V CADDIS-WORMS 249 



Supposing that the lateral processes are mainly- 

 used for securing the body to the inside of the 

 sheath, the use of the adjustable median process can 

 be guessed at. It is the third screw, so to speak, 

 which tightens the whole arrangement, and regulates 

 the distance of the body from its enclosing tube. 

 The thoracic legs, it is obvious, are otherwise employed 

 when the animal is creeping or feeding. 



I have lately seen a passage of Dr. Schmidt- 

 Schwedt's,^ which shows that he had arrived at a 

 similar explanation of the processes in question. 



It is singular that Reaumur should offer the im- 

 probable suggestion that these retractile processes are 

 respiratory organs, and then go on immediately to 

 describe the true respiratory organs of the larva. 



"The rest of the abdominal rings are provided with 

 organs whose use it is not easy to describe. They 

 consist of a great many white and soft filaments, of 

 which there are two bunches in the upper half of each 

 ring. At times the Insect moves them about, or 

 spreads them like a plume. At other times they are 

 laid flat, and meet across the back. I am much 

 inclined to suppose," says Reaumur, " that these offer 

 some analogy with the gills of Fishes." [The tracheal 

 gills of Caddis-worms vary in their arrangement in 



^ He says that the larva moves its abdomen up and down, 

 most probably to renew the water which bathes its body, and 

 that the fringed sides, by increasing the breadth of the abdomen, 

 add to the effect. " The fleshy processes of the first abdominal 

 segment find their use here ; the body is kept by them more in 

 the middle of the sheath, and the flow of water on all sides of 

 it is thus greatly promoted." Zacharias' Tier-und PJlaiizenwelt 

 dcs SiissTvassers, Bd. II. p. 97 (1891). 



