ZOOLOGY, ENTOMOLOGY, AND MEDICINE 227 



surrounded by jelly, and often form a ring around a twig, or 

 other small object in the water. The eggs usually hatch in 

 four or five days, and the larvae, which are called caddis- 

 worms, are at first free-swimmers. The larvae and their hab- 

 its are the chief cause for our interest in this order, because the 

 adults are so small, and not particularly beautiful. The larvae 

 have the general appearance of caterpillars. They have no 

 antennae, the legs are very long, and there are no prolegs but 

 the anal pair. The body has a varying number of filaments 

 containing tracheae, which serve for respiration. Shortly after 

 hatching each larva begin to construct a case in which, with 

 certain alterations, most of them spend their larval and pupal 

 life. These cases are made of small bits of leaves or twigs, 

 sand, pebbles, or any other small fragments of material avail- 

 able. These are fastened together with silk spun from the 

 mouth of the larva. Some of the species of Trichopetra carry 

 their cases around with them, while in others the cases are 

 fixed. The larvae living in movable cases have a large swell- 

 ing on the dorsum of the first abdominal segment, and usually 

 two smaller swellings on the ventral surface. These swellings 

 are furnished with hooks, which hold the larva in its case. 

 The hooks on the anal prolegs are supposed to serve the same 

 purpose, and it is certainly very difficult to pull a larva out of 

 its case. The cases are large enough to allow for the passage 

 of water through the tube and for the movement of the res- 

 piratory filaments. Most of these cases have a thin web of 

 silk spun across the caudal end of the tube. When the larva 

 moves it protrudes the head and legs from the tube and crawls 

 along. There is also a species which bores into fallen twigs 

 found at the bottom of streams. The larvae line the burrows 

 with silk, and these twigs form portable cases which the larvae 

 drag along, or which are allo\ve;l to float down stream with 

 the current. When the twig is too long for the occupant it is 

 girdled with a circular incision, which is deepened until the end 

 is cut away. It is said that this is the method the larvae 

 employ in altering the cases to suit their growth. The cases 

 are nearly always larger at the caudal end, so the larvae cut off 

 the small end and add to the large end. 



The life history of these insects has not been worked upon 

 sufficiently to determine the number of molts in the larvae. 



