202 ME. E. B. POULTON ON THE EXTERNAL 



selected from very favourable individuals, pi-ove that both male and female external 

 generative organs of Rhopalocera are essentially similar to those of the Heterocera. 



A still more typical example is afforded by the pupa of Piet^ls brassica?, of which the 

 male is shown in woodcut 12, x 7, and the female, from two points of view, in woodcuts 

 13 and 14, x 7. 



The constancy and distinctness of the median prolongation of the tenth abdominal 

 and the relation of its apex to one of the generative apertures indicates that it possesses 

 some important morphological significance. Its shape suggests that it may represent an 

 ancestral ovipositor formed as an anterior ventral extension of the tenth abdominal, 

 and now fused to the pupa in its position of rest. Just as the male intromittent organ 

 seems to be now only represented by the cuticle of that part of it which appeared on 

 the surface when it was withdrawn, so the ancestral ovipositor is only represented by its 

 external cuticular layer. The slight shifting of the generative opening, shown in wood- 

 cut 10, does not appear to disprove this hypothesis. 



This hypothesis also explains the fact tliat there is a separate opening into the bursa 

 cojnUatrix. Copulation would be almost impossible if the female aperture were placed 

 on the apex of a conical process ; hence the necessity for a more accessible aperture. This 

 arrangement having been once set up, would be retained after the disappearance of the 

 necessity under which it originally arose, because of the many co-adaptations which 

 woidd have been entailed in both sexes. 



DESCRIPTION OF PLATES XX. & XXI. 



Representing the characters of the terminal abdominal segments and the external reproductive organs 

 of Lepidopterous pupae. 



(The abdominal segments are indicated by Roman numerals, the anus by A, the terminal spine by Sp. 

 Other structural features are described with reference to their segments, and will be identified easily.) 



Plate XX. 



All the figures on this Plate represent the pupae of Heterocera. 



Figs. 1-7. The terminal abdominal segments of the pupa of Smerinthus ocellatus, showing the form of 



the external reproductive organs. 



Fig. 1. X 7. The last three abdominal segments of a female pupa, seen from the ventral aspect. These 

 segments, being the 8th, 9th, and 10th abdominal, are indicated by the numbers viii., ix., x. 

 The number viii. is placed close to the last spiracle, rudimentary in the pupa, although 

 functional in the larva. The rough terminal spine (Sp.) forms the extremity of the pupa; 

 immediately in front of it (in the ventral line) is the anus (A), which is very distinctly indicated. 

 The boundary between the 9th and 10th segments is prolonged forwards in the ventral line. 



