172 NATURAL HISTORY OF AQUATIC INSECTS CH. 



Most of the segments are thickened behind, and on 

 each of these annular thickenings a circle of stiff, 

 backward-directed setae (bristles) is borne. Like the 

 differently arranged setae of the Earth-worm, these 

 give a sufficient hold to enable the animal to advance 

 by alternate expansion and contraction of its seg- 

 ments. The action is aided by three pairs of hook- 

 bearing prominences, or pseudopods, of the same kind 

 as the prolegs of caterpillars. 



The body is provided with numerous sensory hairs, 

 and also with two pairs of small vesicles situated on 



the sides of the body in the 

 tenth and eleventh seg- 

 ments. These are described 

 by Grobben l as filled with 

 a fluid, in which float re- 

 fractive and transparent 



FIG. 57. Trachea (air-tube) of larva rrlobllles three as a rule to 

 of Ptychoptera, showing constric- JUlCh, III 



tion. The upper figure shows a o r U vPQlVlf* A ?npria1 

 cross section of the same, and eacn VCSlClC. /\ bpCUdl 



illustrates the provision for readily C iir>r1ip the VPQtVlp 

 enlarging or contract mg the tube. SUppllCS IllC VCSlClC, 



dilates to form a ganglion- 

 cell, and then tapers to its termination in the wall 

 of the vesicle. These vesicles are considered by 

 Grobben to be auditory, but no adequate reason is 

 assigned. 



At the root of the long tail there is on each side a 

 long and slender appendage, which contains a rela- 

 tively large trachea. This is no doubt, as Grobben 

 says, a tracheal gill. A more important respiratory 

 organ is the long and retractile tail itself, which can, 



1 Sitz. d. k. Akad. d. wiss. Wien. 1875. 



