32 



Note on the Pupation of Aglais urticae and Vanessa io. 



(Plate II.) 



By E. J. BuNNETT, M.A., F.E.^.—Read Jannanj 8tli, 1920. 



In the summer of 1901 I came across a single brood of larvje of 

 1'. io, about five days old, on the summit of a nettle. 



Of this brood I bred through 124 imagines, making notes and 

 photographs during the successive stages. The larvae being all of 

 the same brood, their transformations vi^ere nearly contemporaneous. 

 My notes on this occasion state that, wheij it is all but free from its 

 larval skin, " the pupa hangs on to the larval slough merely by 

 pinching it between its own posterior segments." A few yeai's later 

 I was able to supplement the notes referred to by an observation on 

 the final act in the pupation of Af/lois nrticce. 



At the anal extremity of the pupa of the Nymphalids ma\' be seen 

 a pair of finger-like organs, having their ends more or less curved 

 towards one another. fPl. ii., Figs. 8, 4). 



In the empty pupa case, or in a live pupa shortly after pupation 

 is complete, these " fingers " lie flat upon, and, to some extent, 

 embedded in the last one or two anal segments. 



But in the act of pupation, and in the supremel}^ critical moment 

 when the chrysalis body is dependent from the larval skin, these 

 finger-like processes are free and can be erected or lowered at will. 



They act as a kind of ratchet. The creature raises the " fingers," 

 thrusts them into or upon the crumpled larval integument, and 

 then the act of closing them down again is sufficient to raise the 

 chrysalis body. 



A short succession of such movements produces the desired effect 

 of enabling the anal extremity (cremaster) of the chrysalis to reach 

 the silken mat on the leaf-stem, or other support, to which the 

 anal claspers of the larva are already hooked. 



The mystery of the complete withdrawal of the pupal body from 

 the suspended larval envelope is thus explained. The fact of the 

 complete withdrawal is demonstrated in two of the subjoined 

 photographs (of io pupa) (PL ii., figs. 1-2.), which were taken by 

 nie in 1901, as well as by frequent observations of the phenomenal 

 manoeuvre, but it was not, as I have said, until later that I 

 witnessed the use made of the external processes. 



The two actions, the pinching of the larval skin between the 

 pupal segments and the ratchet motion of the finger-like processes, 

 are performed simultaneously. 



This final act of the process of pupation evidently requires 

 prodigious effort and the straining of every muscle in the pupal 



