KN()\vi.i;i)r.i':. 



Siii'TKMniik, 1912. 



Normal Scale of Tliermobia 

 domestica X 400. 



I'iCLKii 373. 

 Scale shewing both radial 

 and longitudinal tubes X 400. 



sitcclutrina these longitudinal striae were formerly supposed 

 to consist of ribs united by a membrane, but tlun in im doubl 



(or "exclamation"! markings then disappear, and in 

 .some cases, by partial immersion of the scale in the 

 moimtant. both sets of striae may be clearly seen (see 

 Figure 3731. The so-called secondary markings are 

 apparently caused by two factors : one, minute images 

 formed in the interstices between the two sets of crossing 

 striae, and two, images formed by the actual points of 

 contact of the crossing tubes. By crossing two scales 

 at an angle a complicated system of interrupted markings 

 and second.iry structure may be observed (see Figure 374). 

 In T. domestica the obli(|ne striae are next the insect 

 and the longitudinal ones outside. The scale is slightly 

 concave-convex in shape, as if fitted to the curvature of 

 the insect's body, this structure being most marked at 

 the margin where the cuneate markings appear. The 

 latter are mere optical appearances, and are produced 

 by oblitiue illumination of the two sets of crossing striae 

 or tubes. The oblique illumination is caused by the 

 curvature of the marginal portion of the scale, and is 

 independent of the source or manner of the illuminant 

 to a great extent. 



A careful examination of the interrupted markings with 



the one-tenth inch dry objective, the scales being in 



optical contact with the cover-glass, shewed that the 



markings changed their form with exceedingly slight alteration 



if till fdciising. By slow downward focusing, four distinct 





Figure 375. 



First stage in the evolution of the scale 



markings — " connected beads." 



The etiect of crossing two 

 scales. 



Figure 376. 



Second stage — " separate elongated 

 beads." 



whatever that they are really a series of hollow tubes, the 

 juxtaposition of two neighbouring tube-walls producing tlie 



effect of a rib. This is best seen 



at the pedicle end of the scale, 

 where the tubes are closed by 

 rounded ends. Towards the 

 pedicle, and radiating from the 

 latter, signs of another obli(|ue 

 set of striae may be observed in 

 the normal scale of T. domestica. 

 These oblique striae were the 

 apparent cause of the interrupted 

 appearance of the longitudinal 

 striae at the margin of the scale. 

 That these oblique striae really 

 cover the whole of one side of 

 the scale, was proved by mounting 

 the latter in the gummy residue 

 obtained by the distillation of 

 commercial oil of turpentine. This 

 method of mounting shows up the 

 structures of both sides of the 

 scale without optical interference 

 of one set of markings with the 

 other. The marginal cuneate 



stages in the evolution of the markings were obser\'ed, as 

 represented by the accompanying drawings: 1, connected 



Third stage- 



" exclamation or clubbed 

 markings." 



F^iGURi; 378. 



Fourth stage — " cuneate or wedge-shaped 



markings." 



