WATEE-BEETLES. 219 



among useful or choice aquatic plants as it would be to asso- 

 ciate an unfed D. marginalis with fish and other defenceless 

 animals. Though if. piceus will not, as a rule, hurt any of 

 the inhabitants of an ordinary aquarium, it will be more 

 interesting in a separate tank, where it can lead the retired 

 life it loves among the plants it feeds upon. As 3. piceus 

 has no need to pursue its food, it is only adapted for travelling 

 somewhat slowly through the water by the alternate move- 

 ments of its legs. It spends a great portion of its time either 

 at rest among the weeds or crawling over them. Though this 

 beetle is generally described as herbivorous, it wiU occasionally, 

 at any rate whUe in captivity, partake of animal food. It 

 destroys a great deal of the plant upon which it feeds, and 

 therefore it is wise to provide it with Anacharis or some 

 other quickly-growing and easily-obtained weed.^ A pair of 

 these beetles will live, to all appearance perfectly happy, in 

 an inverted propagating-glass, or a small rectangular tank, 

 well stocked with suitable vegetation, and there the female 

 will very likely construct her nest or cocoon (fe and c. 

 Fig. 140), and lay her eggs. The nest is woven by the 

 help of two small spinnerets, which are placed near the 

 apex of the abdomen of the insect; it is shaped some- 

 what like an ordinary retort. This egg-sac is formed of a 

 substance which is perfectly impervious to water, and which 

 has rather the appearance of paper. Inside this pouch, 

 which at its largest part is about fin. in diameter, eggs to 

 the number of fifty or sixty are inclosed, and are hatched, 

 according to the temperatm-e of the water, in about six weeks. 

 The young lai-vse (d. Fig. 140) escape through the bottom 

 of the sac, and seem from the first to be quite vigorous. This 

 sac or pouch is so constructed that it wiU float upon the 

 water with the stem or horn-like part of it uppermost, and 

 is attached by the beetle to some portion of an aquatic 

 plant which is upon the surface (/, Fig. 140). The spike or 

 horn-like part, which is hollow, rises above the water, and 

 thus admits air to the interior of the sac. The insect is 

 sometimes able to spin her cocoon and lay her eggs within 

 three hours. 



