162 MR. D. SHABP ON THE BESPIRATOET 



tlie higtly developed forms of the family, Di/tiscus marginalis, the 

 corresponding ratio is about 12 to 1. The amount of food the 

 Pelohius takes is surprisingly small ; and I have kept the speci- 

 mens for weeks without giving them any food, and without, I 

 believe, their obtaining any in the water in which they were 

 placed ; but they did not appear to suffer from the deprivation. 

 These habits afford a striking example of the truth of Herbert 

 Spencer's generalization, that the grade of development of an or- 

 ganism bears a direct proportion to its activity. 



Sydrovatus clypealis is a species that moves extremely little and 

 slowly ; and its motion is rather that of running or gliding than 

 of swimming. It glides over the surface at the bottom of the 

 water, and climbs up weeds for the purpose of breathing — and 

 was observed on one occasion, instead of ascending to the sur- 

 face, to make use of a bubble of gas adhering to a plant ; the 

 gas, I suppose, would probably be pure oxygen. 



Hyphydrus ovatus is one of the most specialized forms of the 

 Dytiscidae, but of a form which is of a low type compared with 

 others of the family ; and its habits appear to correspond with 

 its structural peculiarities. 



The species of Hydroporus were only very insufficiently ob- 

 served ; but it would appear from such observations as were made 

 that great diversity will probably be found to exist in the habits 

 of the extremely numerous species of this genus, and that such 

 observations may assist in the difficult task of classifying in a 

 natural manner the species of this extensive genus. 



The observations on Dytiscus marginalis suggest that the male 

 of this species is more active and breathes more frequently than 

 the female — a fact which quite agrees with the structural peculi- 

 arities of the species ; for the male is rather larger than the 

 female, and has the swimming-legs very much more developed. 

 This difference between the activity of the sexes is probably general 

 throughout the family. I think it possible that further observa- 

 tions with reference to this fact might throw some light on what 

 has been hitherto an insoluble puzzle to entomologists, viz. the 

 existence in several species of Dytiscus of two forms of the female, 

 one of these two forms resembling the male in some of its peculi- 

 arities. The genus Dytiscus is remarkable in the family, from 

 the great development of the posterior breathing-orifices ; but I 

 have failed as yet to obtain any clue to the relation of this struc- 

 tural peculiarity with the habits of the species. 



