An Evolutionary Study of the Feeding Biology of Aenetus Virescens Larvae (Lepidoptera: Hepialidae)

Abstract

Aspects of the feeding biology of A. virescens larvae are described. The host-plant relationship of A. virescens involves a transition from fungal feeding (fruiting bodies and dead wood) to the callus tissue of live angiosperm trees and shrubs. This pattern contrasts with most Lepidoptera where the larva specialises on one or other substrate. The transition between from fungal to callus feeding is characterised by the formation of a temporary morph which has a distinct pinacula pattern and colour. Enzyme analysis of larval guts has demonstrated the presence of a digestive capacity similar to that of obligate fungivores, but which is comparatively rare in phytophagous Lepidoptera. A digestive capacity to degrade starch (fungal and green-plant storage compound) and laminarin (fungal and phloem constituent) was recorded for both fungal and phytophagous larval stages. It is suggested that the laminarin degrading capacity may function in callose digestion when larvae are feeding on callus tissue in live trees. The life cycle of A. virescens is approximately 1 to 4 years with adult emergence occurring mainly in the spring and early summer months. A 2 to 3 month delay between adult emergence and entry of larvae into trees represents the combined period of egg maturation and development of fungal feeding instars. Although the period of fungal feeding is temporary, it is an obligate stage of the life cycle. The role of biogeographic analysis and interpretation is explored in reference to the origin of the host range of of A. virescens. It is argued that the host range evolved as a consequence of dispersal involving geological changes in the distribution of a generalized Aenetus ancestor. This origin is possibly centered on what is now the Indian Ocean and occurred in relation to tectonic changes during the Cretaceous. Utilisation of various plants is regarded as primarily the outcome of orthogenetic abilities rather than a progressive coevolution in the sense of reciprocal adaptations. The evolutionary history of A. virescens tunnelling and feeding is evaluated at 3 levels of comparison: the genus Aenetus, the Hepialidae, and the Lepidoptera in general. The feeding and tunnelling habits of Aenetus involve excavation of a larval tunnel in the woody stems and branches of trees and shrubs. The tunnel is approximately "7" shaped and is open to the host surface. The larva feeds on callus tissue around the tunnel entrance. The vicariant distribution of Aenetus and the arboreal genus Endoclita is regarded as evidence for the evolution of each genus over a broad geographic front. This suggests that the species in each genus have evolved from an ancestral "type of organization" which preceded both genera. The evolution of different tunnelling and feeding habits in Aenetus is interpreted as the result of a reorganisation or "recombination" of ancestral characters. The distribution of ancestral characters was a major factor in determining the relative distribution and diversity of the modern genera. It is suggested that the immediate evolutionary history of Aenetus originated in the late Jurassic or early Cretaceous in association with the origin of the Indian Ocean. Larval feeding in the Hepialidae is reviewed with particular emphasis on the developmental transition of larvae between different diets. The combined diet of fungal and live vascular plant tissue is probably widespread in the Hepialidae. The dual feeding capability of hepialid larvae is explained in terms of a generalised ancestor. It is suggested that the ancestor included both fungi and higher plants in its diet and during the evolution of taxa below the subordinal level there was an overall trend towards a taxonomic separation of the two diets. Groups such as the Hepialidae appear to have retained the dual capacity although often in a specialised form where mycophagy is the sole feeding habit of early instars. Hypotheses on the relative age of origin for the Lepidoptera and its taxa are examined from a palaeobotanical and panbiogeographic perspective. It is suggested that the diversification of Lepidoptera may have pre-dated the appearance of the angiosperms and many of the Lepidoptera/angiosperm host-plant relationships were, therefore, initiated when the host-plants were pre-angiosperms. The origin of angiosperms in a Jurassic or earlier "mangrove" habitat is considered in relation to ancestral lepidopteran feeding habits. It is suggested that the ancestral Lepidoptera were associated with pre-angiosperms living at the "mangrove" interface between land and sea in the Jurassic or earlier (possibly Carboniferous). The moisture tolerant feeding habits of Hepialidae and many other Lepidoptera may be a terrestrialised expression of an ancestral mangrove origin. The appendix includes discussion of evolutionary mechanisms with respect to the contribution of biogeographic analysis. The evolutionary significance of orthogenesis to modern biology is discussed. Orthogenesis is presented as a non-mystical concept of evolutionary change and orientation which is independent of natural selection. It is argued that biogeographic analysis of distribution patterns can lead to the conclusion that orthogenetic factors are fundamental to the evolution of organisms. This role of orthogenesis cannot be fully appreciated from the study of biological form in isolation from biogeographic aspects of evolution. It is Leon Croizat's "panbiogeography" in particular which has made a major contribution to investigation and interpretation of orthogenetic factors in space and time. The general validity of an orthogenetic point of view is examined by looking at a plant/animal relationship in a completely different context to that of A. virescens. The interaction of the South American Azteca ants with the host-plant Cecropia is generally regarded as a classic example of coevolution where the organisms exist in close ecological association and each exhibits a series of mutually adaptive features. This is contrasted to the view that the specialised ant adaptations of Cecropia involve an orthogenetic transition of a pre-angiosperm ancestor into its modern form. A biogeographic analysis of the Azteca/Cecropia relationship shows that the interpretation applied to A. virescens feeding is equally applicable to highly specialised plant/animal interactions.