J. W. H. Harrison 271 



silvery white central area provides the male, then that pattern is dominant, 

 and the insect obtained differs in no respect from autumnata save in its 

 cool grey ground. Similarly certain of the other autumnata patterns, 

 for instance that of the distinctly barred sandbergi, show like tendencies 

 but their dominance is never complete. 



In the female the fluctuating variation is likewise great, although, 

 as I have indicated previously, but rarely would this sex be deemed 

 other than pure dilutata whether they were melanic or non-melanic — a 

 fact that would not prevent the eye of the trained observer from de- 

 tecting differences in many cases. 



The indications of other structural characters are quite in harmony 

 with those of the wing markings. The male antennae are absolutely 

 intermediate between the finely jointed antennae of autumnata and the 

 much coarser ones of dilutata, and the genitalia simply continue the tale. 

 The hook on the valve is developed but remains quite small, and the 

 head of the labides is intermediate in size. So, too, in the female the 

 signa of the bursa copulatrix (Figs. 1, 2, 3, 4) on the whole favour 

 dilutata, but some hint of autumnata may be gleaned from the sculp- 

 turing of the upper one. 



In making the necessary dissections for the study of the female 

 genitalia a very important feature was revealed ; that the ovaries of the 

 insect were lacking. And herein lies the explanation of the precocious 

 appearance of the females, the two factors of emergence and completed 

 gametogenesis being so correlated that the female leaves the pupa soon 

 after the operation of oogenesis is finished. No oogenesis being possible 

 with this hybrid, the insect develops and emerges at once. That delayed 

 gametogenesis accompanies delayed emergence I can readily prove. In 

 1917 I had two lots of dilutata pupae, one of which had contract.ed a 

 bacterial disease during the last larval skin which in many cases did not 

 prove fatal ; the other lot was not infected. Both lots were freely drawn 

 on for material for cy tological work, on the assumption that larvae which 

 had successfully pupated could not be diseased. When microscopical 

 preparations of the gonads were made it was found that in the healthy 

 lot the maturation divisions were completed by the first week in 

 September, whilst in the other the process had barely commenced at 

 the end of the month. Then every germ cell, from oogonium to ovum, 

 from spermatogonium to spermatozoon, was crowded with bacteria. 



During that season the first wild imago of dilutata was seen in the 

 woods on September 24th, and for practical purposes the species was over 

 by October 13th, when on my searching for wild females for stock only 



