82 MR. F. BALFOUR-BROWNE ON THE LIFE-HISTORY 



although in some of these, e. g. "Wexford, there were many ideal 

 ponds, and although I was specially looking for this species, I 

 never found a specimen. I therefore regard the Irish individuals 

 as having strayed from the English breeding centres, the species 

 so far having failed to establish itself west of the Irish Sea. 



(3) The Imago, (a) Habitat and Habits. 



Pelobius is a pond species, never-, in my experience, occurring 

 in drains, lakes, or running water, but it seems to be very par- 

 ticular as to the type of pond it frequents. It is apparently only 

 found in ponds where the bottom is covered with a fine " ooze," 

 so that gravel- or marl-pits are ideal habitats. In some cases the 

 water of such ponds is clear and bright, while in others the pond 

 is a favourite haunt of cattle during hot weather, the water then 

 becoming thick and turbid and containing a high percentage of 

 ammonia. The (rattle pond is quite as thickly populated as the 

 clean pond, so that it is the "ooze" which seems to be the im- 

 portant characteristic of the habitat. 



In this "ooze" the beetle frequently buries itself, so that only 

 the apex of the body, often with a protruding air-bubble, is 

 visible, and, during the summer, it may remain thus buried for 

 even more than thirty minutes at a time, coming up rapidly to 

 the surface to renew its air-supply, which takes from five to ten 

 seconds, and then once more disappearing in the mud. 



By keeping the beetles in tumblers of water half filled with 

 pond mud, I have found that at irregular intervals the air-bubble 

 protruding from the apex of the body is released, and another 

 then appears in its place and gradually increases in size. At first 

 I was satisfied to explain this on the supposition that these were 

 bubbles of exhausted air, but I can find no statement as to the 

 mechanism of respiration in the Water- Beetle, or as to how the 

 sub-elytral air is utilized ; and I am not now inclined to speculate 

 on the subject beyond saying that possibly the released bubbles 

 are merely due to the expansion of the air in the sub-elytral air- 

 space, due either to the body temperature of the insect or to the 

 tem.perature or pressure of the mud. 



The food of the imago consists of insect larvae and worms, and 

 while I was working at the life-history I kept my specimens in 

 tumblers, feeding them mainly upon chopped-up earthworm, which 

 they took readily. Agrionid dragonfly nymphs, Chironomus 

 larvae, Sialis larvae, and various other forms, were readily 

 devoured, but more active types, such as Chloeon, could usually 

 keep out of the way. 



The mouth-parts are typically mandibulate, and in nature the 

 insect hunts for its food, swimming over the surface of the ground 

 and poking into hollows and under stones. It seems to have a 

 good sense of »mell, since, in swimming over the surface of the 

 mud, it will suddenly check and dive downwards ; and, in such 

 circumstances, if, the beetle is dug up, it usually has some food 



