290 MERISTIC VARIATION, [part i. 



upper and lower surfaces of the same wing, in the several eye-spots 

 along the margin of the same wing, or in all of these, as the case 

 may be. The evidence will shew that there is sometimes a close 

 correspondence between the variations of eye-spots in these several 

 positions. 



But though these are the matters with which we have now 

 the more direct concern it will be convenient to speak at the same 

 time more generally of eye-spots. It should be remembered first 

 that there are eye-spots of various complexity. In the simplest 

 all the bands are circular, having one centre ; the ocellus is then 

 as a rule complete in one cell of the wing, though sometimes the 

 outer zones of colour overspread parts of the adjacent cells. In 

 some cases the spot is double, having two centres, the bands being- 

 disposed round them in an hour-glass shape. As to the visible 

 structure of eye-spots it can be seen with the microscope that the 

 colour of the eye-spot lies in the colours of the scales. The scales are 

 arranged in parallel rows running (with little crossing or anasto- 

 mosing) as nearly as possible at right angles to the nearest nervures, 

 being disposed in regard to them much as the circular threads of a 

 cobweb are in regard to the radial threads. Across these rows of 

 scales run the colour-zones, in no way limited or guided by them. 

 On the other hand it can be seen that the patterns are almost 

 wholly made up by the colours of single scales, each having its 

 own colour, particoloured scales being exceptional. The effect 

 thus seen is very like that of a mosaic picture made of similar 

 pieces, or of a design worked in cross-stitch on canvas, all the 

 stitches being in rows and each stitch having its own colour. 



As regards the position of eye-spots it should be noticed that 

 the simpler sort, e.g. those of Morpho or of Satyridaa, are usually 

 placed in such a position that each of their centres is on the line of 

 one of the creases or fold-marks of the wing, and it sometimes 

 happens that these creases seem to begin from the centre of an 

 ocellus. From the fact that the creases for the most part run 

 evenly between two nervures, bisecting a cell, it commonly results 

 that the centre of the eye-spot is exactly halfway between two 

 nervures. The large spots on the hind wings of some Pieridas, e.g. 

 Parnassius apollo, are an exception to this rule. 



In that cell of the hind wing which lies between the submedian 

 and first median nervures in many ocellated forms (Satyridse, 

 Morpho, &c.) there are two creases, and it is especially interesting to 

 notice that in this cell there are commonly two ocelli, one on each 

 crease ; but if there is only one ocellus its centre does not corre- 

 spond with the middle of the cell but is nearer to the first median 

 nervure, being placed exactly on the anterior of the two creases. 

 In spite of the excessive variability of ocelli, in for instance Satyr- 

 idse, it appears that they are not formed in situations other than 

 these, being so far as I have seen always on one of the creases \ 

 1 These remarks refer to simple ocelli with one or more definite centres. 



