46 COLORATION IN POUSTRS. 



In the appendages, again, the position of the pigmented areas is inva- 

 riable, though there are differences in the extent of these areas. They 

 always mark, however, the attachment of the leg muscles. 



Microscopic examination of the chitin shows it to be striated in two 

 directions and perforated at intervals with openings through which 

 pass spines. These latter have protoplasmic cores which communicate 

 directly with the nerves of the insect ; they thus probably function as 

 tactile organs. 



The transverse striations represent the various stages of growth of 

 the chitin. This is generally supposed to be an exudation from the 

 hypodermal cells, and to develop by successive depositions on the out- 

 side of the chitin already there (see Bibliography, i6 and 23). These 

 transverse striations are usually more or less wavy and suggest a flow 

 of material which is in a somewhat plastic condition. The perpendic- 

 ular strictions which sometimes appear in the cross- sections of dry chitin 

 are thought to represent minute perforations in the chitin, which in the 

 adult are empty or filled with air. Their significance has been hitherto 

 unknown, unless they aid in the interchange of gases in the insect. The 

 following description of their relations to other structures in a pupa of 

 P. variatus, together with fig. 5, may suggest something further con- 

 cerning the functions of these perforations. 



This section is through the anterior abdominal segment, whose sur- 

 face has already attained a considerable degree of pigmentation. The 

 chitin is thick , and shows very decided transverse striations. The hypo- 

 > dermal layer is several cells in thickness, but interposed between it and 

 the chitin is a structureless zone, which nevertheless stains deeply in 

 hsematoxylin. This deeply stained substance may be traced along the 

 numerous exceedingly fine perpendicular striations to within a short 

 distance of the deeply pigmented surface. From this it will appear that 

 these striations do in reality represent very numerous fine pores, through 

 which is exuded a substance which has been in relation with, and per- 

 haps elaborated by means of the hypodermal cells. 



Fig. 6 represents a section through the mesothorax of the pupa. 

 From this it will be seen that the pigmented chitin presents a coarsely 

 granular appearance, which gradually disappears as we pass toward the 

 less deeply pigmented portion at the right of the section. In propor- 

 tion as the granular character disappears, a striated condition takes its 

 place. Again, the original granular condition of the pigment is largely 

 lost in the adult chitin, although it is here plainly indicated by the 

 irregularity of the surface and striations. 



I interpret these appearances in the following manner : The pig- 

 mented chitin is produced at various points of the surface in the form 



