LAMELLICORNIA. 



Sexual Dhnorjjhism. 



15 



A special characteristic of the Lamellicornia is the tendency of 

 the sexes to differ markedly iu their external features. There is 

 no particular in which the differences may not manifest themselves. 

 Colour, vestiture, size and structure are alike liahle to them, and 

 in many cases there is so little resemblance between male and 

 female that they have been regarded as distinct species and even 

 ffenera. In the Passahdj;; alone are marked external differences 



O 



entirely absent. 



In tiie ScARAByEiDyE there is a tendency to tlie occurrence of 

 horns upon the head and thorax in the male. Such appendages 

 may be possessed by both sexes, but they are very rarely equally 

 developed in both and are generally represented by mere rudiments 

 in the female. Occasionally thearmature is of nearly equal develop- 

 ment but of different form in the two sexes, and only in two known 

 species {Onitis)is it more developed in the female than in the male. 

 Such structures ai'e found in their fullest development in the 

 males of the large beetles belonging to the Subfamily Dynastin.!;. 

 These flourish chiefly in Tropical America, but the well-known 

 Xylotrupes gideon, which is abundant throughout Tropical Asia, 

 and Chalcosoma atlas, shown at Plate II, fig. 12, are excellent 

 examples. Although generally of smaller size, some of the species 

 of the Subfamily Coprin^.t: exhibit still more extraordinary forms 

 of armature upon the head and thorax of the male. 



To the same category belong the enormously enlarged mandibles 

 sometimes characterising the males. These are ahnost universal 

 in the Soag-beetles (LrCANiM:) and occur more rarely iu various 

 groups of ScAEAB^BiD^, Dicaulocepluilus falcifer being a striking 

 Indian example. Tlie two forms of armature are never found 

 together. Although the mandibles are normally highly-developed 

 in the Dynastin^, in the males of which horns are so common a 

 feature, no sexual development there takes place in the mandibles 

 except in a few hornless forms (e. g. Aiicognatha), and such 

 hypertrophy of the mandibles, wherever it is found, in the 

 LucAiNiD^, in Geotrupin.e, HYBOSORiNiE, Melolonthin^, or 

 KuTELiKvi;, is never accompanied by cephalic or thoracic outgrowths. 



These structures are in some cases used as weapons of offence 

 in contests between males of the same species, and in some others 

 of the less extravagant forms serve as tools in the task of nest- 

 construction, as M. Pabre has described in Coprh hispanus and 

 Geotnipes Ujpliams. But, although they are commonly assumed to 

 be all explainable in a similar manner, there are many reasons for 

 believing that these uses are secondary and afford no explanation 

 of the origin of the armament. The horns are never sharp or 

 capable of inflicting injury upon such well-protected bodies as all 

 these beetles possess, and they are sometimes extremely slender 

 and brittle and directed backwards so that no practical use of any 

 kind can be imagined for them. Thus the male of the African 



