ZXB,YM OF WATBE-FLIES. 241 



occasionally, under the water itself. The eggs are inclosed in 

 a gelatinous enrelope, and hatch in a few days. The young 

 larvae remain in the glutinous mass for a short time before 

 they venture to brave the dangers of their watery world. 

 They are elongated, and more or less cylindrical in shape, and 

 of a yellowish-white colour. Their bodies are soft, with the 

 exception of the head and the thoracic segments, which are 

 homy. They have branchial filaments, placed either in bunches 

 or singly (according to the species) on the sides of their bodies, 

 six legs, and strong jaws. These insects, which are in them- 

 selves so defenceless, would soon faU victims to fish and other 

 aquatic animals were they not endowed with an instinct which 

 makes them about the most interesting inhabitants of the 

 fresh-water aquarium. Under the influence of this instinct, 

 they build for themselves houses, portable in most cases, but 

 stationary in others, in which they are more or less secure 

 from the attacks of their numerous enemies. These houses 

 are formed in various shapes, and are made of various 

 materials, according, as a rule, to the species of the larva 

 which constructs them. Sometimes the materials depend, to 

 a great extent, upon the locality. The caddis-cases, as they 

 are generally caUed, are made of pieces of stick, leaves, sand, 

 small stones, shells of molluscs, moss, rushes, seeds, and the 

 like. These cases are formed either of all one kind of sub- 

 stance or of two or more different kinds. But whatever 

 material is used, the larvae generally manage to make these 

 places of refuge of the same specific gravity as the water, so 

 that they may have no difficulty in moving from place to place. 

 The insects fasten then* building materials together by the 

 help of a viscous secretion, which on exposure hardens into 

 a kind of silk. The insides of the caddis-cases are also lined 

 with this silk-like substance. The larvae are able to cling 

 most tenaciously to their tubes by means of small hooks placed 

 at the extremity of their bodies, by the third pair of legs, 

 which are often longer than the rest, and in many species by 

 the help of three smaller humps situated upon the first 

 segment of the soft part of their bodies. So firmly, indeed, 

 will these creatures hold to their cases, that an attempt to 



