978 



INSECTS AND AKACHNIDA 



microscopists on account of its beautiful shell-like sculpture. When 

 viewed under a low magnifying power it presents a beautiful 

 ' watered-silk ' appearance, which, with higher amplification, is found 

 to depend (as Mr. R. Beck first pointed out) 1 upon the intersection 

 of two sets of strife, representing the different structural arrange- 

 ments of its two superficial membranes. One of its surfaces (since 

 ascertained by Mr. Joseph Beck 2 to be the under or attached 

 surface of the scale) is raised, either by corrugation or thickening, 

 into a series of strongly marked longitudinal ribs, which run nearly 

 parallel from one end of the scale to the other, and are particularly 

 distinct at its margins arid at its free extremity ; whilst the other 



surface (the free or outer, according 

 to Mr. J. Beck) presents a set of less 

 definite corrugations, radiating from 

 the pedicle, where they are strongest, 

 towards the sides and free extremity 

 of the scale, and therefore crossing 

 the parallel ribs at angles more or 

 less acute (fig. 726). It was further 

 pointed out by Mr. R. Beck that the 

 intersection of these tw r o sets of cor- 

 rugations at different angles produces 

 most curious effects upon the appear- 

 ances which optically represent them. 

 For where the diverging ribs cross 

 the longitudinal ribs very obliquely, 

 as they do near the free extremity of 

 the scale, the longitudinal ribs seem 

 broken up into a series of ' excla- 

 mation markings,' like those of the 

 Podura ; but where the crossing is 

 transverse or nearly so, as at the 

 sides of the scale, an appearance is 

 presented as of successions of large 

 bright beads. The conclusion drawn 

 by the Messrs. Beck, that these in- 

 terrupted appearances are ' produced 



by two sets of uninterrupted lines on different surfaces,' has been 

 confirmed by the careful investigations of Mr. Moorhouse. 3 The 

 minute beaded structure observed by Dr. Royston-Pigott 4 alike in 

 the ribs and in the intervening spaces may now be certainly re- 

 garded as an optical effect of diffraction. In the scale of a type 

 nearly allied to Lepisma, the Machilis polypoda, the very distinct 

 ribbing (fig. 727) is produced by the corrugation of the under mem- 

 branous lamina alone, the upper or exposed lamina being smooth, 

 with the exception of slight undulations near the pedicle, and the 

 cross-markings being due to structure between the superposed 



FIG. 726. Scale of Lepisma 

 saccharina. 



1 The Achromatic Microscope, p. 50. 



2 See his appendix to Sir John Lubbock's Monograph. 



Monthly Microscopical Journal, vol. xi. 1874, p. 13, and vol. xviii. 1877, p. 31. 

 4 Ibid. vol. ix. 1873, p. 63. 



