ATTAC1DES. 283 



the size of these organs in both sexes, before degeneration of the one 

 and concomitant development of the other commenced, or, considering 

 how very common it is among moths for the male antennae to be 

 more developed than those of the ? , only part of the sexual difference 

 between pupal antennae may date from the beginning of the rapid 

 degeneration and development which certain species have undergone 

 in the two sexes respectively ; but it is most likely that the general 

 and wide-spread difference between these imaginal organs in the two 

 sexes is due to precisely similar causes acting slowly and only up 

 to a certain point —due, in fact, to the competition among the males 

 being keener than that among the females," etc. (toe. cit., pp. 230 — 

 231). Chapman points out that, in his opinion, the explanation 

 seems rather to be that the male and female antennae tend to be 

 alike, especially in the pupa. Wherever sexual dimorphism (or, mutatis 

 mutandis, any dimorphism) occurs, it is as if we had two species mingled, 

 which are separately responding to the effects of natural selection, 

 and each tending to vary in a different direction, and yet unable to 

 avoid a certain amount of crossing, making each to a certain extent 

 follow the other. So if the male moth gets a large antenna by 

 selection, the female tends to do so by inheritance from the male. 

 Selection, however, keeps action on the female antenna to prevent its 

 further development, but does not act so strongly on the pupal 

 antenna, which is a quiescent and unimportant structure, and so 

 the pupal antenna outstrips the imaginal one " (Proc. Sth. Loud. Ent. 

 Soc, 1899, p. 2). Considerable difference in the sexual organs is 

 to be observed in the Attacid pupae, and Poulton figures and describes 

 (Ext. Morph. Lep. Pupa, p. 209, pi. xxi., figs. 15, 16) the terminal 

 segments of the $ pupa of Aglia tan, as follows : 



1. Fig'. 15x7. — The surface sculpture is represented ; tne generative openings 

 are unusually distinct and separate from each other. The anterior (bursa copulatrix) 

 occupies the entire breadth of the 8th abdominal ; its margin is very prominent, 

 and much resembles the appearance of the male organ. The posterior opening 

 (oviduct) similarly occupies the entire breadth of the 9th abdominal ; its margin 

 is not so distinct as that of the anterior opening. The median prolongation of 

 the 10th abdominal is short and broad.. The anus is placed on an oval convex area. 

 Behind this area the base of the terminal spine is separated from the anal part of 

 the 10th abdominal by a distinct furrow. The spine is rough and bristles with 

 irregularly twisted thread-like processes. Its ventral surface (seen in the figure) 

 is characterised by a large oval concavity, marked by concentric lines. 



' 2. Fig. 16 x 7. — The last four segments seen from the right side. The 

 functional spiracle on the 7th abdominal differs from the rudimentary one upon the 

 8th in its oblique position. All the visible functional spiracles are oblique like that 

 shown in the figure. The 1st abdominal * is the only concealed spiracle in the 

 pupa, for even the prothoracic is clearly exposed to view. The 10th abdominal is 

 distinctly divided into a dorsal and ventral part. The terminal spine is not, how- 

 ever, uninterruptedly continuous with the dorsal part, but is separated from the 

 latter by a furrow which extends dorsally from that which was shown in the last 

 figure (15) and surrounds the base of the spine. [This tendency towards the 

 separation of the terminal spine from the 10th abdominal is carried further in certain 

 Geometrids.] 



The Attacid imagines are, in many instances, the giant 

 lepidoptera of the world, and the various families have an almost 

 world-wide distribution, and considerable variation exists in many 

 species, e.g., Samia cecropia is said (Can. Ent., 1876, p. 166) to vary 



* Poulton says " thoracic " an evident lapsus calami. 



