BESPIEATOBT ACTION OF THE CAEXIYOROUS WATER-BEETLES. 161 



Observations on the Respiratory Action of the Carnivorous 

 Water-Beetles (Dytiscidae). By D. Shabp, Esq., M.B. (Com- 

 municated by H. W. Bates, Esq., E.L.S. &c.) 



[Read November 2, 1876.] 



The observations here recorded were commenced by me some 

 time ago, to see if I could get any insight into the peculiarities of 

 the aeration or respiration of the carnivorous Water-beetles, or 

 Dytiscidae, at a time when I was hoping I should be able to make 

 some inquiries about the function of respiration in the Insecta ge- 

 nerally. I very soon found, however, that the subject was so vast, 

 and the difficulties of making accurate minute investigations as to 

 one of the functions of creatures so small as the insects I could 

 procure were so great, that I abandoned my intention. But as 

 the observations I made, though of a desultory character, are not 

 without suggestiveness on certain points, I have thought it worth 

 while they should be placed on record. I think that if such 

 observartions were carried out very much more fully and systema- 

 tically, they would serve as material to enable us to fill up some 

 of the vast gaps which exist in our knowledge of the physiology 

 of this highly organized class of the Invertebrata. 



The two most interesting species of all those I observed are 

 undoubtedly Pelohius Hermanni and Hj/drovatus clypealis*. 

 These species are, in their structure, much less highl}^ developed 

 for moving through the water than our other indigenous Water- 

 beetles, and are, in my opinion, to be considered (together with the 

 North- American Ampliizoa) the most rudimentary or primitive of 

 the existing forms of Dytiscidse. Their habits quite accord with 

 their structural peculiarities. Pelohius Hermanni, though it is a 

 powerful swimmer, moves its limbs in such a rapid manner that it 

 must be incapable of any long-sustained efforts — and, in point of 

 fact, passes its life, in the perfect state, concealed in soft mud, 

 from which it suddenly rises to the surface to take air, and de- 

 scends again to its concealment with great rapidity. According 

 to my observations, the time it is concealed bears to the time it 

 is exposed for breathing a ratio of 375 to 1 ; whereas in one of 



* I have much pleasure here in acknowledging the kindness of Henry Mon- 

 creaff, Esq., who procured and forwarded to me living individuals of these two 

 species, neither of which occurs in Scotland. 



