OF ARTS AND SCIENCES. 237 



insects has lately been sliown by careful and convincing experiments 

 by Dorfmeister,* Weismann.t W. II. Edwards,^ and others. 



The remarkable influence of a wet or dry atmosphere on the colors 

 will be considered later. The interesting albinism and its counterpart, 

 melanism, have not yet been studied to such an extent as to allow of 

 decided conclusions. Albinism is much rarer among insects than 

 among birds ; but it is obvious that a satisfactory explanation of these 

 aberrations would permit conclusions about the nature and origin of 

 the colors. 



Besides chemical colors, there exist a very different kind, optical 

 colors, which we will consider first. 



Optical Colors. 



Optical colors, produced by the interference of light, are by no 

 means rare among insects, but they are solely optical phenomena. 

 Colors by the interference of light are produced in two different ways : 

 either by thin superposed lamelliP, or by many very fine lines or small 

 impressions in very near juxtaposition. 



There must be present at least two superposed lamellce to bring 

 forth colors by interference. There cannot be more than four layers 

 in the wings and scales, which show principally such colors in insects, 

 two external ones belonging to the cuticula, and two internal ones 

 belonging to the hypodermis. The naked wings of Diptera and Neu- 

 roptera often show beautiful interference colors. 



The scales of Entimus and other Curculionidje are well known 

 for their brilliancy, and it is interesting to remark that when dry 

 scales are examined with the microscope, many are found partly in- 

 jured, which give in different places different colors, according to the 

 number of layers which remained. The elytra of some Chrysomelina 

 and other beetles with iridescent colors orobably belong to the same 

 category. 



* G. Dorfmeister, Mittheil. des naturwiss. Ver. f. Steiermark, 1870, pp. .3-8; 

 Ueher den Einfluss der Tcniperatur bei der Erzeugung der Sohmetterlings 

 Varietiiten [Vaness^a AtaJanta), and ibid. 1864 (not seen by nie); Ueber die 

 Einwirkung verschiedener wahrend der Entwicklungsperiode angewendeter 

 Warmegrade auf die Fiirbung und Zeichnung der Schmetterlinge. 



t Ueber Saison-Dimorphismus der Scbiiietterlinge, 1875, vol. i. 



X Mr. D. W. Edwards' (Coalburgh, West Virginia) papers are published, 

 Canad. Entomol. 1875, vol. vii. p. 228; 1877, vol. ix. pp. 18, 203; vol. xiv. 

 1882, p. 21 ; Psyche, 1880, vol. iii. Nos. 69, 70; 1881, vol. iii. No. 83. 



