f^58 DTIfASTm^. 



Sexual Dimorpliism. 



The existence of horns in the male, which in the previous group 

 is of exceptional occurrence, here becomes characteristic and in 

 some of the large species these appendages reach a size unequalled 

 in any other insects. They usually take the form of a slender 

 recurved horn upon the front of the head, sometimes toothed or 

 bifurcated, and generally represented only by a slight tubercle in 

 the female ; and upon the pronotum one, two or more processes 

 directed forwards or upwards, and often rising from the margin of 

 a dorsal cavity. Such a cavity may be present without any pro- 

 cesses and it may exist in both sexes but differ in shape, as in 

 Eo^iihiJeurus chlnensis. In the very largest Dynastin.i, in \\-hich 

 the armature of the male attains its maximum development, there 

 is no cavity, but the pronotum of the male on the contrary is much 

 elevated or humped. In the smallest forms again, as in the genus 

 Heteronyclms, there is no trace of such a sexual armature. 



Although generally distinctive of the male, the possession of 

 horns is not invariably so, for in some cases, as in Oryctes 

 rhinoceros, the well-known Cocoanut Beetle, both sexes are horned, 

 but some distinctive difference of form is always discoverable if a 

 sufficient series of specimens is studied. There is no group of 

 insects in which it is more necessary that a good series should be 

 examined in order to obtain a correct idea of the characteristic 

 features of a species. In the early days of Entomology, when only 

 occasional specimens of these insects had yet reached Europe, the 

 variability of the armature and even its sexual character was un- 

 recognised, and almost every specimen which came into the hands 

 of Linnaeus, Fabricius and their contemporaries was regarded as 

 the representative of a different species and given a distinctive 

 name. It has not been considered necessary to include all these 

 names in the present work. 



Eemarkable anomalies occur in some species in the development 

 of the horns, as seen by a comparison of specimens of different 

 size. Horns which at their highest development are slender and 

 simple may in minor examples be knobbed, forked or toothed in 

 various inexplicable ways ; and it was almost inevitable that 

 Chalcosoma atJax, for instance, when known only from a few 

 examples brought from different localities should be considered 

 to form several distinct species. In the structure of the legs 

 there are two opposite tendencies characterising the males of 

 different groups of genera. In one, containing the most striking 

 forms, the legs become elongated to a greater or less extent, 

 while in another the front legs, and especially the tarsi, are con- 

 tracted, the others remaining like those of the female. In the 

 latter case the anterior claws are also modified in the males, the 

 inner claw benig thickened and bent and frequently giving off a 

 broad tooth. This formation is never found in the Cetomix.e but 

 in some degree it is almost general in the Edtelixje. In other 

 genera of Dynastin^, which occupy an intermediate position, the 



