THE FISHES, MOLLUSCA, AND OTHER OBJECTS. 45 



Fig. 38. 



Cases of Molanna 

 angustata. 



forms an ingenious covering whenever the larva is 

 forced to protrude its head in search of food. These 

 larvae generally live on vegetable 

 matters, although they have 

 been said to be not indifferent 

 to the ova of fishes, &c. The 

 mechanism of the tubes of caddis- 

 worms is, geologically speaking, 

 very ancient; for similar cases 

 are found in such abundance in 

 the Miocene strata of Central 

 France, that actual rocks are 

 composed almost wholly of their remains. 



The larvae of the Ephemera and Water-beetle 

 (Dytiscus) inhabit the same water, as many small 

 fishes find out to their cost, for these creatures are 

 as ferocious, after their kind, as the greater land 

 carnivora. Dytiscus feeds on tadpoles, and keeps 

 down the tendency to swarm of these amphibian 

 progeny. The Dytiscus is especially fierce, and, 

 when kept in an aquarium, will not hesitate to 

 attack the stick that is pointed near it. One grip is 

 generally sufficient to settle a poor tadpole, as the 

 jaws actually meet through its sides. To the mi- 

 croscopist the breathing tubes (iracliede) of this 

 insect are very interesting. The best way to obtain 

 them is to make a careful incision along the centre 

 of the back with a pair of fine scissors. After a 

 prolonged soaking in acetic acid, and repeated 

 washing, the skin may be removed almost entire, 



