ACTIOK OP THE CAENIVOROUS -WA-TEE-BBBTLEa. 



163 



The observations on tlie two species of Acilius appear to show 

 a great difference in habits between the two closely allied spe- 

 cies observed ; but I think that further and more extensive obser- 

 . vations would probably greatly reduce this discrepancy ; for I 

 believe that the time of day, the season of the year, the condition 

 of sexual activity, the sex of the individual, the process of diges- 

 tion, and the condition of hunger, all modify the activity of the 

 Dytiscidse. 



It appears probable that most of the species are much more 

 active by night than by day. 



I regret very much that 1 have not been able to make any ob- 

 servations on Gyhister Hoeselii ; for I consider the genus Ct/bister 

 to be, all points considered, the most highly developed form of 

 the family. It is known that some of the species of Gyhister are 

 enormous devourers of animal food ; and I think it probable 

 that observations would show these to be the most active by far 

 of all the members of the family. No species of the genus oc- 

 curs in Britain ; so that it is not likely I shall be able myself to 

 make observations on any of the species. 



I have reduced most of the observations made, by a system 

 of averages, so as to allow of their being easily compared with 

 one another, and have given the averages obtained of most of 

 the species. 



I. Pelobius Heemanni. 



No. 1. Breathing-observation made on four individuals, two 

 (S , two 2, August 29th, 1875. — These specimens were completely 

 concealed in soft mud ; this they left for breathing, rose rapidly 

 to the surface, and, after breathing, descended with great rapi- 

 dity and buried themselves in the mud. 



Observation commenced at 11.46 a.m. : — 



11.57, only very short time up. 

 12.9 remained up 2 sees. 

 12.10i „ 4 „ 



11 " « 



12.17* 



12.26 



1 sec. 



12.36 remained up 3 sees. 

 12.38i „ 13 „ 



12.42 „ 5 „ 



12.50 „ 3 „ 



12.51 „ 3 „ 



Observation ceased at 1 p.m. Thus each individual spent only 

 10 seconds out of 74 minutes at the surface in breathing. 



No. 2. Aug. 29th, 1875. — The same four specimens were under 

 surveillance, commencing at 3.40 p.m. and ceasing at 4.30 p.m. : — 



