THE STUDY OF LIFE-HISTORY. 119 



lating areas of other beetles, nor is there on any other part of the 

 body, so far as I could find. Since receiving your letter I have 

 again examined a Dytiscus I had by me, but I have not been 

 more successful than before. There is a series of short trans- 

 verse ridges near the edge of the upper surface of the first 

 abdominal segment in Dytiscus, but the ridges are very coarse, 

 unlike any occurring on true stiidulating areas of other beetles, 

 and, so fur as I can see, it is not possible for them to come into 

 play against the elytra." I might add that Dr. Sharp, our 

 greatest authority on the carnivorous water-beetles (Dytiscidse^ 

 makes no mention of stridulating organs other than those of 

 Pelobius (Camb. Nat. Hist.). 



Since writing the above Mr. Gahan has been kind enough to 

 again communicate with me on the matter, calling my attention 

 to a paper entitled " Die Tonapparate der Dytiscidse," by Her- 

 mann Eeeker (' Archiv fiir Naturgeschichte,' 1891, vol. Ivii. 

 pp. 105, et seq.), in which he states that stridulation is produced 

 in Dytiscus, Pelobius, and other members of the family by rub- 

 bing one of the large outer wing nervures against the elvtra, the 

 transverse ridges on these nervures being very prominent, and, 

 according to Reeker, more numerous in the Dytiscidse than in 

 other beetles. But " many other beetles," writes Mr. Gahan, 

 " which are not known to stridulate, or else are well known to do 

 so in a different way, have the corresponding wing-nervure just 

 as prominent and as strongly ribbed across ; so that I am some- 

 what inclined to share Dr. Sharp's view that Keeker's explana- 

 tion is erroneous. I certainly believe with Sharp that it is 

 erroneous so far as it applies to Pelobius, but he may, of course, 

 be right in regard to Dytiscus ; and it would be well therefore to 

 call attention to his explanation." 



Here then is one of the enigmas awaiting solution by the 

 diligent naturalist, and any young member here present who may 

 be anxious to solve the mystery will meet with very little trouble 

 in obtaining his beetles, have little time to wait ere hearing them 

 stridulate, and experience little difficulty in obtaining permission 

 to communicate in the form of a paper at an early meeting of 

 the Society the successful results of his investigations. 



Still confining ourselves to the same insect, we may further 

 note that no satisfactory explanation has hitherto been advanced 

 to account for the dimorphism exhibited by the females of this 

 genus. Of our six British beetles, the females are dimorphic in 

 four species. Camerano, probably quoting from Simmermacher, 

 says that the sculptured females belong more particularly to 

 northern districts ; and the observations of Redtenbacher, who 

 states that near Vienna the smooth form of female is as general 

 as the rough, appears to support this assertion. But the fact, 

 even if established, fails to furnish us with any clue as to either 

 cause or purpose. In the allied terrestrial family of Carabidse 



