Morphology and sensilla of the enigmatic Bhutan pine aphid Pseudessigella brachychaeta Hille Ris Lambers (Hemiptera: Aphididae) - A SEM study

The aphid tribe Eulachnini is unusual in being both diverse in species number and specializing on conifer hosts. The Bhutan pine aphid Pseudessigella brachychaeta Hille Ris Lambers, 1966; the only known species of its genus and native of the Himalayan region, displays characteristics of both the Palearctic genus Eulachnus and the Nearctic Essigella. Species of all three genera, as well as eulachnine Cinara subgenus Schizolachnus, share the same ecological niche, feeding almost exclusively on the needles of true pines. To obtain a better understanding of this enigmatic aphid and its biological context within the Eulachnini, we conducted a scanning electron microscopy study of the fine morphology and sensilla of the live-bearing and egg-laying forms of the species. The general morphology most resembles that of species of the genus Eulachnus but in detail Pseudessigella also displays unique features. Many types of sensilla, including coeloconic, campaniform, placoid and trichoid sensilla occur at different locations on the antennae, mouthparts, and legs. The rhinariolum on the second antennal segment is described and illustrated for the first time in Lachninae and a second rhinariolum-like sensillum, hitherto unknown in aphids, is also described on the second antennal segment. We discuss the morphological features of Pseudessigella and the distribution of antennal and labial sensilla with respect of their possible taxonomic and functional significance.