2 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



A detailed description of each species studied will be given, commencing, not 

 with the lower insect groups and spiders, but with the highest group studied, viz : 

 the Lepidoptera. 



General conclusions and comparisons will be reserved for the closing paragraphs 

 of this paper. 



LEPIDOPTERA. 



The Lepidoptera were represented in this study by Thyridopteryx ephemerai- 

 formis or the common bag worm. 



The observations made on the embryology of this insect owing chiefly to 

 abundance of material were more complete than those made on any other insect 

 studied by the writer. Consequently the description of its development is given 

 first that it may serve to fill out as far as possible the incomplete aconmts given of 

 the development of other orders of insects. The development of Thyridopteryx 

 from the earlier stage when all the important organs of the embryo had been formed 

 was carefully followed. The study of the embryology of this insect, if it has brought 

 to light nothing new, has, in the opinion of the writer at least, settled some important 

 points connected with the embryology of this group of insects. 



The manner in which Thryridopteryx, commonly known as the bag worm, lays 

 its eggs, has been recently described by Professor Riley. A female kept in confine- 

 ment left the cocoon after laying her eggs in it, crawled a short distance and died. 

 Professor Riley states that in a state of nature the female leaves the cocoon, falls 

 to the ground and dies. The unsegmented ovarian egg of Thridopteryx (Fig. I & I') 

 consists of an outer protoplasmic stratum shading off into the central yolk spherules 

 which are probably invested with protoplasmic ramifications extending from the 

 outer stratum. 



Referring to figure I, which represents a highly magnified portion of the section 

 represented by figure I, it will be seen that the egg is invested by a thick chorion 

 (CH in Fig). The chorion is perforated at the pole of the egg by a conspicuous 

 micropyle which is not represented in the figure. 



Outside the chorion the ovarian epithelium (OE in Fig.) is seen. It con- 

 sists of flattened cells with large oval granular nuclei. The granular protoplasmic 

 stratum of the egg (P, in Figs.) lies beneath the chorion and extends between the 



