1879. J 151 



during the .critical last moments of pupation. And such is also the 

 opinion of Mr. "W. H. Edwards, as communicated to me by letter. 

 On September 9th he writes : " I do not believe that the rectum has 

 any especial bearing on pupation. All my observations indicate that 

 the "Osborne" membrane alone supports the pupa, and cutting this, the 

 pupa always falls ; cut the rectum, and no efBect is produced, so far as 

 I have seen." 



In the cases mentioned by Prof. Riley, when the dissection was 

 made " an hour or more after suspension," I can only suppose that the 

 interference was premature, and the intestinal canal not yet completely 

 disengaged ; but this is not the normal condition towards the end of 

 pupation. In the chrysalis of V. tirticce, the last three or four ab- 

 dominal segments are wedge-shaped, the thin ends of the wedges 

 lying close together on the venter near the knobs to which the ligament 

 is attached. The terminal or anal surface of the last segment has 

 lying on it the ridges which terminate in these knobs, and the tail- 

 piece (with the hooks), making an obtuse angle with the ridges, and 

 forming with them a sort of bent lever — (viewed sideways, not unlike 

 the open jaws of a serpent, having on its nose the hooks for attach- 

 ment to the silk). The extension and contraction of the abdominal seg- 

 ments, then, cause the tail-piece ("cremaster " of Prof. Eiley) to move 

 through the arc of a circle, whose centre is at the point of suspension 

 of the chrjrsalis from the ligament, which ligament itself is highly 

 elastic. Now, with this mechanism, it is not difficult to conceive how 

 the tail of the chrysalis may work its way out of the pocket of 

 shrivelled larva skin, stretching the elastic ligament as far as necessary, 

 and then be thrust up around that shrunk-up packet of old skin so as 

 to reach the silk. But if the exuviated rectum is to be considered as 

 the suspensory ligament, how is this to be accomplished ? In the first 

 place, so much of the rectum must be " paid out " of the anus as will 

 lower the pupa sufficiently to get its tail clear out of the larva skin. 

 But if it is drawn up again, it will be drawn up vertically in the same 

 straight line, and still impinge against the intervening skin. And by 

 what power is it to be drawn up ? For Prof. Riley's views on this 

 point, I must refer to the paper in " Psyche." "The principal means 

 by which the chrysalis holds on, and rises at the critical moment, is a 

 stout ligament, which is, virtually, the shed intestinal canal ; not alone 

 the lining, but the whole organ, which, as we know, becomes sub- 

 obsolete in the imago state of so many Lepidoptera. The ileum and 

 colon are more particularly serviceable, and the ligament holds with 

 such force around the anus of the cast larval skin, that it cannot well 



