Entomologia Generalis, Vol. 37 (2018), Issues 3–4, 197–230 Article Published in print July 2018 The Phenomenon of Metathetely, formerly known as Prothetely, in Raphidioptera (Insecta: Holometabola: Neuropterida)** Horst Aspöck1, Viktoria Abbt2, Ulrike Aspöck3,4 and Axel Gruppe2* 1 Institute of Specific Prophylaxis and Tropical Medicine, Medical Parasitology, Medical University of Vienna, Kinderspitalgasse 15, 1090 Vienna, Austria 2 Chair of Zoology – Entomology, Technical University of Munich (TUM), Hans-Carl- von-Carlowitz-Platz 2, 85354 Freising, Germany 3 Natural History Museum Vienna, Department of Entomology, Burgring 7, 1010 Vienna, Austria 4 Department of Integrative Zoology, University of Vienna, Althanstraße 14, 1090 Vienna, Austria * Corresponding author: [email protected] With 36 figures and 4 tables Abstract: For completion of their life cycle, most snakefly species require two years, some only one, and others (at least single specimens) three years or more. In most species, the larvae of the final stage hibernate in a state of quiescence, pupate in spring and emerge as adults shortly thereafter. Hibernation starts when the temperature decreases, thus inducing quiescence in the larva. If the temperature decrease is withheld during the last hibernation, the larvae remain active and usually continue to molt, but will not pupate successfully in spring. Moreover, most of them will die prematurely and prior to that will often develop considerable pathomor- phological alterations of the eyes, sometimes also the antennae, some develop wing pads and occasionally even pathomorphological modifications of the last abdominal segments. Until now, this phenomenon in Raphidioptera has been inaccurately referred to as “prothetely”; how- ever, in reality, it represents “metathetely”. The degree and duration of lower temperatures in winter that are required for a normal pupation after hibernation have been presumed to be different among the species. So far, no standardized experiments have been carried out to clarify this. Here, we report on results of chilling the final larval stage of three species – Phaeostigma (Ph.) notata (Fabricius), Raphidia (R.) mediterranea (H. Aspöck, U. Aspöck & Rausch), and Mongoloraphidia (M.) sororcula (H. Aspöck & U. Aspöck) – from +20 °C to +4 °C for 4, 8, 12, 16, and 20 weeks. As expected considerable differences between the species were found: M. sororcula, which occurs in a region with markedly continental climate with a very cold winter, requires the © 2018 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/entomologia/2018/0646 0171-8177/18/0646 $ 8.25 eschweizerbart_xxx 198 Horst Aspöck et al. longest period of chill for a normal metamorphosis. R. mediterranea, which occurs in Mediterranean areas with only short cold periods, needs only short periods of chilling for successful development and normal pupation. Ph. notata, which is distributed in large parts of extramediterranean Europe, necessitates a distinctly greater chilling than R. mediterranea, but less than M. sororcula. At any rate, a +4°C chilling for 20 weeks is sufficient to prevent metathetely in all species. Each metathetelously affected individual, even those from the same species, differs in some detail from all others. Presumably, these pathomorphological alterations are the result of an unsuccessful pupation, rather than an early appearance of imaginal characters. Thus, it is appro- priate to term the phenomenon “metathetely” rather than “prothetely”. The physiological back- ground of pathomorphological alterations due to withholding the chill is still entirely unknown. Various forms of metathetely in the three species are shown in 28 figures. The decrease of temperature as a precondition of pupation or – generally spoken – of normal and successful metamorphosis of snakeflies is convincingly correlated with the distribution of extant Raphidioptera in the world. Keywords: Agulla (A.) astuta (Banks, 1911), Agulla (A.) bicolor (Albarda, 1891), Agulla (A.) bractea (Carpenter, 1936), Mongoloraphidia (M.) sororcula (H. Aspöck & U. Aspöck, 1966), Phaeostigma (Ph.) notata (Fabricius, 1781), Phaeostigma (Magnoraphidia) major (Burmeister, 1839), Raphidia (R.) mediterranea H. Aspöck, U. Aspöck & Rausch, 1977, chilling duration, development, distribution, hibernation, larvae, pathomorphology, pupation, quiescence, rear- ing, temperature. **Dedication This publication is dedicated to the memory of Professor August Wilhelm Steffan (23/05/1933–03/11/2016), founder and former Editor-in-Chief of Entomologia Germanica, later Entomologia Generalis. He was famous and sometimes dreaded for his critical, meticulous and often painstaking scrutiny in editing manuscripts includ- ing the extensive usage of Latin