341 



SELENIA ILLUSTRARIA (TETRALUNARIA) AND ITS 



BEOODS. 



By F. Merrifield, F.E.S. 



Mr. Garratt's experience, as given in your October number 

 [ante, p. 288), is not an uncommon one. Sometimes a whole 

 brood, hatched from the egg in May, will " go over " until 

 spring, more often only a few of them. When a brood " splits " 

 in this way, those which go over are always on the average much 

 larger than those which emerge in July. This is no doubt 

 because there is a similar difference in size between the natural 

 spring emergence and the natural summer emergence ; and the 

 larvae which go over belong physiologically to the spring form, 

 one of the attributes of which is largeness. Experiments tried 

 by me on a large scale have shown that the question to which of 

 the two forms — the large spring one or the small summer one — 

 tlie individual will belong is decided in a very early period of the 

 larval life, but not — at all events unalterably — earlier. I have 

 had, by forcing fpm the beginning of the oval stage, five 

 generations in the year, of the allied species S. hilunaria (illit- 

 naria), all of the summer form, but have not been able to thus 

 change the destination by forcing half-grown larvae or pupae. I 

 had some large broods this year of S. tetralimaria larvae, from 

 eggs laid by the summer emergence early in July, which were 

 beginning to pupate just before I went abroad on Aug. 16th, and 

 when I returned on Sept. 20th I found many more had pupated, 

 and by the beginning of October almost all had done so, giving 

 me nearly 170 pupae. Those which pupated before and during 

 my absence had been placed in a refrigerator, as I felt sure some 

 would otherwise emerge before my return. When nearly all had 

 pupated— about a fortnight since — I picked out twenty-four, 

 which I judged from their small size to belong to the summer- 

 emerging division, and forced twelve of them. All but two of 

 the largest of these emerged (except two which died) in less than 

 a week. These two largest, after a fortnight's forcing, are 

 lively, and show no signs of approaching emergence, so I have 

 added them to the majority, which will remain in pupa until 

 next spring unless I force them ; and this, my experience tells 

 me, it will be of no use to do until about January. It may be of 

 interest to mention that the individuals belonging to this major 

 division are of very large size, averaging in weight: males, '3112 

 gramme (nearly 5 grains) ; females, 'SOOO gramme (over 6 grains). 

 I have two other broods of practically the same weight ; the 

 heaviest individuals are : males, '3900 gramme ; females, *5300 

 gramme (about 8 grains). I have no doubt that these results are 

 owing to my having crossed individuals from Germany with 

 some from England, for they are more than 50 per cent, heavier 



BNTOM. — DECEMBER, 1901. 2 E 



