268 INVEETEBEATA CHAP. 



second and third segments of the thorax there are pairs of similar 

 imaginal discs which later become infolded, and from the bottoms 

 of these discs, in the later larval life, small buds grow up which are 

 the rudiments of the wings. 



The larva when hatched is 1 mm. long ; it moults about 

 seven times and grows rapidly after each moult, so that when ready 

 to metamorphose it is 8-10 mm. long. It then stops eating, since the 

 epithelium of the mid-gut has already been cast off, and it moves more 

 and more slowly, for the muscles moving its limbs are beginning 

 to undergo destruction. Then it becomes bent double ; it remains 

 so for a period of twenty-four hours, and during that time the 

 muscles of the legs are completely destroyed, and the salivary glands 

 share the same fate. Finally, the cuticle is moulted and the larva 

 becomes a pupa, which in external form and proportions resembles 

 the adult beetle ; the antennae and the legs have reached the 

 definite proportions attained in the adult, but the wings are short 

 and motionless. Eight or nine days afterwards a second moult 

 occurs, and the perfect insect, or imago, emerges. 



The principal part of Poyarkoff "s investigations concern the histol- 

 ogical changes which occur during the metamorphosis. He established 

 the fact that, broadly speaking, the larval ectoderm becomes the adult 

 ectoderm, but that the nucleus of each cell expels part of its 

 chromatin, which passes to the base of the cell, surrounds itself with 

 a ball of cytoplasm, and is then cut off from the rest of the cell. 

 This ball lies between the bases of neighbouring ectoderm cells, and 

 while in this position is devoured by amoebocytes. 



In the case of the " tendon " ectoderm cells, in which a portion of 

 the cell is transformed into fibres to which larval muscles are 

 attached, this tendinous part is thrown off from the rest of the cell 

 and is then devoured by amoebocytes like the balls derived from the 

 other ectoderm cells. In some ectoderm cells, however, the nucleus 

 is absorbed in situ, and then the whole cell degenerates and is eaten 

 by amoebocytes. Many of the ectoderm cells are glandular; they 

 have narrow necks and enlarged basal portions with three nuclei, and 

 they persist in the pupa. 



When the larval cuticle is thrown off the basal part of the 

 pedicel of the gland cell grows and forms a connection with the cuticle 

 of the pupa. There are a series of gland cells situated in the neck fold 

 of the larva, between the head and the first segment of the thorax, 

 each of which has several ducts; these round themselves off into 

 balls, drop off, and are devoured by amoebocytes. Even those glands 

 which, as we have seen, persist into the pupa stage, undergo destruc- 

 tion in a similar fashion when the pupa moults in order to become 

 an imago. The glands of the adult are developed out of ordinary 

 ectoderm cells. 



The changes which occur in the alimentary canal are as follows. 

 The first thing to be noticed at the onset of metamorphoses is an 

 increase of the cells on both sides of the valve separating stomodaeum 



