CADDIS-FLIES. 91 



position to that shown in the figure. It is, however, 

 prevented from actually leaving the case by its abdomen, 

 which is too large to be withdrawn from either end. 



The larva has the head and thorax horny, and pale brown in colour ; 

 the two anterior legs are rather short, and apparently prehensible in 

 function ; the other two pairs are very long and slender, and employed 

 exclusively in walking. The abdomen is very large, soft, dark green in 

 colour, with the segments very deeply excised. At the extremity of the 

 body there are two large breathing orifices, surrounded by a number of 

 long bristles. Two convoluted tracheas are plainly visible extending up- 

 wards from these organs, and ramifying through the body of the insect 

 in all directions. 



This larva is very active, pulling its comparatively 

 large and heavy case through the green slime weed, 

 amongst which it always lives, and no doubt feeds upon. 

 When alarmed, the insect can entirely retreat into its 

 case, which is very roomy, and enables the occupant not 

 only to turn round and project its head and anterior 

 segments through the lower aperture, but also to move 

 about quite freely, whilst entirely within the case. 



The methods, followed by the larva in the construc- 

 tion and subsequent enlargement of its case, are not 

 known at present ; but there is no doubt that the case is 

 made of a viscous fluid, which hardens when exposed to 

 the water. This material is secreted by the larva, and is 

 analogous to silk. 



When about to change into a pupa, the insect fixes 

 its case down by four ligaments, two at each end, the 

 extremities of these being firmly fastened to a stone ; 

 the small aperture is then closed, and after this the larva 

 constructs an arch-shaped partition of dense material 

 inside the case, a short distance from its broad end. In 

 about a week the insect is transformed into a pupa, 

 having the limbs free from the body, but incapable 

 of motion. The fixing down of the case prior to the 

 change is easily performed by the larva through each of 

 the apertures, which are, for that purpose, left open until 

 the last. When mature, the pupa breaks through the 

 partition at the broad end of the case, and wriggles to the 

 surface, the imago ascending a blade of grass to dry 

 and expand its wings. The little skins of the pupa^ may 

 often be noticed floating on the water, and the empty 

 cases are very conspicuous on the sides of a glass 

 aquarium, where the insects generally fix them down 

 when in captivit}'. These fixed cases may often be 

 found in thousands attached to the stones in the streams. 



