I AQUATIC BEETLES 33 



perpendicular sides of the glass, and it takes them 

 about half an hour to rise so high. Failures do not 

 discourage them, and no matter how long they 

 have been kept in captivity they are never weary of 

 endeavouring to creep out of the tank. 



When handled, they give off a milky fluid of un- 

 pleasant odour from nearly all the joints of their 

 body, but especially from the fore and hind edges of 

 the thorax. The scent is rather like that given off by 

 Cockroaches. According to most authors, the perfect 

 Gyrinus is largely carnivorous. Westwood says that 

 he has ascertained that its food consists of small dead 

 floating Insects. Fowler remarks that the broad blunt 

 mandibles point to a partially vegetable diet, though 

 the sharp sickle-shaped maxillae favour a different 

 conclusion. In captivity they feed upon water-plants. 1 



The body of the adult Beetle is of oval shape, 

 convex above and flat beneath. The head is sunk in 

 the thorax. The end of the abdomen projects, and 

 forms a short tail upon which are carried two small 

 retractile prominences. Between the tips of the 

 wing-covers and the extremity of the body rests the 

 air-bubble which the Beetle carries with it when it 

 dives. 



1 Air. W. F. Baker favours me with the following observa- 

 tion : " I was once watching some Gyrini, when a gad-fly came 

 flying round my head. I struck at it, and knocked it into the 

 water, stunned if not dead. Two or three Gyrini seized it, and 

 shortly afterwards the whole swarm clustered round. In a 

 short time nothing was left except the wings of the fly, which 

 were allowed to drift away. This is the only occasion on which 

 I have seen anything of the sort. I could never get captive 

 Gyrini to eat dead flies." 



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