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IV. Revision of the species included in the genm Tropi- 

 sternus (fam. Hydrophilidae). By D. Sharp. 



[Bead December 6th, 1882.] 



In working out the Hydrophilidce for the 'Biologia Cen- 

 trali- Americana,' I found so much difficulty in discrimi- 

 nating the species of Tropisternus, and such great con- 

 fusion prevailing about those already described, or, 

 perhaps I should rather say, named, that I have been 

 obliged to submit the whole of the genus to a careful 

 revision. For this purpose I have used only the speci- 

 mens extant in my own collection, and a set of types of 

 the North American species kindly lent me by Dr. Horn : 

 but as several of the older French collections have come 

 into my possession, I have had sufficient material to 

 enable me to elucidate most of the species enumerated 

 in the Munich Catalogue, and to distinguish a few new 

 ones. The difficulty of identifying the older descriptions 

 is extreme ; for these insects, like most other water- 

 beetles, are very similar to one another in colour, form, 

 and such minor characters as in other families of 

 Coleoptera are readily perceived, and make the dis- 

 tinction of species by cursory inspection easier than it is 

 in the case of the water-beetles. Careful examination 

 has revealed some important characters for grouping the 

 species, and when the structural characters, by which 

 the members of the different groups are characterised, 

 are appreciated, the task of discriminating the species 

 becomes comparatively easy, for it is frequently species 

 of two quite different groups that bear the greatest super- 

 ficial resemblance to one another. Even after all my 

 efforts, however, I leave a great many questions of 

 specific identity unsolved, and it is quite possible that 

 the species are considerably more numerous than those 

 the method I have adopted has induced me to promul- 

 gate. Among the first points to which I directed my 

 attention was the finding of an external means of dis- 

 tinguishing the two sexes ; for the front feet, which are 

 usually of such assistance for this purpose, are in Tropi- 



TRANS. ENT. SOC. 1883. — PART II. (JUNE.) I 



