64 WHEELER. [Vol. VI 1 1. 



during revolution. The only explanation I have to offer, will 

 be given in connection with a discussion of the movements of 

 the germ-band. In that connection the variations in the devel- 

 opment and amputation of the envelopes in the different groups 

 of insects may also be treated to greater advantage. 



2 The Yolk. 



To my knowledge, the quantity of yolk in the insect &g^ 

 has not been made the subject of comparative study. It has 

 long been vaguely stated (^nde Brauer, '69 and '70) that the eggs 

 of Ametabolous insects contain relatively more yolk than the 

 eggs of the Metabola. In other groups of animals (Crustacea, 

 Annelida, Mollusca, Vertebrata) Jt is often observed that ab- 

 sence of yolk is correlated with free larval development, while 

 in eggs provided with an abundance of yolk the larval stages are 

 either lacking or considerably modified. This same law obtains 

 also in the Hexapoda, though it can hardly be formulated so 

 concisely as in other groups of animals. And this is not sur- 

 prising when we stop to consider that, as regards complexity 

 of organization, the difference between the simplest insect 

 larvae, such as those of the Muscidae and their highly special- 

 ized imagines, is far from being as great as the differences 

 between the trochophore and the Annelid, or the Nauplius and 

 the crustacean. 



Beginning with the Orthoptera we find that the ^gg is pro- 

 vided with an abundance of yolk, — the germ-band when first 

 formed in most cases covers only a very small portion of its 

 surface, and when it reaches its maximum length before revolu- 

 tion is no longer than, and usually not so long as, the egg.^ 

 The period of embryonic development is greatly prolonged; 

 most of the species are monogoneutic and oviposit in the fall, 

 the larvae not hatching till the following spring or summer. 

 There is practically no metamorphosis. 



In the most highly metabolic insects (Muscidae) on the other 

 hand, the quantity of yolk is comparatively limited. The germ- 

 band before revolution is nearly double the length of the Qg%, 



1 To this rule G)yllotalpa seems to be a noteworthy exception. 



