168 Dr. T. A. Chapman on the 



In evagination the prop begins the eversion at its outer 

 margin ; phases in the process may be noted in various of 

 the Plates, as e. g. XXXII, fig. 1 ; XL, fig. 2 ; XLI, fig. 1 ; 

 XLVIII, fig. 2. 



Within the rein may usually be easily seen the cervix 

 (Stitz), passing backwards from the terminal plate to the 

 bursa copulatrix ; it is sometimes possible by traction basad 

 on the bursa to withdraw the rein within the prop, which 

 at the same time, of course, invaginates ; the opportunity 

 to do this often occurs in preparing the specimens. 



What are these eversible structures, the prop and the 

 rein ? What segments do they belong to, and what parts 

 of the segments are they ? I have been able to discover 

 only one detail in their structure that seems to throw any 

 light on this. The membranous surface of the basal 

 portion, the prop, is apparently simple and free from any 

 cutaneous structures such as hairs, skin-points, etc. The 

 rein, however, is regularly studded with points that have 

 all the appearance of abortive hairs or scales, i. e. they 

 are something more than mere skin-points, though if they 

 were the latter their significance would be the same. 



The membrane forming the tube of the rein must there- 

 fore be a portion of the sternite of the 8th abdominal 

 segment, that of the prop a portion of the membrane 

 between the 7th and 8th segments. The chitinous loop 

 in the prop is possibly not easy to explain on this hypo- 

 thesis, but its explanation on any other seems equally 

 difficult. 



The terminal chitinous plate containing the ostium is 

 surrounded by the hair-point-studded tube, i. e. by the 

 surface of the 8th abdominal sternite ; it must therefore 

 be situated in the sternite and not at either margin. 

 This is an interesting conclusion to arrive at, as the usual 

 evanescence of both plates of this segment, in practically 

 nearly (or quite) all other cases, leaves us without any 

 data on this point. 



It is a secondary, but useful, circumstance, that each 

 species of Plebeiid butterfly seems to have a distinctive 

 form for the small terminal plate. This appears in most 

 of the photographs presented herewith, though in some 

 cases the smallness of the differences, the occasional 

 variation by the plate not being exactly in the plane of 

 the picture, and the loss of definition in reproducing the 

 photographs, may prevent this being evident. 



