INSECTS AND AKACHNIDA. 223 



tndinal striae, which seem due to ribbed elevations of one of the superfi- 

 cial layers. There is also an appearance of transverse striation, which 

 cannot be seen at all with an inferior objective, but becomes very decided 

 with a good objective of medium focus; and this is found, when sub- 

 mitted to the test of a high power and good illumination, to depend 

 upon the presence of transverse thickenings or corrugations (Fig. 414), 

 probably on the internal surface of one of the membranes. The large 

 scales of the Polyommatus argus (' azure-blue ' butterfly) resemble those 

 of the Menelaus in form and structure, but are more delicately marked 

 (Fig. 415)., Their ribs are more nearly parallel than those of the 

 Menelaus scale, and do not show the same transverse striation. When 

 one of these scales lies partly over another, the effect of the optical inter- 

 section of the two sets of ribs at an oblique angle is to produce a set of 

 interrupted striations (#), very much resembling those of the Podura- 

 scale. The same Butterfly furnishes smaller scales, which are commonly 

 termed the t battledore ' scales, from their resemblance in form to that 

 object (Fig. 415, a). These scales, which occur in the males of several 

 genera of the family Lyccenidm, and present a considerable variety of 

 shape, 1 are marked by narrow longitudinal ribbings, which at intervals 

 seem to expand into rounded or oval elevations that give to the scales a 

 dotted appearance (Fig. 416); at the lower part of the scale, however, 

 these dots are wanting. Dr. Anthony describes and figures them as 

 elevated bodies, somewhat resembling dumb-bells or shirt-studs, ranged 

 along the ribs, and standing out from the general surface. 2 Other good 

 observers, however, whilst recognizing the stud-like bodies described by 

 Dr. Anthony, regard them as not projecting from the external surface 

 of the scale, but as interposed between its two lamellae; 3 and this view 

 seems to the author to be more conformable than Dr. Anthony's to gen- 

 eral probability. 



622. The more difficult 'test-scales' are furnished by little wingless 

 insects ranked together by Latreille in the order Thysanura, but now 

 separated by Sir John Lubbock, 4 on account of important differences in 

 internal structure, into the two groups Collembola and true Tliysanura. 

 Of the former of these, the LepiamidcB constitute the typical family; and 

 the scale of the common Lepisma saccharina, or ( sugar-louse, 5 very early 

 attracted the attention of 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. E. Beck first pointed out)' upon the intersec- 

 tion of two sets of striae, representing the different structural arrange- 

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

 tained by Mr. Joseph Beck 7 to be the under or attached surface of the 



1 See Watson, loo. cit. 



2 ' The Markings on the Battledore Scales of some of the Lepidoptera,' in 

 " Monthly Microsc. Journal," Vol. vii. (1872), pp. 1, 250. 



3 See " Proceedings of the Microscopical Society," op. cit., p. 278. 



4 See his "Monograph of tha Collembola and Thysanura," published by the 

 Ray Society, 3872. 



6 This insect may be found in most old houses, frequenting damp warm cup- 

 boards, and especially such as contain sweets; it may be readily caught in a small 

 pill-box, which should have a few pin-holes in the lid; and if a drop of chloro- 

 form be put over the holes, the inmate will soon become insensible, and may be 

 then turned out upon a piece of clean paper, and some of its scales transferred to 

 -a slip of glass by simply pressing this gently on its body. 



6 "The Achromatic Microscope," p. 50. 



7 See his Appendix to Sir John Lubbock's " Monograph." 



