226 E. A. HARTLEY Vol. XXII, No. 8 



in transmitted light. The corion is very thin and transparent; 

 showing the finely granular structure of the contents, which is 

 homogeneous throughout for a few hours after the egg is laid. 

 Soon development begins to appear by the characteristic form 

 of the growing embryo. At the end of twenty-four hours the 

 egg clears up slightly, except for certain parts of the embryo, 

 which show darker in the form of two blotches; the one in the 

 anterior portion of the egg having a long and wavy form which 

 bends around like a hook near the end of the egg; the other in 

 the posterior portion is more rounded. Both of these blotches 

 finally join together by the growth of the mesenteron, shown by 

 two faint lines. When the egg is forty-six to forty-eight hours 

 old, it becomes still more transparent, and only one elliptical 

 dark spot appears near the center. It is now almost ready to 

 hatch. (See Figs. 3, 4, and 5, Plate I.). 



Position in Host. — The egg floats loose in the hsemocoel of 

 the host, where it may be seen on dissection under the high- 

 power of a binocular microscope. It is quite large in size 

 compared with the size of the adult. 



Length of Stage. — The period of time spent in the egg stage 

 is very near three days (72 hours) for all the observations. 



Number Laid. — The only actual count made of the number 

 of eggs laid by a single individual in the course of its life, gave 

 507. (Fig. 1). Other partial counts would lead one to believe 

 that this is a good high average. The conditions under which 

 the eggs were deposited may be considered almost optimum, 

 since the parasite was closely confined with a number of par- 

 asite-free aphids of the most desirable stage. It is difficult to 

 make an estimate of what the total number would be under 

 more natural conditions out of doors, but it is probably much 

 less than the actual counts indicate for caged material. From 

 the number of blackened aphids obtained from the different 

 cages, one might estimate the total number parasitized by 

 one adult female to be around 200. 



Like many other parasitic Hymenoptera, there are only a 

 few mature eggs in the ovarian tubes at a time; about ten for 

 Aphelinus. The other eggs mature from day to day in such 

 numbers as required, which is probably regulated by some 

 external stimulus, like the presence of a large number of hosts. 



