124 NATURAL HISTORY OF AQUATIC INSECTS CH. 



Another pair of hook-bearing appendages is carried on 

 the last segment of the body, and is probably used for 

 holding on to the burrow. Caddis-worms too possess 

 a pair of hooked feet at the hinder end of the body, 

 and no doubt use them for the same purpose. The 

 last segment but one bears two pairs of long and 

 flexible tubes. These are filled with blood, which is 

 continually renewed by the pulsations of the heart, 

 and the exchange of gases necessary for respiration 

 is probably effected through the thin wall of the tube. 

 Four other smaller prominences on the last seg- 

 ment have a similar structure, and very likely are 

 used in the same way. The last segment also car- 

 ries on its upper surface two bunches of fine hairs 

 which are supplied by a pair of nerve-cells, and are 

 believed to be sensory in function. 



When the larva is concealed within its burrow and 

 apparently at rest, an undulatory up-and-down move- 

 ment of the body is kept up, which continually renews 

 the water. At such times the four tubes are probably 

 actively engaged in the exchange of gases. The 

 same movement is practised by other case-dwelling 

 aquatic larvae, such as Caddis-worms and the cater- 

 pillar of Paraponyx. 



The head, which is small -in proportion to the body, 

 bears a short pair of jointed antennae, and two pairs 

 of eye-spots, which are mere pigmented patches, 

 without lenses. As usual, a sort of flap, the labrum, 

 hangs down in front of the mouth. This is furnished 

 with a very elaborate set of teeth, hooks and spines, 

 some of which are probably used to guide the fila- 

 ments of silk as they issue from the salivary glands. 



