222 



THE MICROSCOPE AND ITS REVELATIONS. 



and in discussing it, regard should be had to what we are taught by the 

 study of the larger and more strongly marked forms of Insect-scales, as 

 to what scales are. That they are in reality flattened cells, analogous to 

 the Epidermic cells of higher animals ( 671), can scarcely be doubted 

 by any Physiologist. Their ordinary flattening is simply the result of 

 their drying up; and the exception presented by the * plumules' and 

 1 battledore' scales (Fig. 416), which have the two surfaces separated by 

 a considerable cavity, helps to prove the rule. It is perfectly clear in 

 some of these, that the membranous wall of the cell is strengthened by 

 longitudinal ribs, which diverge from the peduncle; as is particularly 

 well seen in the plumules of two West African butterflies, Pieris Aga- 

 tliina and Pieris Phloris, in which the plumules are as much as l-300th 

 of an inch in length (large enough to be studied under the Binocular 

 Microscope), and are of cylindrical form, save that they are drawn in as 



FIG. 414. 



FIG. 415. 



FIG. 416. 



Scale of Morpho Menelaus. 



Scales of Pelyommatus argus (Azure- Battledore Scale of 

 blue); a, battledore-scale; b, inerfer- Polyommatus argus 

 ence striae. (Azure-blue). 



if by a cord at about one-half or one-third of their length, the ribs 

 curving inwards to this constriction. 1 In ordinary scales we find similar 

 ribs, sometimes running parallel to each other, or nearly so (Figs. 414, 

 415), and occasionally connected by distinct cross-bars (Fig. 418), but 

 sometimes diverging from the ' quill; ' and where, as in Lepisma (Fig. 

 417), the ribs are parallel on one surface and divergent on the other, a 

 very curious set of appearances is presented by their optical intersection, 

 which throws considerable light on the meaning of the Podura-msn- 

 ings. 



621. The easier test-scales are furnished by the order Lepidoptera 

 (Butterflies and Moths); and among the most beautiful of these, both 

 for color and for regularity of marking, are those of the Morpho Mene- 

 laus (Fig. 414). These are of a rich blue tint, and exhibit strong longi- 



1 See Watson, loc. cit., p. 75. 



