1418 THE STORY OF THE UNIVERSE 



larvae and form for themselves cylindrical cases or 

 tubes, built up of sand, little stones, bits of stick, 

 leaves, or even shells. They generally feed on vege- 

 table substances, but will also attack minute fresh- 

 water animals. When full grown, the larva fastens 

 its case to a stone, the stem of a plant, or some other 

 fixed substance, and closes the two ends with an open 

 grating of silken threads, so as to admit the free ac- 

 cess of water, while excluding enemies. It then turns 

 into a pupa which bears some resemblance to the per- 

 fect insect, "except that the antennae, palpi, wings, 

 and legs are shorter, inclosed in separate sheaths, and 

 arranged upon the breast." The pupa remains quiet 

 in the tube until nearly ready to emerge, when it 

 comes to the surface, and in some cases creeps out of 

 the water. It is not therefore so completely motion- 

 less as the pupae of Lepidoptera. 



The Diptera, or flies, comprise insects with two 

 wings only, the hinder pair being represented by mi- 

 nute club-shaped organs called "halteres." Flies 

 quit the egg generally in the form of fat, fleshy, 

 legless grubs. They feed principally on decaying 

 animal or vegetable matter, and are no doubt useful 

 as scavengers. Other species, as the gadflies, deposit 

 their eggs on the bodies of animals, within which the 

 grubs feed, when hatched. The mouth is generally 

 furnished with two hooks which serve instead of jaws. 

 The pupae of Diptera are of two kinds. In the true 

 flies, the outer skin of the full-grown larva is not shed, 

 but contracts and hardens, thus assuming the appear- 

 ance of an oval brownish shell or case, within which 

 the insect changes into a chrysalis. The pupae of the 



