144 PEOE. "W. H. JACKSON ON THE 



practical purposes without the labour of counting by the fact that its stigmata are 

 abortive as compared with the stigmata of preceding somites. The characters which 

 determine the sex are to be found on the ventral or sternal aspect of the ninth somite in 

 the male, and in the corresponding region of both the eighth and ninth somites in the 

 female. They were first recognized by myself in 1875, and I have used in succeeding 

 years whatever opportunities have presented themselves of verifying their truth not only 

 by the inspection of specimens but also by means of breeding and dissections. 



The male sex is indicated by a linear depression in the sternal region of the ninth 

 somite, a depression which represents the aperture of the ductus ejaculatorius of the 

 imago. It presents itself in one of three slightly differing shapes. 



(1) It is a fine line situated in a raised area and provided with two oval lips, one 

 right, the other left. This is the form which is seen in Pieris and is shown in PI. XV. 

 fig. 1. 



(2) It is a more strongly marked line inclosed in a nearly circular cup-shaped area 

 with edges strongly raised. The sternal region of the ninth somite is a smoother 

 continuation of the lateral portions of the somite. This form is the one seen in the 

 genus Vanessa (V. urticce, lo, Atalanta, polychloros) and is shown in PI. XV. figs. 4 

 and 5. In some specimens the edges of the inclosing area are more strongly pronounced 

 on the right and left, forming a couple of lips, and then the area has a more or less 

 oblong aspect. 



(3) It is a very well defined linear depression, guarded by two lips, one on the right, 

 the other on the left. These lips are tumid, broad at their centres, and pointed at either 

 extremity ; or, in other words, tbey are either oval or somewhat triangular, the bases of 

 the triangles being the linear depression itself. The whole structure lies either in the 

 centre of the sternal region or at its posterior limit. Examples are Papilio Machaon 

 (PI. XV. fig. S) and Sphinx lUjustri (PI. XV. fig. 11). The latter figure is taken from 

 a spirit-specimen of an individual killed immediately on pupation ; in older living 

 specimens the dark colour and the thickening of the pupal cuticle rather obscure the 

 structure. All the Heterocera which I have examined possess this third shape. 



The female sex is to be recognized by the aspect of the sternal regions of the eighth 

 and ninth somites. There are typically and primitively two linear depressions, one in 

 each of the sternal regions named, but they may be hidden or become confluent. These 

 depressions coincide respectively with two pairs of vesicles invaginated from the hypo- 

 dermis ; see infra, pp. 159-160. There are several types of structure to be distinguished. 



(1) The eighth and ninth sterna are each marked by a short fine longitudinal line, 

 surrounded by an oval and slightly raised area. This type is seen in Pieris and is 

 shown in PI. XV. fig. 2. 



(2) The eighth and ninth sterna are marked by a longitudinal line which is apparently 

 continuous. The sternal region of the ninth somite is, as in the male, a smoother 

 continuation of the lateral portions of the somite. This type is seen in the various 

 species of the genus Vanessa and is exemplified in PI. XV. figs. 6 and 7, taken from 

 V. Io. The essential difference between it and the preceding type depends on the 

 formation of a median furrow corresponding to the course of the azygos oviduct by 



