120 THE ENTOMOLOGIST. 



the males as well as the females are usually rough, an event of 

 rare occurrence in the Dytisciflje ; whilst in Hyderodes, the 

 closely allied genus which combines with Dijtiscus to constitute 

 the tribe Dytiscina, the females, although dimorphous, have 

 almost invariably smooth elytra like the males. Early authors 

 have generally associated the roughness of the females with the 

 perpetuation of the species, and regarded the grooved elytra in 

 Dytiscns as a special provision of Providence, and accessory to 

 the suckers on the fore feet of the male beetles. But Plateau 

 was the first of several naturalists whose experiments greatly 

 tended to shake our faith in these earlier interpretations. The 

 results of Plateau's investigations have been summarized by 

 Miall, whilst the subsequent tests of Dr. Lowne formed the sub- 

 ject of a paper communicated to the Pioyal Microscopical Society 

 in 1871. It will be sufficient for our purpose to-night to quote 

 the former : — " (1) Furrows on the elytra diminish instead of 

 increasing the holding power of suckers ; ground glass and paper 

 do not on trial act so well as polished glass. (2) The suckers of 

 the male are not applied to the furrowed part of the elytra at all, 

 but to the prothorax and the smooth edges of the elytra. (3) 

 Females with smooth elytra occur now and then in England, and 

 the male can hold these as well as the common form." 



The subject of dimorphism has, since the publication of 

 Darwin's ' Origin of Species,' received a good deal of attention, 

 more especially from botanists and lepidopterists. Sexual 

 dimorphism is shown in some degree by a large portion of our 

 British Lepidoptera, especially amongst the blue and hairstreak 

 butterHies ; and two forms of the same sex, as in Dytiscus, are 

 not uncommon : thus in the clouded yellow {Colias edtisa) one 

 form of tbe female resembles the male, save that the broad dark 

 border which runs round the wings in the latter sex is broken by 

 yellow spots, whilst the other is of a more or less unicolorous 

 creamy-yellow tint. Again, in the silver-washed fritillary {Dryas 

 j)aphia), the female may be either dark brown or blackish green. 

 Whether Mr. Tuit, Mr. Pierce, or other authority has successfully 

 solved the reason for these persistent variations in the same sex 

 amongst lepidopterous females, I am, I regret to say, insufficiently 

 acquainted with that order of insects to state ; but no convincing 

 or satisfactory explanation has hitherto been advanced to account 

 for the dimorphous females in Dytiscus, and I have therefore 

 pleasure in further calling the attention of some of our more 

 enthusiastic members to this interesting fact. 



It is a far cry from the rotund and solidly constructed 

 Dytiscus to the fragile form of Cldronomus plumosns, the earlier 

 life of which dipteron furnishes those of us who have microscopes 

 with opportunity for very careful observation and minute re- 

 search, for the problem here set us falls to the province of those 

 who combine with the study of the living insect, pure and 



