water-insects. 255 



Dyticus marginalis, or Water-Beetle. 

 (Plate XIX. fig. 4.) 



Lively and interesting, but dangerous inmates, are these 

 Beetles of the fresh-water tanks, in which they are quite 

 at home. Tor in their natural haunts they prefer stagnant 

 or still, to running water. They swim with very great 

 agility, their hind legs acting in concert, and looking like 

 oars. I have noticed tliat when the Beetles are descend- 

 ing from the surface, they carry, wrapped round the end of 

 their abdomens, a bubble of air, which apparently assists 

 them in keeping the head downwards in diving. When 

 at the bottom, a portion, or even the whole, of this bubble 

 becomes disengaged or absorbed, and they mount, head up- 

 wards. The bubble is very bright and silvery in appear- 

 ance, and is no doubt retained by means of a few fine hairs 

 on the abdomen. These Beetles are very voracious in their 

 habits, seizing and devouring small aquatic insects and 

 moUusca, and sometimes destroying young fish. The keeper 

 of the Chelsea Botanic Gardens complains of these insects 

 destroying his gold and silver fish by nibbling at their 

 dorsal and pectoral fins. 



Mr. Westwood relates that a specimen of D. marginalis, 



