THE EVOLUTION AND ADAPTIVE SIGNIFICANCE OF THE LEAF-MINING HABIT

Feeding inside leaves by ''leaf-mining'' insects has been viewed as an adaptive innovation for consuming foliage. The leaf mine presumably p rovides its occupant shelter from the detrimental effects of the physi cal environment, protection from attack by natural enemies, and potent ially a means of avoiding plant defenses concentrated in particular la yers of leaf tissue. We examine several hypotheses concerning the adap tive significance of leaf mining using both the published literature a nd experiments. We also examine the notion that leaf mining is an inno vation that has led to an adaptive radiation by examining the phylogen y and species richness of leaf-mining lineages and their sister groups . Both advantages and disadvantages accrue to insects that feed inside leaf mines in comparison to external-feeding folivores. The advantage s of feeding within a leaf mine include: a lower incidence of disease infection, a microenvironment with lower evaporative demand and theref ore protection from desiccation, protection from the direct and indire ct effects of UV radiation by the leaf-mine epidermis, and avoidance o f plant defenses resulting in higher feeding efficiencies. The disadva ntages of feeding inside a leaf mine include: lower mobility resulting in higher mortality from parasites, higher mortality associated with premature leaf-abscission, and smaller average body size and lower fec undity. Leaf mining is certainly a viable means of feeding upon foliag e, and in particular instances it might be the most successful means o f feeding on leaves. However, the lower species richness of leaf-minin g lineages in comparison to their external-feeding sister groups indic ates that the evolution of leaf mining does not represent an innovatio n that has led to an adaptive radiation. Perhaps the evolution of leaf -feeding per se and the evolution of the ability to feed externally on leaves rather than concealed feeding modes such and mining and gallin g represent innovations that have resulted in adaptive radiations in t he Insecta.