tNTRODUCTION. 23 



short triangular mandibles which indicate the usually pre- 

 dominant variable phase. In this case the length of the jaws 

 and of the insects themselves is less constant tlian usual in 

 the isolated phase. It seems probable that this phase is 

 replacing the other as the normal form of the species, and 

 that the more primitive form is in course of disappearance. 

 In many Lucanid^, as in those belonging to the genus 

 Lueanus, the primitive tyjie of the male mandibles, meeting 

 at the inner edge, is not found, and it may be that it has been 

 rej)laced by the later-evolved phase which, originally constant, 

 has now become variable, like the form it has replaced. 



No similar dimorphism is found amongst female beetles. 

 In those Lamelhcorns remarkable for the horns borne upon 

 the head or thorax, these, although generally distinctive of 

 the males, are in some cases well developed in females also, 

 but the occurrence of two phases, as in the mandibles of 

 LucANiD^, is confined to male horns. A single instance has 

 been noted amongst these beetles of the combination of 

 the two phases in the horns of one individual. This is in a 

 South American beetle, Megoxeras jason *, the males of which 

 have a slender horn upon the head and a very massive one 

 upon the thorax. Thirty-eight male specimens of this collected 

 in Ecuador were found to show two horn-j)hases, 18 of all 

 sizes belonging to the variable phase and lb large specimens 

 to an isolated phase, while one example shows the two phases 

 on opposite sides of the body. 



It is probable that this strange form of polymorphism is 

 less uncommon than appears at present. At least, it is not 

 pecuUar to the LameUicorn beetles. In the magnificent 

 Longicorn beetles of tropical America, belonging to the genus 

 Psalidognathus, the females of which are without wings, and 

 in the related genus Frionocalus, of which both sexes are 

 wingless, enlargement of the male mandibles occurs exactly 

 as in the Stag-beetles, and a similar transition can be traced 

 from small to large individuals. The females of these insects 

 have broad mandibles with sharp cutting edges, which meet 

 and cross one anotlier like scissor-blades, the front half of 

 the imier edge straight, the hinder half a little jagged. The 

 great males, which may be three inches in length, have long, 

 curved calliper-hke mandibles, which meet only at their tips. 

 But in males of very small size the mandibles are precisely 

 like those of the females and the calUper shape only appears 

 gradually as we examuie larger and larger specimens. Exactly 

 similar conditions are found in a South African Long-horn, 

 Cacosceles newniani, the female of which has scissor-hkc 

 mandibles, while in large males they are much longer and 



* (Seo Proc. Zool. Soc, wr. A, vol. cxii, 1943, p. 113, pi. i. figs. 3-5.) 



