Scales from Thermobia domcstica {Packard). 719 



a projection, which is evidently the point of attachment. Between 

 the two sets of striae there is a thin central membrane. The scales 

 appear to contain some highly refractive substance, and the radial 

 stria? start from the pedicle in such a manner as to suggest a sys- 

 tem of capillary tubes supplied with fluid through the point of 

 attachment. 



On the longitudinal tubes and between the broken markings a 

 minute secondary structure may be observed in the form of little 

 oblique streaks and minute circles. These are due to an effect 

 produced by the light passing through the points of contact of the 

 two sets of crossing striae. The same structure may be observed, 

 on a larger scale, by crossing two scales at right angles to each other, 

 in which case it is produced by the crossing of the longitudinal 

 tubes. A similar structure of minute circular markings was pro- 

 duced on a much larger scale by crossing two sets of fine glass 

 capillary tubes, filled with water and moistened at their points of 

 contact. The form of the secondary structure of the scales is modi- 

 fied by the method of illumination. 



Cause of the Beaded and Cuneate Markings. — That the broken 

 markings are due to the crossing of the two sets of tubes or striae 

 is quite evident under a high power (yL in.). Towards the margin 

 of the free end of the scale by careful and very slow focusing 

 through the former the evolution of the cuneate markings may be 

 observed. First, there is a bead on the longitudinal tube- walls 

 where the radial capillaries cross. This bead lengthens out and 

 becomes twisted obliquely. Then one end of the twisted bead 

 widens out, thus producing an " exclamation " marking. Finally, 

 the wide end of the latter opens out, leaving a cuneate shape. 

 That the radial striae really pass all over the scale is proved by 

 obliterating the longitudinal striae. This was done by Bryan (loc. 

 tit.) by placing a grease-spot on the scale. Where the grease-spot 

 was applied the longitudinal striae disappeared, leaving the radial 

 striae alone visible. A much better method of accomplishing this 

 end was discovered by the writer after some experimenting. A slip 

 is ringed with shellac cement and the ring filled with turpentine. 

 Commercial turpentine, which leaves an oily or resinous residue 

 when evaporated, must be used. The turpentine is allowed to 

 evaporate in a warm room and the scales are placed on the oily or 

 gummy residue. The radial striae then become very clear, while 

 the longitudinal ones to a great extent disappear. It is probable 

 that the application of this method will shed light on the structure 

 of other scales, such as Podura. In the case of T. domestica, in my 

 opinion, two factors are at work in producing the optical effect of 

 the cuneate markings. These are (1) light passing through the 

 two sets of striae ; (2) the slightly concavo-convex shape of the 

 scale, which causes the light to pass through obliquely at the mar- 

 ginal area of the scales. 



3 a -2 



