LIMNOPIIILA. 197 



The difficulty consists in discovering the proper characters for 

 a subdivision. Some characters, very striking at first sight, 

 prove, upon comparison, to be of a secondary value. We find, 

 for instance, a number of LwinopMlae which, in the male sex, 

 have the antenme much longer than in the female, and of a differ- 

 ent structure. This would seem a good character for a sub- 

 division. But we soon discover that L. fenuipes Say, with long 

 antennae in the male, is very closely related to the European L. 

 discicollis Meigen, and to the North American L. recondita, 

 which have short antennas in both sexes, whereas it is much 

 less related to some other species with long antennoe in the male 

 sex. In the same way, the number of posterior cells is a character 

 of a very secondary value for any subdivision above a specific 

 one ; I believe, for instance, that L. quadrata, with four posterior 

 cells, is more related to L. tenuipes, which has five, than to some 

 other species with four posterior cells. The presence of a cross- 

 vein in the second basal cell, upon which Macquart has based his 

 genus Idioptera, is not a sufficient character to be used, unsup- 

 ported by others, for the establishment of a genus. The species 

 which Macquart would have placed in this genus are more closely 

 related to some species without such a cross-vein (to L. poetica, 

 for instance), than to the subgenus Epjhelia, which is also dis- 

 tinguished by this cross-vein. 



The most reliable characters to guide us are those taken from 

 the structure of the male forceps ; but in order to be available, 

 they must be supported by characters supplied by other parts of 

 the organization. Those Limnopliilse which, like the subgenera 

 Dacfylolahis, Prionolabis, and EjjJielia, have a forceps of a very 

 peculiar structure, are the best entitled to a separation. The 

 remaining Limnophilse, with a forceps of the typical shape (Tab. 

 lY, fig. 24, 25), would then form a still numerous genus, sub- 

 divided in groups, indicative of different degrees of relationship 

 between the species. I have to confine myself for the present 

 to an accouht more historical than critical, of the subdivisions 

 hitherto adopted by other authors as well as by myself; I will 

 add to it suggestions about some affinities which I perceive, but 

 which are of too vague a nature yet as to be available immedi- 

 ately. 



1. The subgenus Prionolabis 0. S. {Proc. Acad. Nat. Sci. 

 Philad. 1859, p. 239), has Livinophila rufihaais 0. S. for type ; 



