302 



THE DEVELOPMENT OF ARTICULATA. 



spots, (o,) there is a beginning of the formation of the heart, by a 

 hollowing of a longitudinal furrow (/t, A 1 ) along the middle of 

 the back, and in the midst of the remains of the yolk mass (y}. 

 By one more step we come upon that condition of things in 

 which the proportions of the embryo are nearly the same as 

 those which it has when hatched, and commencing the first of 

 its three phases of self-sustaining life, i. e., its worm state. Al- 

 though the body has completed its unrolling, the throat (th) be- 

 come a distinctly marked channel, and the furrow (/<) in which 

 the heart forms is nearly or completely covered over, nothing has 

 been added which renders the embryo any more insect-like than in 

 the last stage ; but one or two characters have become developed 

 by which its relations to the Caddice-worms may be inferred. 



The principal of these is exemplified 

 in the forked tail, (fig. 203, d 10 ,) upon 

 the prongs of which are eventually 

 developed a hook or claw by means 

 of which the larva clings to the inside 

 of its tubular, aquatic tenement. The 

 other feature is sot forth in the very 

 weak, unjaw-like jaws, (6, c,) which, 

 even in the adult state of the Caddice- 

 fly are scarcely more than feeble organs of prehension, and are 

 called by that name because they occupy the same position as 

 the genuine masticatory organs of other insects, rather than on 

 account of the office which they perform. 



When hatched which happens in the water the larva 

 constructs about itself a cylindrical tube of bits of sticks, which 

 it glues together with a sort of glairy substance similar to that 

 which silk'Worms produce to construct their cocoons. After re- 

 siding for a time in their portable domiciles, and becoming fully 



Fig. 203. The same as figs. 198 to 202, just before the period of hatching. 

 220 diam. t, the head; o, the eye; tf, the feeler; b, the upper jaw; c, the lower 

 jaw ; gn, the lower lip; e, f, g, the legs; r/ 10 , the last joint of the abdomen ; th, 

 the throat; y, the remnant of the yolk; 7i, the furrow in which the heart is de- 

 veloped. From ZatJihicli. 



c yn <Z 10 



Fig. 203. 



