658 



INSECTS ! SCALE OF PODFRA. 



Fig. 336. 



found amidst the sawdust of •wine-cellars, in garden tool-houses, 

 or near decaying wood, 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 dif- 

 ferent degrees of strength 

 of marking (Fig. 336, a, b), 

 and are therefore by no 

 means of uniform value as 

 tests. The general appear- 

 ance of their surface, under 

 a power not sufficient to 

 resolve their marking, is 

 that of Watered Silk, light 

 and dark bands passing 

 across with wavy irregu- 

 larity ; but a well -corrected 

 Objective of very moderate 

 Angular Aperture now suf- 

 fices to resolve every dark 

 band into a row of short 

 lines, each of these being 

 thick at one end and 

 coming to a point at the 

 other, so that the impres- 

 sion conveyed is that of a 

 set of Spines projecting 

 obliquely from the flat sur- 

 face of the scale, like 

 the teeth of a 'Hackle.' 

 Under a well-corrected 1 - 8th 

 inch Objective, the appear- 

 ance of the markings by transmitted light is that which is repre- 

 sented in Plate n. , fig. 2 ; when, however, they are illuminated by 

 oblique light from above (the Scales being placed under the objective 

 without any cover, so as to avoid the loss of light by reflection 

 from its surface), the appearances presented are those shown in 

 fig. 4 when the markings are at right angles to the direction of 

 the light, and in fig. 5 when they lie in the same direction as 

 the light with their narrow ends pointing to it. When this last 

 direction is reversed, the light from the points is so slight that 

 the scales appear to have lost their markings altogether. When 

 moisture insinuates itself between the Scale and the Covering-glass, 

 the markings disappear entirely, as shown in fig. 3. All these 

 phenomena seem best explained upon the supposition that the 

 markings of the Podura-scale (between which, notwithstanding 

 their transverse sinuosity, a longitudinal continuity may be traced) 



Scales of Podura plumbea : — a, large 

 strongly-marked scale; b, small scale, 

 more faintly -marked. 



