INTRODUCTION. 235 



living in the forebt regions of tlio tropics. Superficially 

 resembling rather closely certain Tenebrionid.«, they have 

 structurally no close relationshi]) with any other family. 

 With the LuCANiD.^, with which, as anotlier Lamellicom 

 group of generally large-sized insects of similar habits, it is 

 natural to associate them, they have really little hi common, 

 as has been already said. The entire absence of any external 

 difference between males and females throughout the P.\ssal- 

 ID^ is almost as remarkable as the nearly complete absence of 

 visible features common to the two sexes in many Lucanidje. 



There is another very marked difl^"erence between the two 

 grou})s. The Lucanid.^ show astonishing hiconstancy of size 

 within the species. It is quite usual for a large example to have 

 several times the bidk and weight of a small s})ecimen of the 

 same kind. This great variability in size is rather character- 

 istic of those insects which feed ui)on wood, and is probably 

 related to the varyhig nutritional value of their food. This is 

 not the case with the Passalid^ however, for the size of each 

 species is, on the whole, rather constant. Social uisects seem 

 generally to vary little hi size, and we can, perhaps, attribute 

 the constancy of the Passalid.^ in this respect to the larvae 

 being ])rovided by their ])arents with a uniform and regular 

 suj)ply of food. 



Another notal)le feature of the grouj) is the faculty of 

 stridulation, ajjparently possessed by all its members in both 

 the larval and adult stages, and not in the former only, as in 

 the LuCANiD^. That this faculty is of special importance to 

 these insects, seems to be proved by its universality throughout 

 the family, which is quite exceptional, by the profound 

 structural changes which have acconi])anied its acquisition, 

 and by the strikhig fact that in many sjiecies the power of 

 flight has been sacrificed for the greater efficiency of the 

 stridulatory ap])aratus, tlie wings being used for that jnirpose 

 alone. In the larva\ In* a still more remarkable alteration, the 

 tlurd-])air of legs for the same end have lost every trace of 

 their original form. 



Although the members of both the families dealt with in this 

 volume live in a similar environment and are dependent upon 

 similar food, viz., decaying wood, the organs of the mouth 

 differ widely. Except that the mandibles in l)oth are strong 

 and exposed, there is little resemblance. The mouth-organs 

 of the LrcANlD.E serve chiefly for the absor])tion of liquid 

 nourishment, those of the Passalid.^ are obviously useless for 

 that ])urpose, and are much more powerful, as rec|uired for 

 dealing with solid matter. The maxilla% which in the 

 LucANiDiE form brushes for the absorj^tion of liquids, are here 

 auxiliarj^ masticatory organs, sharj) and horny, and the labrum, 

 or upper lip. whicli has almost vanis^hed in the Lucanid.^, is- 



