96 



On Scale Evolution, as shown in Ithomia diasia. 



By J. E. Ingpen, F.R.M.S. 



{Read May Vjth, 1895.) 



Plate IV. 



Variation of form in insect scales, though not considered 

 by systematic entomologists as of much importance, has always 

 been a subject of interest to microscopists, and the question of the 

 line of demarcation (if there be one) between scales and hairs has 

 frequently been discussed. The description of a specimen showing 

 an unusual number of gradations of form may therefore be not 

 altogether useless or uninteresting. 



While hunting for illustrations of iridescence, I came across 

 some specimens of Ithomia diasia, a clear-winged Columbian 

 butterfly. Though not a showy form, it is very delicate and 

 pretty, the iridescence of the clear parts of the wings being set off 

 by the black ribs and borders. On the underside there is a dull 

 orange and yellow margin to the hind wings, and there are a few 

 dusky white spots on both front and hind wings. On examining 

 the clear parts to ascertain how far the iridescence might be due 

 to plates, lines, dots, etc., I could not help noticing the hairs 

 thinly scattered over them. Some of these were single, others 

 forked, some white, most of them black, the white hairs becoming 

 transparent when mounted in Canada balsam, the black ones a 

 dark brown. These hairs were only present on the clear parts, the 

 ribs and borders being thickly covered with well-defined scales, but 

 close to the ribs and borders numerous transition forms between 

 the hairs and the scales occurred, and it is to the illustration and 

 description of these that I desire to call your attention. 



1 and 2. — Single and forked hairs, white and black. Apparently 

 -•tubular, as shown by the character of the air bubbles expelled in 

 tb£p. course of mounting. No appearance of interior granulation or 

 exterior marking. 



8. — The part near the fork a little wider and thicker, showing a 



