VARIATION IN THE DERMAPTERA. 215 



B. Macrolabiism. 



This is a form of structural variation that is ahtiost, if not 

 quite, confined to the male sex, since it consists in often extreme 

 elongation of the forceps, which are almost invariabl}^ more ornate 

 and complex in the male than in the female ; they may be compared 

 to the antlers of deer. 



There are two forms of the forceps in most species, a normal, short 

 or "low" forni, and a very elongate, or "high" form, but intermediates 

 are rare or unknown. j\Iessrs. Bateson and Brindley (1892) examined 

 a thousand specimens of the common earwig (F. atiriciilaria, L.), col- 

 lected in one day on three small islands known as the Knocksies and 

 Widerpern, in the Fame Islands, off the coast of Northumberland, and 

 found that 583 were males ; the length of the forceps varied between 

 2-5mm. and 9mm., the commonest length is about 3-5mm. and about 

 7mm.; the mean form, 4mm. -6mm., is compa.ratively rare; in the 

 female the length of the forceps scarcely varied at all. We quote 

 Messrs. Bateson and'Brin ley's obssrvations upon these results: — 



"It is perhaps unneci - .iry to say that this result is of considerable 

 importance to an appreciaoion of the M^ay in which variation may 

 occur. There is here a group of individuals living in close communion 

 with each other, high and low, under the same stones. No external 

 circumstance can be seen to divide them, and yet they are found to 

 consist of two well marked groups, containing about equal numbers. 

 To those who are acquainted with the chapter on Organic Stability in 

 Galton's Natural Inheritance, this will be recognised as an instance of 

 variation about two positions of stability, the intermediate position 

 being one of less stability. In the common language of naturalists, 

 the facts of this case suggest that there is, for some wholly unknown 

 reason, a dimorphism among the males of these Earwigs, maintained 

 though all live together. In cases of dimorphism some have thought 

 lit to speculate on the possible utility of the phenomenon. We know 

 no basis of fact from which these discussions may be properly attempted, 

 and we leave these matters to those who are satisfied with such methods 

 of biological inquiry and have leisure and ingenuity to pursue them." 



When the forceps are thus elongate, the effect is just as though 

 they Avere elastic and had been stretched, each part being extended in 

 proportion, but in this genus B'orficala, L., and many others, the male 

 forceps have two distinct portions, a basal dilated portion, and an at- 

 tenuate apical portion ; the proportions between these two parts is 

 often of specific value ; in the macrolabious forms, the dilated part is 

 duly extended with the slender apical part, so that the proportions are 

 approximately preserved. Thus, a vertical tooth on the upper surface 

 of the forceps in S/calistes lutjubris is drawn out into a compressed and 

 acute ridge or crest in the macrolabious form which is known as the 

 " var. iiietrica.'" 



The varietal name forcipata is often given to the macrolabious 

 form, but " var. iiiacmlabia " occurs too ; for the sake of uniformity, 

 and on account of the very general occurrence of the phenomen, I 

 prefer not to use the varietal names, but to speak of the macrolabious 

 and cyclolabious forms respectively. 



The macrolabious form of the common earwig is well-known, and 

 generally called var. forn'iiata, Steph. It is curious that it seems to be 



