326 E. H. STRICKLAND. 



pendicularly on rocks to which they are attached by a strong anal 

 adhesive disc, and kept in position by silken threads secreted by 

 the salivary glands. While thus anchored they spread out a pair 

 of cephalic fans which act as strainers and collect small particles of 

 food from the water. The head capsule is moulted independently 

 of the body cuticle and exposes a new capsule which is at first 

 white with a few dark spots on the vertex, but which rapidly 

 becomes uniformly darkened all over. The thorax bears un- 

 usually well defined and large histoblasts of the imaginal wings, 

 halteres and legs, and also on either side a histoblast of the 

 pupal respiratory filaments, which by turning black when the 

 larva is mature becomes very conspicuous at this stage of growth. 

 The larvae are infested by two parasites, namely a Mermis and a 

 Sporozoon, both of which live in the body cavity. 



The Mermis does not affect the larval development to any 

 extent, except by slightly increasing its size, but it inhibits the 

 development of the histoblasts to such an extent that pupation 

 becomes impossible. 



The embryo worms are probably caught by the cephalic fans 

 of the larvae and pass into the alimentary tract, through the 

 walls of which they" bore and live in the body cavity of the 

 host till the latter matures. They then rupture the abdominal 

 cuticle and pass into the water where they live a free life under 

 stones in the bed of the stream. The number of worms con- 

 tained by a single larva is usually only one, but as many as 

 twelve have been found. A single worm measures 3 cm., which 

 is about three times the length of tin- ho>l. In some streams 

 25 per cent, of the larvae were infested with this parasite. Para- 

 sitized larvae never pupate, bin are killed by the worms when they 

 escape. 



The retardation in the development of the histoblasts is the 

 opposite condition to that met with in prothetely which is usually 

 caused by keeping larvae at an abnormally high temeprature. 

 This probably results in an increased supply of the cnzyjnes 

 which cause these histoblasts to develop. The Mermis ap- 

 parently excretes some substance which lessens the supply or 

 action of these enzymes and leads to metathetely. 



The Sporozoon parasite occurs in several forms in different 