terminology. Moreover, he was a convinced and also a convincing advocate of the use of German language in scientific publications. One of us (H.A.) had the honour to act as a guest editor of an issue of Entomologia Generalis devoted to German as a language in science. Over the years Prof. Steffan came to admit that English has become the lingua franca of our days, which enables scientists the world over to communicate more easily. Indeed, we have to be grateful for this international medium, nevertheless national languages must remain as valu- able options to publish scientific results. Two of us (H.A. and U.A.) had the pleasure of knowing Prof. Steffan personally; we had many substantive conversations with him and enjoyed his originality. Prof. Steffan was an outstanding figure in our branch of science, he will remain unforgettable. eschweizerbart_xxx The Phenomenon of Metathetely 199 1 Introduction 1.1 Raphidioptera: Overview Raphidioptera (snakeflies), the smallest order of Holometabola, comprises two fami- lies: Raphidiidae (with about 200 species) and Inocelliidae (with about 40 species) (H. Aspöck & U. Aspöck 2014b) (Figs. 1–6). The distributions of the two families in the world are similar: They comprise the arboreal parts of the Palaearctic region (includ- ing Northern parts of the Oriental realm at high altitudes), as well as arboreal parts of the west and southwest Nearctic region (including transition zones to the Neotropics at high elevations) (Fig. 7) (H. Aspöck & U. Aspöck 2012, 2013b, U. Aspöck et al. 1992, Haring et al. 2011). Snakeflies are lacking entirely in the Southern Hemisphere. Snakeflies are often apostrophized as “living fossils” since they had their greatest period in the Mesozoic, possibly with thousands of species and with a much larger dis- tribution comprising tropical regions and the Southern Hemisphere. It was hypothesized that the K/T-impact (when an asteroid of a diameter of about 10 km slammed into our planet causing a dramatic climatic change, which led to the extinction of numerous organisms) had extinguished the majority of Raphidioptera, particularly those in the Southern Hemisphere. The extant snakeflies are restricted to regions in the Northern Hemisphere characterized by a distinct decrease of temperature in winter. This seems to be necessary for the development of all snakeflies to the imago stage in both families (H. Aspöck 1998, 2000, H. Aspöck & U. Aspöck 2009, 2013b, 2014, H. Aspöck et al. 1991, 2012, 2014, U. Aspöck & H. Aspöck 2007, 2009, U. Aspöck et al. 2014b). The development from egg to imago lasts in most species for two years, in a few species (possibly in all Agulla species and several Palaearctic species) only one year, in many species, however, longer, up to several years. Even within a species (possibly even within a population of a species) there is a surprising variation in the length of development, since some individuals need only one year, some two, some more (H. Aspöck 2002, U. Aspöck et al. 1994, Gruppe & Abbt in press). During hibernation, larvae exhibit dormancy at low temperatures in the form of quiescence in the sense of Müller (1970, 1992). At a sufficiently decreased tempera- ture, the larva becomes inactive; however, if it is transferred to a higher (e.g. room) temperature – even from a temperature of −15 °C (or lower) – it will become active within seconds. In most cases the last overwintering stage is the final larval stage, and pupation occurs in spring. However, in a few genera (or clades) of Raphidiidae pupation takes place already in summer or autumn, yet in these cases a cold period is likewise necessary for the overwintering pupa to develop to the imago (H. Aspöck 2002, U. Aspöck et al. 1994). However, it is not known whether and how often larvae molt after the last hibernation and before pupation. If the final larval stage is not subjected to a decrease of temperature in winter, it will remain alive, be active, feed and molt, but after weeks or months it will molt to a peculiar larva with more or less markedly developed pupal characters. These larvae may molt again, however, sooner or later they die. This kind of disordered metamor- phosis has usually, although incorrectly, been termed “prothetely”. eschweizerbart_xxx 200 Horst Aspöck et al. Fig. 1. Phaostigma (Ph.) notata (Fabricius, 1781), male. Germany, Bavaria, Nürnberg, Stein, 9 May 2009. L. Weltner leg. (Photo: Leo Weltner, Nürnberg). Length of forewing: 11 mm. Fig. 2. Raphidia (R.) mediterranea H. Aspöck, U. Aspöck & Rausch, 1977, female. Austria, Upper Austria, Mühlviertel, Pelmberg near Hellmonsödt, 24 June 2014. H. & U. Aspöck leg. (Photo: Harald Bruckner, Vienna). Length of forewing: 9 mm. eschweizerbart_xxx The Phenomenon of Metathetely 201 Fig. 3.