DIMORPHISM. 53 



descri])0(l as a separate species, C. adcidata. Mr. B. D. Walsh 

 considers the two sets of females as dimorphic forms, and he 

 thinks that C. aciculata lays eggs wliich produce C. quercus- 

 S2)onyiJica. 



Huber supposes there are two sizes of the three forms (i. e. 

 male, female, and worker) of Bombus, one set being a little 

 larger than the other. 



Alfred Wallace has discovered that there are two forms of 

 lemales of Papilio Memnon of the East Indies ; one is normal, 

 having its wings tailed and resembles a closel}' allied species, 

 Papilio Coon, which is not dimorphous, while the other is tail- 

 less, resembling its tailless male. PcqMo Pammon has three 

 sorts of females, and is hence '^trimorphic." One of its forms 

 predominates in Sumatra, and a second in Java, while a third, 

 (described as P. Romulus) al)ounds in India and Ceylon. P. 

 Ormenus is trimorphic, as Mr. Wallace ol)tained in the island 

 of Waignion, "a third female quite distinct from either of the 

 others, and in some degree intermediate between the ordinary' 

 male and female." Much the same thing occurs in the North 

 American P. Turnus. Papilio Glancns is now known to be a 

 dimorphic form of the former butterfly, both having, according 

 to Mr. Uhler, been bred from the same batch of eggs. Mr 

 W. H. Edwards has found that Papilio Ajax is polymorphous, 

 the same batch of eggs giving rise to P. Ajax, and varieties 

 Walshii, Telamonides, and Marcellus. The male sex also pre- 

 sents dimorphic forms. Mr. Pascoe states that there are di- 

 morphic forms of Anthribidce; that they occur in the males 

 of Stenocerus and Micoceros. Six species of Dytiscus have two 

 female forms, the most common having the elytra deeply sul- 

 cate, while in the rarer forms the elytra are smooth as in the 

 male. 



There is a tendency, we would observe, in the more abnor- 

 mal of the two sexual forms, to revert to a lower type. Thus 

 the agamic Aphis is more generally wingless, and the tailless 

 female butterfly mimics the members of a lower genus, Pieris. 

 The final cause of Dimorpliism, like that of agamic reproduc- 

 tion, is the continuance of the species, and is, so far as yet 

 known, an exceptional occurrence. 



Mimetic forms. Many insects often resemble, in a remark- 



