Some Aspects of Beetle Pollination in the Evolution of Flowering Plants

Abstract

Pollination by beetles seems to have strongly influenced the evolution of angiosperm flowers. Beetles are a predominant group of potential visitors and pollinators of flowers since earliest times, so cantharophily is apparent within groups of flowering plants of the most diverse evolutionary levels. In the subclass Magnoliidae, cantharophily is a dominant feature of many families, but even in these archaic groups a more open, unspecialized type of beetle pollination and a more specialized one can be distinguished. Specialization here was probably connected with increase in flower size, numerical increase and grouping and flattening of the sexual organs, etc. The strongly protogynous flower attracts beetles through imitative odours, to which the insects are already conditioned in their other activities. Secondary polyandry, such as occurs in the more basic groups of Rosidae, Dilleniidae and Caryophyllidae, is in many cases related to cantharophily and may find its functional explanation in this mode of pollination. This probably somewhat more recent radiation into beetle-pollination might have caused the stamens to increase in number in order to save some of them from the crude visitors. Cantharophily in secondarily polyandrous groups is also frequently connected with the proto-gynous condition of flowers and with odours which act directly on the instincts of the visitors. During these new waves of flower-biological radiation other insects besides beetles must have already been in existence. The pollination of primitive Rosidae, Dilleniidae and Caryophyllidae is therefore much less exclusive compared with the Magnoliidae, viz., beetles and other insects often frequent flowers jointly. Since beetles have continued as the predominant insect group until today, cantharophily can be observed also in advanced groups of angiosperms. In this case, cantharophily is no longer a sign of primitiveness as it is in the Magnoliidae and to some extent probably also in secondarily polyandrous groups, but a relatively recent adaptation into a still existing ecological niche.