COLEOI'TERA — BEETLES. 229 



feature ; but among the Dytiscidce, the typical genus 

 Dytiscus must be avoided, or kept apart, on ac- 

 count of their voracious habits, as they will not only 

 destroy other aquatic insects, but even small fish, 

 where any are kept. The late Curator of the old 

 " Physic Garden " at Chelsea is known to have com- 

 plained that these predatory Water Beetles greatly 

 annoyed him in the ponds of the garden by eating 

 off the fins of gold and silver fish, even of large size. 

 But notwithstanding these objections, Dytiscus 

 Marginalis is worthy of a place in the Vivarium if 

 he can be kept separate, for his activity in swim- 

 ming is very attractive, and he is in some cases 

 very long-lived. Esper cites a case in which a 

 specimen was kept for three years and a half, being 

 occasionally fed upon small pieces of meat. But 

 Kirby and Spence attribute this degree of longevity 

 to his having been kept solitary, and so not 

 allowed to pair. Some of this tribe, Acilius Sulcatus 

 and others, have the power of making a humming 

 noise, which is not produced, as formerly supposed, 

 by rubbing the elytra, or hard wing-cases, against 

 the abdomen, but rather, as Mr. Westwood supposes, 

 by the action of the air upon the alulcts. 



Water Beetles may be observed to remain deeper 

 in the water as the weather becomes colder, and to 



