ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 443 



opaque they may entirely conceal the surface on wliicli tliey are in- 

 serted, as the white hairs hide the bronze surface of the sides of the 

 thorax in Cicindela dorsalis, and as the white scales of Alaus oculatus 

 hide the black surface beneath the rings on the thorax ; or they may 

 only partly conceal the surface of the insect, giving rise to coarser and 

 finer mixtures and shades of colour. Opaque scales, or hairs, of more 

 than one colour, may cause figuration, whether they imbricate as on the 

 wings of lepidoptera, or are separated as on Anthrenus scrophularice. 



The possibilities of varying effects of colour are many with ojjaque 

 scales and hairs, but with transparent ones, especially if they are 

 coloured, the effects of colour can be multiplied still further. With 

 hairs the effects are not so remarkable as with scales. The scale, by 

 its form, increases the number of layers of the surface of an insect 

 which are available for coloration purposes. Where the siirface of 

 an elytron had previously a cuticular and hypodermal layer, by the 

 addition of a scale of the simplest type there is an addition of two 

 cuticular and, theoretically at least, two hypodermal or sub-cuticular 

 layers ; in all six layers, without counting overlappings of imbricated 

 scales. Some of these surfaces may have pigments, strife, hairs, and 

 other appliances to produce colours, and other surfaces may have other 

 striae and contrivances to act on the colours produced. 



Next to the consideration of how the colour and presence of 

 scales and hairs affect the appearance of surfaces to which they 

 are attached is the not less interesting question of the causes of 

 coloration in scales themselves. But before considering the causes 

 of colour, properly speaking, the author deals with the causes which 

 produce silvery and milk-white appearances in scales and on insects. 

 Leydig was the first, in 1855, to call attention to the presence of air 

 between or beneath their chitin layers as a cause for certain silvery 

 spots and scales on insects. He speaks of air in the finer pore-canals 

 of Ixodes testudinis giving these canals a black appearance, but 

 causing the whitish grey colour of the skin. So, too, he mentions 

 silvery scales on a spider, Salticus, and glistening hairs on another spider, 

 Cluhione claustraria, which appendages owe their silvery whiteness to air 

 within them. Again he mentions hairs which contain air on spiders 

 of the genera Epeira and Theridium. 



Leydig accounted for silvery glistening scales and surfaces and 

 for milk-white coloration among insects, but he fails to account for 

 the difference between these two 'kinds of coloration. Of the white 

 scales of Pieris rapce and the silvery scales on the under side 

 of the posterior wings of Argynnis idalia, neither contain any 

 appreciable colouring matter, and both contain air; both, too, are 

 simply milk-white by transmitted light. The difference is that there 

 must be in the silvery scales a polished surface towards the observer. 

 Ground glass does not appear silvery, but what is the surface of the 

 smoothest polished plate of glass but finely ground glass ? Ground 

 glass differs from polished glass only in degree : in ground glass the 

 scratches are so coarse and so abundant as to turn most of the light- 

 waves into the glass again, where they are lost. In polished glass 

 the scratches are still present, but have become so small that even the 



