Royal Microscopical Society. 9 



I believe the Thysanura give the most remarkable examples, 

 perhaps the only ones, where the change of skin takes place at a 

 time "when the scales are in all stages of development. From that 

 moment, I believe, the growth of the scales is arrested ; they dry 

 up, and, by forming a somewhat thick layer of elastic corrugated 

 plates over the delicate skin of their owner, provide a protective 

 covering admirably adapted to the wants of the fragile creature 

 they adorn. Their slender attachment to the skin is an additional 

 evidence of Creative design; for were they to remain attached in 

 spite of all the concussions the Podurae are subject to in their active 

 leaps, the tiny creatures must suffer endless contusions. 



Now the smallest scales, as I see them, appear to be mere filmy 

 sacs ; those a little larger give some evidence that the membranes 

 collapsed in drying, but they are so delicate that the wrinklings 

 which such a process would give rise to are very indistinct.* Larger 

 scales display the feature more and more strongly, and in the largest 

 of all we see those wrinklings have acquired importance enough to 

 occupy the patient attention and study of hundreds of clever ob- 

 servers. So much for the Podurid^. 



In the Lepismidse (Lepisma and Petrobius) we get yet more 

 information. The organization of these insects being of a higher 

 type, it is reasonable to expect to find a higher and more compli- 

 cated development in their scales; and this I thmk we do find. 

 As I see them, and in my former paper I have dwelt on this point 

 in detail, they form the link between the lower forms of scales and 

 those possessed by the Lepidoptera, Diptera, &c. 



The unintended length of this communication precludes my 

 touching upon Iridescence, a subject which may at some future 

 time be attempted, at least so far as bringing forward some curious 

 effects in iridescent scales. I will therefore sum up the principal 

 notions that the above experiments have led me to form, previous to 

 making a few comments on Dr. Pigott's paper. 



1. That the principal structural feature in insect scales is cor- 

 rugated membranes — a plan ensuring the maximum amount of 

 strength and elasticity with the minimum of weight. 



2. That there are a few scales having one surface hackled. 



3. That the ornamental requirements of scales are fulfilled 

 either by iridescence or the possession of pigment granules m or 

 upon the membranes. 



4. That the beaded appearances seen in scales are due to the 

 following causes, either singly or collectively : — 



a. Corrugations, taking the form of hemispherical embossings. 



fe. Pigments. 



c. Shadows of projections, or folds in the membranes, either 



within or beyond the focus of the object-glass. 

 * Of course there is a predisposition for wrinklings of a definite pattern. 



