The reciprocal relations of flowers and insects form a subject which has attracted many workers since Darwin and others showed the importance of this phase of biology. In the whole field of adaptation we have no better materials than those afforded by the morphology of flowers and insects considered in their relation to each other. The field has been most exploited by botanists, but some entomologists have also made valuable contributions to the literature of this subject, and their assistance has always been necessary for the identification of visitors observed.