Biogeographical implications in species richness, biological diversity, and evolution of gall-inducing insects of the orient and the eastern palearctic

Because biogeography offers insights into distribution patterns, species richness, and biological diversity of organisms, and more importantly, into the dynamics of organic evolution, this article explores the biogeography of specific groups of insects, which induce galls on plants in the Orient and the eastern Palearctic. The capability to induce galls occurs only in specific groups of insects. Leaf-mining habit in plant-feeding insects from the Upper Cretaceous possibly led insects to seek new food sources and their larvae preferring specialized, but limited, feeding sites on plants of the Eocene, which must have coincided with the diversification and establishment of angiosperms. Integration of monophagous host-selection behavior and evolution of other related adaptive strategies in insects, sometime in the Miocene-Pleiocene, must have predisposed gall-inducing behavior among insects. Nonetheless, discovery of a holometabolan-induced gall on the fronds of Psaronius (Filicopsida: Marattiales) from the Carboniferous period has challenged the current premise that gall-inducing habit originated simultaneously with the diversification of angiosperms in the Cretaceous. Among modem insects, the capability to induce galls does not occur uniformly in all species; so much so, gall-susceptible plants also do not exhibit any discernible pattern because galls develop on plant species of unrelated natural orders. Asteraceae emerges as a common natural order that hosts gall-inducing insect species in both tropical and temperate worlds, mostly accommodating the larvae of Coleoptera, Lepidoptera, and Tephritidae that tunnel the relatively soft, axial organs (stems and roots) and induce swellings (= galls). When the Eocene-Oligocene origin of Asteraceae and gall-inducing habit are viewed in conjunction, gall-inducing capability can be construed to have evolved in the predominantly arborescent families (e.g., Fagaceae, Lauraceae) earlier, where the insect mostly induced galls on appendicular organs, than in the predominantly herbaceous families (e.g., Asteraceae), where the insect mostly induced galls on axial organs. Against a brief background on the patterns of distribution and radiation of gall-inducing insects of the world, a further exploration has been made in this article on the patterns of distribution and radiation of gall-inducing insects in the Indian subcontinent, by analyzing that of 20 species of Cecidomyiidae confined to Anacardiaceae. Out of the nearly 2000 galls and inducing insects documented in the Indian subcontinent, cecidogenous relationships between gall midges and Anacardiaceae are striking, because species of Anacardiaceae produce diverse secondary chemicals, which possibly play a defense role against insect herbivores, and a cecidomyiid-induced fossil gall on leaves analogous to modem Mangifera indica (Anacardiaceae) from the Upper Palaeocene has been found recently. Among the modem Indian Cecidomyiidae, fourteen species induce galls on leaves, four on flowers, one on stems, and one both on stems and leaves of Anacardiaceae. Radiation of these species seems to have occurred conservatively with a majority of them remaining tied to leaves, retaining the behavior of the fossil gall inducer on the leaves of the M. indica analog. Conservative radiation is not only evident among the gall midges that infest M. indica, but also among those that infest Holigarna arnottiana (Anacardiaceae).