ACTION OF THE CAENIVOEOUS WATEE-BEETLES. 167 



plants to the surface, breathed for 15 seconds, then descended and 

 remained quiet on the sand, but not concealed. At 9.56 again rose 

 and breathed for 2 sees., then descended and concealed itself, rose 

 again at 10.7 and breathed for 5 sees., after which it gently moved 

 about. The observation ceased at 10.11 p.m. 



The data resultant from these 5 observations are as subjoined : — , 



Summary. — A female of Hydrovatus clypealis, as the result of 

 five observations extending over a period of 370 minutes, waa 

 only observed to breathe seven times, or an average of about 

 once in 53 minutes ; it has been observed to be 100 minutes 

 without breathing, while, on the other hand, it has been observed 

 that there is sometimes only an interval of 6 minutes between 

 two respirations ; the period it remained breathing at the surface 

 varies from 1 sec. or less to 15 sees. 



It is probable that the species is more active at night than 

 during the day. 



This species appears sometimes to take advantage of a bubble 

 of gas attached to a plant for its breathing, and has been observed 

 remaining for 10 minutes in a position enabling it to breathe such 

 a bubble. It walks or glides rapidly over the surface at the bottom 

 of the water rather than swims, and likes to rise by guiding 

 itself up the stem of a plant ; when it swims freely through the 

 water it is only for a short distance. 



Ill, Htphtdeus ovatus. 



No. 1, Aug. 29th, 1875.— The female example I here watched 

 was quite quiescent at edge of stone with bubble of gas adhering 

 to its extremity; observation commenced at 3.15 p.m. At 3.52 

 it moved and came up, but almost instantly descended and re- 

 newed its quiescent position ; it again ascended at 4.35, and 

 remained up 6 seconds. Observation ceased at 4.53. Thus only 

 about 6 seconds out of 98 minutes were passed in breathing at 

 the surface ; the bubble of gas .was always present. 



