Entomological Research in Australia

Abstract

MR. R. J. TILLYARD, whose admirable book on “The Biology of Dragonflies ”was recently reviewed in NATURE, has made a further contribution of importance to the study of this order of insects in a series of papers on “The Morphology of the Caudal Gills of the Larvæ of Zygopterid Dragonflies, ” published in the Proceedings of the Linnean Society of New South Wales (vol. xlii., 1917, parts i and 3). Zygopterids are the slender-bodied dragon-flies, often distinguished as “demoiselles, “ the larvæ and nymphs of which are provided with three conspicuous appendages, at the hinder end of the body, traversed by branching air-tubes. A careful comparative study of the structure of these organs in various genera and in successive stages of growth has been made by the author, who concludes that the median dorsal gill-plate in these insects is analogous with the telson in Crustacea, while the paired lateral appendages are cerci, and “therefore the true homologues of the uropods of Crustacea. ”These latter are compared with the filamentous cerci of the well-known stonefly (Perlid) larvæ and nymphs, which they resemble in form in the early stages, becoming more highh specialised as growth proceeds. From his comparative studies Mr. Tillyard is convinced that in the evolution of this group of dragonflies a primitive filamentous condition of the larval telson and cerci was succeeded by the “saccoid ”type, which persists in a few genera such as Diphlebia and Neosticta, this by the “triquetro-quadrate ”type, found in the Calopteryginæ, where the median gill is trapezoidal and the lateral ones are triangular in cross-section, and this by the specialised “lamellar” type characteristic of the larvæ of the great majority of the group, including the familiar Agrioninæ. The author promises further studies on the physiology of these interesting structures, for although their function is doubtless respiratory, the aquatic larvæ which possess them continue to breathe in some way when artificially deprived of them.