Chap. X. INSECTS. 347 



these exceptions are intelligible. Size and strength 

 would be an advantage to the males, which fight for the 

 possession of the female ; and in these cases the males, 

 as with the stag-beetle (Lucanus), are larger than the 

 females. There are, however, other beetles which are 

 not known to fio-ht Wether, of which the males exceed 

 the females in size ; and the meaning of this fact is not 

 known ; but in some of these cases, as with the huge 

 Dynastes and Megasoma, we can at least see that there 

 would be no necessity for the males to be smaller than 

 the females, in order to be matured before them, for 

 these beetles are not short-lived, and there would be 

 ample time for the pairing of the sexes. So, again, 

 male dragon-flies (Libellulidse) are sometimes sensibly 

 larger, and never smaller, than the females ; 16 and 

 they do not, as Mr. MacLachlan believes, generally 

 pair with the females, until a week or fortnight has 

 elapsed, and until they have assumed their proper 

 masculine colours. But the most curious case, shewing 

 on what complex and easily-overlooked relations, so 

 trifling a character as a difference in size between the 

 sexes may depend, is that of the aculeate Hymenoptera ; 

 for Mr. F. Smith informs me that throughout nearly 

 the whole of this large group the males, in accor- 

 dance with the general rule, are smaller than the 

 females and emerge about a week before them ; but 

 amongst the Bees, the males of Apis mellifica, Anthidium 

 manicatum and Anthophora aeervorum, and amongst the 

 Fossores, the males of the Methoca ichneumonides, are 

 larger than the females. The explanation of this ano- 

 maly is that a marriage-flight is absolutely necessary 



16 For this and other statements on the size of the sexes, see Kirhy 

 and Spenoe, ibid. vol. iii. p. 300 ; on the duration of life in insects, 

 see p. 314. 



