6 Transactions of the 



closely analogous, if not identical structures (I furnish a few ex- 

 amples of transitional forms wliicli might be multiplied ad libitum)* 

 — are fully developed before they are exposed to external influences 

 at all, — i. e. from the moment when they become external appen- 

 dages they do not grow. So that I cannot see the necessity, as a 

 rule, for any growing matter in their interior. There are some 

 hairs or spines on the legs of spiders, I know, which form sheaths 

 for new ones, but I do not refer to these : they are more than mere 

 hairs. Let anyone, by way of experiment, examine carefully the 

 hairs of caterpillars, or the more delicate hairs of spiders, and I 

 think he will find them to be dry membranous hollow tubes, often 

 of simple structure, and quite as often very complex. 



Believing as I do that the Podura beads, which are just now so 

 eagerly looked for by microscopists, partake very much of the nature 

 of the illusive appearances I have so far alluded to, I nevertheless 

 do not decry such appearances. Their cause resides in the scales 

 and not in our object-glasses ; in fact, I think the better our object- 

 glasses are, the better they will be shown, provided always that we 

 illuminate the scales in the manner required to throw up these 

 beaded shadows. We see them with the same mixture of delight 

 and incredulity that we applaud the performances of Pepper's Ghost. 



But there are other beaded appearances in scales, the causes of 

 which we can better understand. 



Let us take any Lepidopterous scale, possessing much colour and 

 opacity. Let us illuminate it with a very strong light, and bring 

 high enough amplification to bear upon it ; and we shall find that 

 the substance t which causes the opacity and colour of the scale is 

 granular, and for the most part of regular circular or eUiptical out- 

 line in its minute details. It seems to reside upon, or very near 

 the surface of the membrane, which it renders more or less opaque, 

 without, so far as I can discover, increasing its thickness. In de- 

 scribing the scale of Amathusia Horsejielclii some years ago in the 

 ' Microscopical Journal,' Mr. Warren De La Eue fixed his attention 

 more particularly upon this one feature of scale-structure, and gave 

 an account of it which is, to my mind, quite satisfactory. He con- 

 sidered these granules to be pigment, and spoke of them as such. 

 Since his paper is largely quoted in Quekett, and a copy of the 

 original drawing appears there too, I need only indicate where 

 interested inquirers may read his opinions. 



I think the term " pigment cells " which he uses, is a suitable 

 one to describe these almost omnipresent granules. They are of 

 such general occurrence, and oftentimes of such a pale tint, that I 

 would not venture to assert any scale to be ivholly without them, 

 though clearly in some scales they are very numerous, and in others 



* Scales from Cijphus Gennari, and scales from Inrva of Attagcims. 



t The note by Dr. Maddox appended may alibrd a clue to its chemical nature. 



