SOME NOTES ON SEXUAL DIMORPHISM. 141 



Some notes on sexual dimorphism observed in the scaling of 



Colias cdusa {irith plate). 



By E. M. PRIDEAUX. 



In his interesting account of Colias ediim in the January number 

 of Science (ioxsiii, there is one constant distinguishing sexual character 

 which Dr. Lang does not touch upon — which is hardly to be wondered 

 at considering its very fleeting nature in the butterfly at large, I refer 

 to the presence of a peculiar linear pale yellow scale, with which the en- 

 tire black margin of the S forewings is thickly sprinkled (pi. iv., fig. A) ; 

 conspicuous enough in bred specimens, but absent, or nearly so, in ex- 

 amples, otherwise perfect, which have flown in nature. The effect is very 

 striking, quite a greenish-black resulting in some instances, the differ- 

 ence in this respect from the hindwings of the same specimen (on the 

 margin of which these scales are absent) being very marked. The 

 presence of these scales in bred specimens only of g- 0. edum, made a 

 further investigation seem worth while, when it was found that they 

 differed entirely in shape and structure from the other yellow scales — 

 those of the ground colour and of the wing rays on the black border — 

 being unusually long and narrow, and possessing a peculiarly minute 

 and linear stalk of attachment. It is doubtless this latter feature which 

 causes the premature removal of these scales by the insect's earliest 

 activities, which may be quite insufficient to dislodge scales of the 

 ordinary types, which are less than half as long as these strap-shaped 

 ones, and possess the usually seen peg-like stalk, thickening from the 

 base upwards. These facts recall what has been noticed in some of the 

 "clear-wing" moths, the transparent patches having been found in 

 bred examples to be powdered with ill -attached scales, which the first 

 flight of the insect was sufficient to remove. The black portions of the 

 wings of the ? ( '. ediisa are entirely without these linear yellow scales. 



Some interesting features of comparison were here further in 

 evidence. On looking over a number of specimens of both sexes it is 

 soon recognised that the peculiar depth of velvety black, seen on the 

 borders of the ? , is never approached in intensity even by the hindwing 

 borders of the <? . It is found microscopically that two very distinct 

 forms of scale are responsible for this sexual difference, those of the $ 

 (and also those of the ^ discoidal spot) being of a familiar type, flat, 

 with numerous fine longitudinal striations, and with a crenated apical 

 margin, possessing from three to five rounded teeth, and, at the 

 narrowed base, two projecting lobes with the stalk lying between 

 (pi. iv., fig. B). The (j black borders, on the other laand, are 

 composed almost entirely of much smaller scales, bent convexly 

 towards the observer, and having a few (usually nine) very 

 prominent ridges, these ridges slightly projecting beyond the 

 apex, which, however, is always entire (pi. iv., fig. C). The 

 general shape is that of a " battledore," the base ending in a point 

 from which projects the stalk. These scales seem to be entirely 

 confined to the ^ sex, and I have not succeeded in finding any truly 

 intermediate forms between them and the other black scales. It 

 seems reasonable to suppose that their convexity and the prominent 

 ridges would cause a dispersal of light resulting in a general effect of 

 the somewhat rusty black referred to above. As mentioned, the black 

 discoidal spot of the ^ consists of the flat type first discussed, and a 

 few of the same are also found on the inner edge of the J black border, 

 May 15th, 1901. 



