INTRODUCTION. ll 



lobes, called the paramera, on the dorsal side and a median lol)e 

 ventrally, wliich is often strongl}' bifurcated. It is this terminal 

 part which, in the present subfamily, assumes an extraordinary 

 variety of forms, and in species closely allied usually exhibits 

 differences of shape so well-marked as to afiord more definite and 

 certain evidence of distinctness than any part of the external 

 anatomy. Strange and almost fantastic forms are sometimes 

 found, reminiscent of the external outgrowths of many male 

 Lamellicornia, but without the extreme variability so cliai'ac- 

 teristic of those outgrowths. The sedeagus indeed seems to 

 combine a tendency to more rapid and considerable changes 

 under certain conditions with less variability under other con- 

 ditions than other parts of the body. When a species forms a 

 compact colony of restricted range, although in colour and other 

 external features it may show great variability, the form of the 

 aedeagus appears to be very constant ; but in wide-ranging 

 insects the ae leagus is sometimes found to exhibit in different 

 localities differences of form which, when the extremes are com- 

 pared, are strikingly unlike. Such a case is re])resented in 

 Fachiirrhinadoretus (Plate V, figs. 4-10), in which a remarkable 

 transition in the shape of the organ is traced as we advance from 

 north to south. Another instance is shown in Plate IV, figs. 30-32 

 representing Adoretus costopiJosus, another wide-ranging insect. 

 Eig. 32 is from a typical example from the Andaman Islands, 

 fig. 31 from one taken in the Chittagong Hills, and fig. 30 from the 

 United Provinces. Such variation, according to mj^ observation, 

 is always geographical and not found in individuals from the same 

 place. It must be supposed that this divergence in members of 

 the same community would constitute an obstacle to interbreeding 

 nnd so become immediately checked. We may also assume that 

 the isolation of outlying colonies of such wide-ranging species by 

 t!ie elimination, from any cause, of the species in a connecting 

 district would result in their complete separation from the rest 

 and so give rise to independent species, which would not neces- 

 sarily show any external distinctive features. These organs are 

 therefore of great importance and, while the occurrence of con- 

 siderable variability must not be overlooked, it may be stated as 

 a general rule that specimens with the same form of sedeagus, 

 although otherwise dissimilar, are conspecific, and that specimens 

 from the same locality with a different form of sedeagus, although 

 they may be otherwise similar, are specifically different. The 

 external characters separating the species in many groups of the 

 genus Anomala, for instance, are so minute that, unless con- 

 siderable series are compared with extreme care, their significance 

 can scarcely be appreciated; but when such series ai-e not available 

 the examination of the a3deagi will often reveal marked differences 

 which unmistakably indicate that more than one species is 

 present. The value of the sedeagus for the purpose of classi- 

 fication consists, not in any essential difference in its degree of 

 constancy as compared with otiier features, but in the fact that 



