INTROnrCTION. 25 



of which thf two sexes can be distiiiguished l>y any external 

 (lifFerence. 



Tliis coinj)lete contrast between the two groups is no doubt 

 to be explained by tiie dili'erence in the mode of life. xVfter 

 reaching the adidt stage those Lucanid^ whose life-liistories 

 are known, leave the rotting wood in which they have been 

 living and feeding and henceforth live in the open. Although 

 the female returns to dej)osit her eggs, the male, with his 

 unwieldy jaws and long legs, is (|uite incapable of burrowing 

 into even the softest material. The Passalio.^s, on the 

 contrary, on reaching maturity, continue to inhat:)it the same 

 places, their narrow compact bodies, short legs, and sharp 

 stout jaws enabling them to penetrate and masticate the 

 woody material. A few members of the group (e. g. ('eracupes 

 and Aulacocydus) have processes upon the mandibles and 

 head but tliese have not developed so far as to hinder and 

 may conceivably assist them in the perfi^rmance of their 

 functions. Certain Lucanid.-e also have processes upon the 

 mandibles which are common to both sexes instead of being 

 confined to the males (e. g. ^igidins). These also have 

 attained only a small degree of development. It is evident 

 that the growth of the mandibular i)rocesses of the Pas.salid.e 

 to anything resembling the fantastic structures acquired by 

 many Lucanids would effectively prevent them continuing 

 their burrowing activities and, unless accomj)anied by a 

 simultani'ous change in their niode of life, must bring about 

 extermination. Similarly, the continued develo])ment of sucli 

 outgrowths in l)oth sexes of LrcAMD-l;: would ultimately 

 result in hindering the fi'inales from reaching the proper 

 situations in A\hich to tlcj^osit their eggs. Since the males 

 take no part in this operation, continuance of the species 

 requires only restricted development in the female sex. In 

 the other family male and female live side by side, and there 

 is good reason for believing that their offspring are to a greater 

 or less extent de])endent u[)on both ])arents. The undue 

 development of any a])])endages which hindered their free 

 movements would therefore affect tluj next generation harm- 

 fully, whether in one j)arent or both. 



In dealing with the Copkin^e, 1 described * an' investigation 

 of the evidence afforded by the wearing down of the teeth 

 upon the tibiai as to the share, borne b}' male and female 

 respectively, in the necessary labours of the species. This 

 investigation led me to the conclusion that " where the two 

 sexes have similarly developed armatures, or when that of 

 the male is of moderate development only, both sexes are 

 likely to show the effects of use in the forelegs in a similar 

 degree ; but where they are very dissimilar and the male 



* See Arrow, l-'aiiiiu of Jiiit ish Itidiii. ( 'o|)riii;i'. p. .'{(i. 



