PLAXT-IiOUSE ALEVRODES. 89 



the yolk substance within the kimen of the gut. The 

 dorsal organ begins to break up, its cells disintegrate, their 

 substance being used for the nourishment of the embryo. 

 The body walls of the embryo grow out laterally and take 

 the place of the amnion as the covering of the dorsal surface. 

 After the dorsal wall of the enteron has closed, a pair of 

 diverticula are given off from it near the anterior end. These 

 increase rapidly in size until they are larger than the original 

 part of the enteron. 



Quite late in the embrvonic life of the insect there are formed 

 in the thoracic region of the embryo five pairs of imaginal discs, 

 three pairs on the ventral and two pairs on the dorsal side. 

 These discs arise as ingrowths of the hypodermis which soon 

 lose their connection with the outer layer and remain just below 

 the surface as closed pouches of ectoderm. The internal layer 

 of the pouches increases in thickness by the multiplication of the 

 cells, but the outer part remains a single celled layer. 



In the latter part of the embryonic development the changes 

 which take place are mostly confined to the readjustment of the 

 organs already present, which gives to the fully formed embryo 

 the proportions seen in the newly hatched larva. These changes 

 are particularly noticeable in the appendages. The larval integ- 

 ument is secreted by the hypodermis. There is an increase in 

 the size of the embryo so great that the Qgg is distended until it 

 is markedly larger than at the time it was deposited. 



On the thirteenth or fourteenth day after the egg is laid the 

 shell splits on the anterior end and the larva appears. The larva 

 when it first comes from the egg is rounded and compressed on 

 account of its having had to conform to the shape of the egg 

 shell. After it is free it soon flattens out and assumes the typical 

 larval form. The larva moves about for a short time after hatch- 

 ing and then settles down to its scale-like immovable existence. 

 After its period of moving is over the legs begin to atrophy 

 and by the time the pupal stage is reached have almost com- 

 pletelv disappeared. The mouth parts of the larva consist of a 

 number of piercing setae which are thrust into the tissues of the 

 leaf, where they remain during the larval stage. The tracheal 

 system shows peculiar adaptation to the mode of life which the 

 larva assumes. The body of the larva is in close contact with 

 the surface of the leaf, so that no air could reach it from that 



