558 



INSECTS AND ARACHNID A. 



FIG. 281. 



tint, and exhibit strong longitudinal stride, which seem due to 

 ribbed elevations of the superficial colored layer. There is also 

 an appearance of transverse striation, which cannot be seen at 

 all with an inferior objective, becomes very decided with a good 

 objective of medium focus, but is found, when submitted to the 

 test of a high power and achromatic condenser, to depend upon 

 a sort of beaded subdivision of the longitudinal ribs, the trans- 

 verse stride that may be seen between these ribs, being apparently 

 produced by the beading of the ribs on the other surface of the 

 scale. The large scales of the Polyommatus argus (azure-blue 

 butterfly) resemble those of the Menelaus in form and structure, 

 but are more delicately marked. The same insect, however, fur- 

 nishes small scales, which are commonly termed the "battledoor" 

 scales, the resemblance which their form presents to that instru- 

 ment being usually much greater than in the specimen repre- 

 sented in Fig. 280 ; these scales, also, are marked by narrow 

 longitudinal ribbings, which at intervals expand into rounded or 

 oval elevations, that give to the scale a dotted appearance ; at 

 the lower part of the scale, however, these dots are wanting ; and 

 in the interval between the two portions, we observe a sort of 

 crescent, formed of minute pigment-granules, crossing the scale 

 transversely. The scales of the Pontia brassica (cabbage butterfly) 



and of the Hipparehia janira (mea- 

 dow-brown butterfly), have longitu- 

 dinal markings of a somewhat simi- 

 lar nature, but less sharply defined ; 

 these are further noticeable for the 

 brush-like appendage which each 

 scale bears at the end furthest from 

 its implantation. The Podura plum- 

 bea or " spring-tail" is a little wing- 

 less insect that is found amidst the 

 sawdust of many wine-cellars, espe- 

 cially such as are damp, leaping about 

 like a flea, by means of that peculiar 

 power of using its tail, from which 

 its name is derived. Its scales are 

 of different sizes and of different 

 degrees of strength of marking 

 (Fig. 281, A, B), and are therefore 

 by no means of uniform value as 

 tests. The general appearance of 

 their surface, under a power not suf- 

 ficient to resolve their marking, is 

 that of watered silk, light and dark 

 bands passing across with wavy ir- 

 regularity; but a well-corrected lens 

 of very moderate angular aperture, now suffices to resolve every 

 dark band into a row of short lines, each of these being thick at 



Scales of Podura plumbea : A, large 

 strongly marked scale; B, small scale, 

 more faintly marked ; c, portion of an 

 injured scale, showing the nature of 

 the markings. 



