236 PASSALID^. 



very large and protruded in the present group. The other 

 organs of the mouth also differ completely, in the two groups. 



The antenna' of the Passalid.e are also of less delicate build, 

 and their sensory lamellae are differently disposed when at rest. 

 Although the terminal leaflets are often more than three in 

 number in Passalid^e, and occasionally more than three in 

 LuCANiDiE, the usual number in both groups is three. In 

 Passalid.^, however, discontinuity is almost alwaj's visible 

 between the 4th and oth joints, affording some ground for the 

 supposition that six leaflets may have 1:!een the primitive 

 number. 



The Oriental and Australian Passalid.^ have been divided 

 by Dr. Gravely into six subfamilies but, as the first of these, 

 the AuLACOCYCLiN^, is more distinctly separated from the rest 

 than these are from each other, the differential features of the 

 latter, most of which only include a single genus in our region, 

 being very slight, I have considered it sufficient, for the present 

 j)urpose, to adopt on\y two primary divisions. 



Asymmetry. 



A remarkable and unusual phenomenon shown by certain 

 Oriental Passalid.^ is asymmetr}' of the head. This is most 

 strongly develoj^ed m the common Indian genus Acemius, in 

 which the right mandible is jieculiarly attenuated, while two 

 jaro cesses from the front margin of the head are much more 

 prominent on the left side than on the right. In a lesser 

 degree, the same peculiarity is found in the genera Episphenns 

 and Pelopides and certam other Asiatic genera. Very few cases 

 of a similar kind are known in Coleoptera, but in the volume 

 of this series dealing with the Erotylid^ and related families 

 I have called attention to certain striking examples in the 

 family Languriid^, the relationship of which to the 

 Passalid^e is extremely remote (see ' Fauna of India ', Coleop- 

 tera, Erotylid^, etc., 1925, pp. 165, 166). In the Langur- 

 iiD^, as in Passalid^, the asymmetry is confined to the head, 

 but in the former it is peculiar to the female and appears to be 

 a consequence of the great enlargement of the left mandible. 

 It seems likely that in both groiqis the employment of the 

 very strong mandibles m dealing with vegetable fibres is in 

 some way facilitated by the distortion of the head. In the 

 Passalid^ the asymmetry is the same in both sexes, and as 

 these beetles are exceptional in the association of the larvae 

 and ]mrents of both sexes, it seems not im])robable that some 

 special feature in the social behaviom- of those species in which 

 it occurs may afford the explanation. 



In a paper dealing with " The Evolution and Distribution of 

 certain Indo-Australian Passalid Coleoptera ". Dr. Gravely has 

 put forward the view that the asymmetry of the mandibles 



