MORPHOLOGY OF THE LEPLDOPTEROUS PUPA. 247 



•which the imaginal sexual differences are even more strongly marked. There is an 

 immense difference between the imaginal antenna? (compare figs. 9 and 10), while the 

 corresponding pupal organs are not widely different (compare figs. 7 and 8). Although 

 the antenna? of the female imago are extremely degenerate, a careful examination reveals 

 traces of the structure which is so elaborately developed in the male (compare fig. 13 

 with figs. 11 and 12). 



Orgyia antlqua affords an interesting comparison with Saturnia and Acjlia, in that the 

 antenna? of the female imago are less degenerate, hut are nevertheless out of all propor- 

 tion to the broad antenna? of the female pupa (compare figs. 17 and 15). The male 

 antenna? form most highly complex and expanded sense-organs (fig. 16), while the 

 corresponding pupal antenna? are not much larger than those of the female pupa 

 (compare figs. 14 and 15). The details of the antenna? of both sexes are shown in 

 figs. 18 and 19. 



Again, the antenna? of the female Cerura vinula exhibit distinct pectination like that 

 of the male, but on a decidedly smaller scale ; while the pupal antenna? are more nearly 

 of the same size (compare figs. 20, 21, 22, and 23). 



Even the extremely degenerate females of the genus Fumea emerge from pupa? with 

 tolerably stout and well-developed antennae (Plate XXVII. fig. 14). The lowest depth of 

 female degeneration is reached in those Psychids which are a mere bag of eggs, without 

 limbs or sense-organs, and utterly unable to emerge from the pupal shell. Distinct traces 

 of antenna? can nevertheless be made out upon the pupa? of some of them (Plate 

 XXVII. fig. 15, A). 



The same facts are well seen among the degraded females of certain Geometrce. The 

 wingless female of Nyssia zonaria possesses thread-like antennae very different from 

 those of the male, but the pupal antenna? do not greatly differ in size (compare Plate 

 XXVI. figs. 24, 25, 26, and 27). The similarly degenerate female of Hybernia defoliaria 

 emerges from a pupa with comparatively broad antenna? (compare Plate XXVII. figs. 8 

 and 9). 



2. The History of the Degeneration of the Antennce in Female Imagines. — These and 

 other examples could be easily arranged in a series leading from a state of sexual 

 equality through stages of increasing female degradation to the culmination reached in 

 the condition of many Psychids. It appears certain that all such cases of sexual in- 

 equality have been gradually reached by the degeneration of one sex attended by a 

 corresponding development of the other. The tendency towards such a change is present 

 in many groups of moths, especially among the Bombyces, and exists whenever the 

 females are less active than the males. When this is the case, the chief competition 

 among the males will be in sense-organs to ascertain the existence of virgin females at 

 as great a distance as possible, and in the power of flight to reach the female before 

 other males. But such competition, ensuring the success of the best-endowed males, and 

 a gradual improvement from generation to generation in their sense-organs and their 

 powers of flight, will therefore cause coitus to take place at a shorter and shorter 

 interval after the emergence of the female from the pupa. But such a result must tend 



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