648 



NOTE. 



Notes on the Markings of the Wing-scales of a certain Butterfly. 



By Dr. Alfred C. Stokes. 



Ix this Journal for August, 1895, Mr. Alfred Letherby, F.R.M.S., 

 published a paper entitled " Notes on the Podura Scale," in which 

 he says that this scale is formed of two membranes : " one a deli- 

 cate hyaline membrane from which the stalk extends ; and one a 

 denser (optically) brownish membrane superimposed upon the 

 other. The latter is perforated all over in the form known as 

 exclamation marks." And he presents photographs to substantiate 

 his contention. 



While every observer of the Podura scale has probably seen 

 the appearances that he pictures and describes, the explanation 

 seems to be original with Mr. Letherby. And from what I have 

 recently observed on the scales of another insect, if I may judge 

 from analogy, Mr. Letherby' s interpretation is correct. 



For a number of years a slide labelled " Butterfly's wing dust," 

 has been in my possession, but until recently it has not been 

 examined with any care. I have no knowledge as to the history 

 of the slide, nor the origin of the wing-scales. This is to be 

 regretted, as a further supply would be desirable, since the struc- 

 ture of the scales, while rather more complicated than that described 

 by Mr. Letherby, in regard to the Podura scale, is important when 

 considered in connection with his interpretation. 



These special wing-scales are formed of three distinct mem- 

 branes, of which the upper and the lower bear longitudinal ribs, 

 between which both membranes are distinctly, even conspicuously 

 perforated by minute apertures arranged in rows more or less hori- 

 zontal. Some of the scales in the preparation have the two 

 margins accidentally turned upward, so that both membranes may 

 be readily examined. Others, accidentally torn, show the postage- 

 stamp fracture with equal distinctness. 



The longitudinal ribs appear to be externally directed folds or 

 elevations of both membranes. Those on one surface pass around 

 the stem-bearing end of the scale, and are continuous with the 

 corresponding ribs on the other. The perforations cease at a con- 

 siderable distance from the posterior, or stem end, so that in that 

 region the membranes show no markings, except these ribs, and, 

 in certain instances, several oblique folds or wrinkles. 



The third membrane is intermediate between the two perfor- 



