Sunday 2 December 2018

Strepsiptera: A larval Twisted-wing Insect from Cretaceous Burmese Amber.

Twisted-wing Insects, Strepsiptera, are a highly specialised group of parasitoid Insects (Insects in which the larval stage is parasitic and matures inside a living host animal). The males are extremely short-lived as adults, they are swift fliers and have highly developed chemoreceptors to help them find the females, but lack any means of feeding themselves, so that the adult stage of their life-cycle often lasts only a few hours. The females do not fly, and in most species never emerge from their host, instead extending their abdomens to mate and give birth to live young. These young are extremely simplified and extremely small, with an average size of 230 μm, comparable to many single-celled Eukaryotes, making them among the smallest known Animals. The relationship of the Strepsiptera to other Insects was for a long time a mystery, as their highly specialised body-plans do not closely resemble any other Insect group, but recent genetic studies have suggested that they are the sister-group to the Beetles, Coleoptera, which would imply they have existed as a separate lineage since at least the Permian, though sheds no light on when they adopted their current life-strategy. The oldest known fossil Twisted-wing Insects are males from the Cretaceous, though these do not reveal anything about the habits of the females and young, both of which are not known from fossils older than the Eocene (some reported fossil larvae from the Cretaceous have now been rejected).

In a paper published in the journal PeerJ on 22 November 2018, Hans Pohl of the Institut für Zoologie und Evolutionsforschung at Friedrich-Schiller Universität Jena, Jan Batelka and Jakub Prokop of the Department of Zoology at Charles University Prague, Patrick Müller of Käshofen in Germany, and Margarita Yavorskaya and Rolf Beutel, also of the Institut für Zoologie und Evolutionsforschung at Friedrich-Schiller Universität Jena, describe a first-instar larval Twisted-wing Insect from Cretaceous Burmese Amber.

Cretaceous ‘Burmese Amber’ has been extensively worked at several sites across northern Myanmar (though mostly in Kachin State) in the last 20 years. The amber is fairly clear, and often found in large chunks, providing an exceptional window into the Middle Cretaceous Insect fauna. This amber is thought to have started out as the resin of a Coniferous Tree, possibly a Cypress or an Araucaria, growing in a moist tropical forest. This amber has been dated to between 105 and 95 million years old, based upon pollen inclusions, and to about 98.8 million years by uranium/lead dating of ash inclusions in the amber.

The larvae is 197 μm in length, excluding the terminal bristles, and has an elongate oval body made up of eleven segments, with two pairs of well developed bristles on the final segment. The head is partially covered by debris, but what can be seen is semi-circular in shape. The dorsal surface is smooth, lacking ornamentation and almost devoid of bristles. Pohl et al. contend that this larvae is almost identical to modern larvae of the modern Strepsipteran group Mengenillidae (which are thought to be the oldest living group of Strepsipterans and which have females that emerge from the host before breeding), leaving little doubt as to the identity of the specimen, and extending the known range of parasitism in the group 50 million years earlier than was previously possible.

Strepsiptera primary larva in Burmese amber, ventral view. (A) Photomicrograph with an Axio Zoom.V16 microscope with a PlanNeoFluar Z 1.0x lens. (B) Drawing based on photomicrographs with an Olympus IX81 inverted fluorescence microscope with UIS2 objective. Abbreviations: af, antennal field; cb, caudal seta; cx, coxa; fe, femur; fs, frontal seta; lcb, lateral caudal seta; mp, maxillary palp; mssp, mesosternal plate; mt, metanotum; mx, maxilla; prsp, prosternal plate; sbsIX/X, segmental border between abdominal sternites IX/X; sbtVIII/IX, segmental border between abdominal tergites VIII/IX; sI-sIX, abdominal sternites I-XI; st, stemmata; ta, tarsus; te, tentorium; ti, tibia; X, abdominal segment X; XI, abdominal segment XI. Pohl et al. (2018).

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