AN INTERPRETATION OF GULAR SIZE IN SOME ADULT COLEOPTERA

An interpretation of the gular width of some adult beetles has been attempted in terms of absolute head size and the functioning of the mounth-parts. In many groups of beetles (e.g Aphodiinae) the gular width to head width ratio remains approximately constant in species of incresing size, whereas in the Staphylininae it declines because the absolute width of the gula remains nearly constant. Thus in very large Staphylininae the gular sulci meet in the midline. It is suggested that this is due to a relative increase in the size of the mandibular adductors which is necessary to maintain the relstive biting force in larger species, this being required for the penetration of tough prey. It is postulated that this need arises because if the head width were doubled, the volume (and weight) of the head would increase about eight times, whilst the cross-sectional area of the mandibular adductors would only quadruple. A variety of species have been examined in relation to this hypothesis. It is suggested that many Carabidae do not conform because of the presence of stipital retractor muscles between the gular fidges. It also appears that because of these muscles some species have developed a mid-gular apodeme to bear the origins of the labial muscles.