OF ARTS AND SCIENCES. 257 



would thus be explained. In fact the larger part of the patterns seem 

 to be produced by expansion or by distraction of the pattern, performed 

 in the bag at a period before transformation. It should have been 

 stated before, that the formation of patterns on the wings of Lepi- 

 doptera must take place at the time when the scales are still little 

 open bags. 



At first ray suggestion about the formation of the pattern in such a 

 manner may seem to be strange, and j)erliaps not admissible. But in 

 thinking over the subject again and again, I have found more and more 

 sup25ort for its adoption. I had been puzzled chiefly about the exist- 

 ence of obstacles in the streams of blood. The excellent paper of 

 C. Semper (Zeitschrift f. wissensch. Zool. vol. viii. pi. xv. f. 1) 

 contains a statement of the formation of a layer to close the wing- 

 bags at the base. This layer is formed by a number of cells united 

 gradually by some projections, and the whole may be compared to a 

 kind of cobweb with larger spots at certain intervals. I believe that 

 the presence of such a layer would explain very well the presence of 

 obstacles in the manner before stated. 



A striking proof of the appearance of dark color and dark lines 

 along an accelerated circulation is given by the dark coloration around 

 the nipple and along the linea alba in pregnant women. This dark 

 color is not formed by pigment cells, but by the common cells of the 

 mucosa. Around the nuclei of these cells is -found deposited fine and 

 homogeneous pigment. 



The colors of the pattern are dermal colors. They may, and in 

 fact do, often cover the whole insect. Leydig stated long ago that 

 the eye-spots of the caterpillars of Papilio are dermal colors. All 

 colors, the pattern excepted, are hypodermal colors. The dermal 

 colors are formed during the transformation, before the integument 

 becomes rigid and before the cuticula has finished to enclose safely 

 the colors. The hypodermal colors are formed either after this period 

 or as a main color in previous stages just after hatching, before any 

 pattern exists. 



I think this is the proper place to mention the interesting fact 

 called mimicry. In treating of mimicry there have been used indis- 

 criminately very different fjictors. I have before endeavored to give 

 some preliminary ideas about it, which are perhaps useful in explahiing 

 this curious fact. Of course I speak here only of mimicry in colors, 

 of colors imitating the surroundings, as, for instance, the excrements 

 of birds, dry leaves, berries, parts of trees, branches, bark, and what- 



VOL. XVII. (n. S. IX.) 17 



