HETEROGONY, MOULTING, AND DIMORPHISM 73 



values for forceps-length against body-length give a good 

 approximation to the formula for constant differential growth- 

 ratio between forceps-length and body-length. When, how- 

 ever, frequency curves for forceps-length are plotted for each 

 class separately, a situation is revealed analogous to that for 

 Inachus female abdomen. The smallest specimens show uni- 

 modal curves, their forceps being all of ' low ' type ; the 

 largest, also uni- 

 modal curves, 

 with forceps all of 

 ' high ' type ; and 

 the medium-sized 

 show b i m o d a 1 

 curves, the num- 

 bers of low forceps 

 diminishing and 

 high forceps in- 

 creasing with in- 

 crease of body- 

 size. It should be 

 noted that low and 

 high types, as with 

 Inachus chela and 

 abdomen, d i ff e r 

 only in size ; fur- 

 ther, that in this 

 case there is slight 

 overlapping of the 

 two modal types 

 (Figs. 40, 42.) 



Przibram (1927) 

 has suggested that 

 the bimodality is 

 due to some male 

 earwigs having 

 one or more extra moults ; but his contentions, as they stand, 

 will not explain the facts, for they should give bimodality of 

 body-length as well as of forceps, and they do not take account 

 of the overlap of ' low ' and ' high ' types, between which there 

 is neither qualitative nor quantitative difference, but merely a 

 modal distinction depending on their frequency distribution. 



Furthermore, it is difficult to see how more than one extra 

 moult can ever occur without leading to tri- or multi-modality. 



2 n — 1 n n -j- 1 



Moult-stages {and Body-size) 



Fig. 



— Diagram to show the possible origin of 



dimorphism of the male forceps in earwigs, Forjicula. 



LM, LM . . . sizes of larval forceps at successive moults ; 

 growth isogonic. X, point at which heterogonic growth of forceps 

 is supposed to be initiated. If the change to the imago occurs at 

 the nth moult, the modal level of adult forceps-size is at A — A 

 ('low' males) ; if at moult (n + i), the forceps, after continuing 

 of larval type for one more instar (B — B), would reach the modal 

 size-level, C — C (' high ' males). 



