22 MOSQUITOES OF NORTH AMERICA 



Megarhinus and the simple male genitalia of Megarhinus and Orihopodomyia. 

 If this view is correct, the absence of the ventral brush in the larva? of the Sabe- 

 thini is to be explained as the loss of a character originally present in the ances- 

 tors of the group. This structure functions as a steering organ or rudder during 

 the progression of the larva through the water, and is present in all the other 

 Culicidffi. In the Culicini the brush is not present in the first larval stage, 

 appearing only at the first ecdysis, so that if the structure were disadvantageous 

 to the Sabethini it may have become entirely suppressed. That it may have 

 been disadvantageous seems reasonable to suppose from the following considera- 

 tion : We will suppose the structure originally present in a larva inhabiting 

 holes in trees where the water, although sometimes scanty, generally has a 

 considerable depth. The structure would then still be useful. However, as the 

 sabethids developed, they became more and more adapted to inhabit the water 

 between the leaves or flowers of plants, which is generally contained in very 

 small or narrow receptacles. In such situations the ventral brush would be in 

 the way, and its elimination would be an advantage. 



Again, if this derivation be correct, we must consider the presence of set[e 

 upon the postnotum as an acquired character and not one of generalization, 

 representing an originally hairy and functional condition of this structure ; for 

 there are no setae upon the postnotum of the anophelines, nor of any of the other 

 Culicini, except in the specialized Dinomimetes and Hcemagogus, which are not 

 related to the Sabethini. We are unaware of the functional importance of this 

 structure, if indeed it has any, and are unable to suggest why it should have 

 arisen. Its sporadic appearance in wholly unrelated genera of Culicini reduces 

 its importance. We observe that setse appear likewise in an apparently sporadic 

 manner upon the postnotum of some Tipulida?, so that perhaps this character is 

 of no fundamental value in the Diptera. However, in the case of the Sabethini, 

 the seta? of the postnotum are correlated with a number of well-marked char- 

 acters, and, while this character seems insignificant, it nevertheless indicates a 

 natural and distinct group. 



Tables of Genera of the Tbibe Sabethini.* 



ADULTS. 



1. Eyes separated by at least a narrow area; prothoracic lobes collar-like 2 



Eyes broadly contiguous above; prothoracic lobes sublateral 6 



2. Prothoracic lobes contiguous or closely approximated dorsally 3 



Prothoracic lobes well separated 5 



3. Front femora shorter than the middle ones. . .Sabethes Robineau-Desvoidy (p. 23) 

 Front femora as long as the middle ones 4 



4. Proboscis moderate, swollen at tip Sabethinus Lutz (p. 31) 



Proboscis long and slender Sabethoides Theobald (p. 37) 



5. Hind tarsi with 2 claws, normal Wyeomyia Theobald (p. 49) 



Hind tarsi with but 1 claw Limatus Theobald (p. 40) 



6. Proboscis long and slender throughout 7 



Proboscis short, swollen at tip; clypeus scaled Prosopolepis Lutz (p. 160) 



7. Clypeus with setae Joblotia Blanchard (p. 175) 



Clypeus nude Lesticocampa Dyar & Knab (p. 162) 



1 See page 185 for remarks on unrecognized genera. 



