 



The first group includes the subgenera Ochlerotatus and Finlaya, and is spoken of 

 by Dyar as the New World type — inappropriately, since it includes the majority 

 of the European mosquitos, and has many representatives in Australia, while Finlaya 

 attains its strongest development in the Oriental region. The second or Old World 

 group (subgenera Ecculex, Aedes and Stegomyia) is almost confined to the tropical 

 and subtropical regions of the Old World. 



The subgenera occurring in the Palaearctic region may be separated (as adults) 

 by the following keys, which will not necessarily hold good for the species of other 

 regions. 



Male Hypopygium. 



1. Claspettes present ; mesosome an unpaired simple structure . . . , 2 

 Claspettes absent ; mesosome a paired, more complicated structure . . 3 



2. Side-piece with more or less distinct apical and basal lobes . . Ochlerotatus. 

 Side-piece without apical or basal lobes . . . . . . . . Finlaya. 



3. Clasper deeply divided, placed before tip of side-piece ; without articu- 



lated spine . , . . . . . . . . . . . . . . Aedes. 



Clasper not divided, placed at tip of side-piece, with distinct articu- 

 lated spine . . . . . . . . . . . . . . . • . . 4 



4. Spine of clasper well before the tip . . . . . . . . . . Ecculex. 



Spine of clasper at the tip (except in A . vittatus) . . . . . . Stegomyia. 



Other Adult Structures. 



1. Proboscis distinctly longer than front femora ; last two joints of male 



palpi distinctly swollen, hairy, and turned downwards . . . . . . 2 



Proboscis not longer than front femora ; male palpi otherwise ; female 

 cerci short . . . . . . . . . . . . . . . • . • 4 



2. Female cerci short, the eighth sternite large and prominent in repose ; 



male palpi somewhat shorter than the proboscis . . . . . . Finlaya. 



Female cerci long, the eighth sternite much smaller and not prominent 

 in repose . . . . . . . . . . . . . . . . . . 3 



3. Male palpi longer than the proboscis . , . . . . . . Ochlerotatus. 



Male palpi not longer than the proboscis . . . . . . . . Ecculex. 



4. Palpi alike in the two sexes, very short ; tarsi dark. . . . . . Aedes. 



Male palpi long, slender, the last two joints upturned and nearly bare ; 



tarsi with white rings at the bases of the joints . . . . . . Stegomyia. 



The adults of these subgenera are treated separately, but it will be more convenient 

 to tabulate all the known larvae together, as follows : — 



Key to the known Larvae of Palaearctic Species o/ Aedes [sens, lat.).* 



1 . Antennal tuft represented by a single minute hair ; shaft devoid of 

 small spines ; eighth abdominal segment with a well-marked comb of 

 teeth set in a single row . . . . . . . . . . . . . . 2 



* This key will not necessarily apply to any but fourth-stage larvae. For the characters 

 of A. dorsalis and A. intrudens I have relied on American descriptions, and for those of A. alpinus 

 on Wesenberg-Lund's description of specimens from Greenland. 



