rs'SECTS : wings and their appendages. 87 



jTce ; tiie foot-stalk, projecting at a similar angle, is not 

 set on the inferior surface, but in the bottom of a 

 deep narrow incision ; and the ribs are invariably di- 

 vergent like those of a fan. In these, however, there is 

 a peculiarity of arrangement, which I have never seen 

 noticed, but which is obvious enough in the specimens 

 before us. The ribs on the two surfaces diverire 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 irreirular dashes. 



Such is the arrangement on these scales, which I 

 prepared myself from the comon Lejyisma y but I have 

 a slide marked " Lepisma," from one of the dealers in 

 microscopic objects, in which the ribs on the two 

 sides concur ; but, on one side, there are obliquely 

 divergent lines visible only near the margin, which 

 appear to be produced by wrinkles of the membrane 

 analoGTOUs to the transverse dashes on those of Ma- 



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 

 bt)dy, like a coiled spring. They are common in cellars, 

 in hot-bed frames, on dunghills, on the surface of water 

 in road-ruts, &c. On the slide before you are some of 

 the smaller scales from one of these insects ; they ai-e 

 exceedingly delicate, and the clearness with which you 



