76 A Method of Dissecting Podura Scales. 



side, or that next the body of the insect. In mounting the scales 

 the cover is laid on the back of the Podura, and those adhering to 

 the glass are drawn off, consequently the outside is next the cover. 

 Now in my specimen the keen knife-edge has first met the raised 

 ribs, and then undercut the membrane farther back. Had this 

 been severed first it must have covered the rib ends, which would 

 not project, but have lain backwards beneath. I shall be happy to 

 show this specimen to anyone interested in the question. 



None of the minute detached sections show any beads, for in the 

 Podura these are illusory. In the entire scale they may be made to 

 appear throughout more or less plainly by conjuring the illumina- 

 tion, particularly if the object-glass is set a little out of proper 

 adjustment, I have on several occasions stated, that a determina- 

 tion of structure in finely-marked transparent entire objects cannot 

 be entirely relied upon for accuracy, as the refraction and inter- 

 ference of one portion influences the adjoining parts. I have an 

 instance of this. Some gum-arabic has run under part of a Podura 

 scale. In the dry portion beyond this for the remaining distance 

 of the scale, the interference bands from the edge of the gum, 

 cause the whole of the ribs to appear as lines of beads, which follow 

 the exact curved outline given by the gum. 



These spurious beads, with a touch of the illumination, appear 

 with a plainness that might delight the eyes of the few that would 

 believe in their reality. 



My accident has shown how clean detached fragments of all sizes 

 and sections may be obtained. Dr. Pigott, in order to sustain his 

 theory, has selected that well-known impracticable object, a mashed 

 Podura scale, which has been jumbled into a bead-like mass or 

 rubbed between two surfaces, reminding me of a lettuce-leaf that 

 has been trodden upon. I consider this more like structure de- 

 stroyed than developed. In this question, beheving as I have 

 done, and in what others have shown before me, I have no pet 

 counter-theory of my own to support. Some respect must be paid 

 to reasoning by analogy from various scales, and the gradual tran- 

 sition of insect hairs into scales. It must be remembered how 

 systematically Mr. Mclntire has gone into the question. Also, in 

 the last Journal, we have a paper by Mr. Gr. W. Morehouse, " On 

 the Structure of the Lepisma Saccharina Scales." The investi- 

 gation has been made calmly and carefully, with the highest 

 powers practically useful, by one who well understands his work. 

 He arrives at the conclusion that both the small and big beads 

 (stated to constitute this object also) are spurious for a similar 

 reason. 



