54 METAMORPHOSIS 



egg, is a definite crisis of paramount significance in the ontogeny 

 of all insects. The theory maintains that, in all Hemimetabola, 

 the three phases alluded to are passed through in the egg, and 

 eclosion takes place when the insect has assumed a post-oligopod 

 phase, termed the nymph. In the Holometabola, on the other 

 hand, the young insects (or larvce) invariably issue from the 

 egg at any earlier ontogenetic stage than the nymphs of the 

 Hemimetabola. The stage in organisation in which eclosion 

 takes place determines in a large measure both the form and 

 subsequent development of any given insect. The more advanced 

 the condition at the time of emergence, the lesser are growth 

 changes required for an insect to attain its ultimate organisation. 

 In the Holometabola, the stage in which eclosion takes place 

 corresponds more or less closely with one or other of the embryonic 

 phases alluded to. Some groups emerge as veritable embryos ; 

 others may be polypod larvae or oligopod forms ; or they may 

 be apodous and have lost all traces of trunk limbs. 



The factors which determine the moment of eclosion from 

 the egg are obscure. Even William Harvey, as long ago as 1651, 

 considered that premature birth in insects is due to paucity of 

 food reserves in the egg. There is little doubt that the main 

 influence is physiological, and depends to a considerable extent 

 upon the amount of yolk available in the egg, in proportion to 

 the size of the embryo. This aspect of embryology has, however, 

 been very little studied, and information respecting relative 

 amounts of yolk in the eggs of diff'erent insects, and its comparative 

 nutritive values, is a practically unexplored field. Precocious 

 emergence is also intimately related with the immediate environ- 

 ment of the newly-born larvae, and the egg-laying instincts of 

 the female parent. If, for example, the eggs are deposited in 

 situations providing the young larvae with an abundance of food 

 immediately at hand the early hatched, and more or less inert, 

 offspring would have chances of survival. On the other hand, 

 if the young larvae have to seek out their food survival would be 

 impossible, unless emergence be delayed until their development 

 had advanced to the stage where they had become endowed with 

 the necessary locomotory and sensory organs. 



