INSECTS: WINGS AND THEIR APPENDAGES 95 



however, there is a peculiarity of arrangement, which IT 

 have never seen noticed, but which is obvious enough in 

 the specimens before us. The ribs on the two surfaces 

 diverge at a different angle, those of the upper surface 

 being the more divergent, divaricating from the foot stalk, 

 while those of the lower membrane are coarser, and much 

 more nearly parallel, their bases ranging along the hind 

 edge of the scale. The effect of the intersection of the 

 sets of lines at so acute an angle is to convey the optical 

 impression that the scale is covered with short irregular 

 dashes. 



Such is the arrangement on these scales, which I pre- 

 pared myself from the common Lepisma; but I have a 

 slide marked "Lepisma,'' from one of the dealers in mi- 

 croscopic objects, in which the ribs on the two sides con- 

 cur; but, on one side, there are obliquely divergent lines 

 visible only near the margin, which appear to be produced 

 by wrinkles of the membrane analogous to the transverse 

 dashes on those of Machilis. 



Scales much more delicate than either of these sorts 

 are found on the Podura, a minute insect of which there 

 are several species; which leap, jerking out the bristles of 

 the tail, that are ordinarily carried under the body, like a 

 coiled spring. They are common in cellars, in hot-bed 

 frames, on dunghills, on the surface of water, in road- 

 ruts, etc. On the slide before you are some of the smaller 

 scales from one of these insects; they are exceedingly deli- 

 cate, and the clearness with which you discern the char- 

 acter of their markings is a proof of the good definition 

 of my microscope; this is what is called a "test-object." 

 At first sight they seem covered with ribs like those of 

 the Machilis and Lepisma; but, by the use of a magnifying 



