Raphidioptera, Navas, 1916

Archibald, S. Bruce & Makarkin, Vladimir N., 2021, Early Eocene snakeflies (Raphidioptera) of western North America from the Okanagan Highlands and Green River Formation, Zootaxa 4951 (1), pp. 41-79 : 70-72

Raphidioptera and the necessity of cold winter

It has been generally thought that extant snakeflies ( Inocelliidae and Raphidiidae ) require a cold interval to mature, explaining their restriction to mid- and higher latitudes of the Northern Hemisphere, reportedly in high, cooler elevations in the southern portions of their range ( Aspöck, 1998, 2000, 2002; Aspöck & Aspöck, 2009, 2014; Abbt et al., 2018). Aspöck (2000) stated that the cold interval is necessary to induce pupation and develop the imago, which must be 0° C or below. Abbt et al. (2018) later found that at least some can mature in the laboratory with a cold interval of 4°C.

This would account for their exclusion from the tropics. In North America, they are found west of the Rocky Mountains from about 53° N in southern British Columbia with some known from as far south as the Mexico-Guatemala border. In the Eastern Hemisphere, they range from Norway at 70° N as far south as parts of Mediterranean North Africa, with scattered records eastward through Asia to the Pacific, southward to the Himalayas, northern Thailand and Taiwan ( Aspöck, 1998; Aspöck et al., 2011, 2012a, b; Aspöck & Aspöck, 2014; Blades, 2019).

The fossil record of Raphidioptera before the Oligocene, especially of the extant families, would then present a biogeographic puzzle. The Eocene/Oligocene boundary represents the end of the globally warm “greenhouse world” climates of the Cretaceous through the Paleocene and Eocene, with higher mean annual temperatures (MAT), a low pole to equator MAT gradient, and lowered temperature seasonality with mild winters. The post-Eocene onset of the modern “icehouse world” regime saw extra-tropical climates with colder MAT, lacking Arctic ice sheets until the later Neogene and Antarctic ice sheets beginning to form at the end of the Eocene, an increased latitudinal MAT gradient, and more severe extra-tropical winters ( Zachos et al., 2008). Raphidioptera of the Paleocene and Eocene did not then experience cold winters, and in the Mesozoic lived under even warmer conditions, some in low latitudes, e.g., Brazil (e.g., Oswald, 1990) and Myanmar (e.g., Engel, 2002; Liu et al., 2016).

Agulla bicolor ( Albarda, 1891) from Texas was successfully raised in laboratory conditions at a constant temperature of 23 ± 3°C ( Kovarik et al., 1991). Raphidiidae of the North American genus Alena and Mexican inocelliid species apparently also don’t require a cold interval to mature, as these pupate in summer and the imago emerges in late summer ( Aspöck, 2002). At least two species of Harraphidia Steinmann in the Iberian Peninsula inhabit low elevation regions without cold winters, some at locations on the Mediterranean coast such as Tarifa, Spain, which has a coldest month mean temperature of 13.0°C and minimum temperature of 8°C ( Monserrat and Papenberg, 2006; AEMET, 2020). A cold interval is then not required for all modern snakeflies.

Inocelliidae and Raphidiidae of the Okanagan Highlands and Green River Formation are from the latter portion of the Ypresian, which had the hottest sustained global MAT values of the Cenozoic, the Early Eocene Climatic Optimum ( Zachos et al., 2008). While nearby coastal climates had hot MATs, the interior Okanagan Highlands localities bearing Raphidioptera were at some elevation, and had cooler, upper microthermal MAT values (microthermal = MAT ≤13°C), similar to that of south-coastal British Columbia today ( Rouse et al., 1970; Greenwood et al., 2005; Tribe, 2005; Archibald et al., 2011). These values would usually be associated with cold winters in the modern seasonal Northern Hemisphere, however, winters there appeared to be mild, without frost days; its forests included plants such as palms and their obligate palm feeding bruchids ( Coleoptera , Chrysomelidae , Pachymerina: Archibald et al. 2014 ), as well as other insects restricted to frost-free regions today, e.g., Megymenum Guérin-Méneville ( Hemiptera , Dinidoridae ), Mastotermitidae (Isoptera) , Myrmeciinae ( Hymenoptera , Formicidae ), and Diplopterinae ( Blattodea , Blaberidae ) (e.g., Archibald et al., 2014, 2018).

The regional tectonic uplift in the Ypresian that raised the Okanagan Highlands created a landscape of great topographic relief, with high mountains and deeply incised valleys in which the fossil-bearing lacustrine depositional basins formed ( Ewing, 1980; Tribe, 2005). Its snakefly fossils are in good general condition, having not suffered the extensive mechanical damage that would be consistent with post-mortem downslope transport from cooler elevations in streams feeding the depositional lakes. Nor can their presence be easily explained by occasional transport of living adults by winds from local higher elevations. The larvae of modern Raphidioptera live from one to six years, usually under bark, but some in litter below trees or bushes ( Aspöck, 2002). Most adults live for a very short while, often a few days, and are weak fliers with low vagility, remaining close to their resident tree or bush ( Aspöck, 1998). We believe that Okanagan Highlands snakeflies lived in close proximity to their depositional lakes.

The Okanagan Highlands communities also included a suite of other insects that today inhabit cooler boreal regions, higher elevations in lower latitudes, or mostly so. These include aphids ( Hemiptera, Aphidoidea ), scorpionflies ( Mecoptera , Panorpidae , species of Panorpa Linnaeus : Archibald et al., 2013b) and sawflies and wood wasps ( Hymenoptera, Symphyta : Tenthredinidae , Siricidae , Pamphiliidae , Cephidae : Archibald & Rasnitsyn, 2015; Archibald et al., 2018).

The Okanagan Highlands green lacewings ( Neuroptera , Chrysopidae ) consist of at least nine species of Nothochrysinae and one of the predominantly Mesozoic Limaiinae , it’s youngest occurrence. Today, Nothochrysinae mostly prefer Mediterranean climates of milder winters ( Archibald et al., 2014). Green lacewings of the Chrysopinae thrive in a wide range of climates, including regions of very cold winters. They appear in the fossil record after Okanagan Highlands time at the end of the Eocene, subsequently diversifying to vastly dominate the family today across most of the globe from the equator to northern Siberia in our post-Eocene icehouse world as the Nothochrysinae became relictual. Nymphinae ( Neuroptera , Nymphidae ), found at two Okanagan Highlands localities ( Archibald et al., 2009; Archibald & Makarkin, 2020) is today restricted to Australia, where it is less seasonal than at equivalent northern latitudes.

At the same time, snakeflies to the east of the Okanagan Highlands from the Green River Formation of Colorado inhabited much warmer lowland climates, with high mesothermal (mesothermal = MAT>13°, <20° C) to megathermal (MAT ≥20° C) MAT value estimates ranging from 19.6°C to 23.0°C (Wilf, 2000; Archibald et al., 2011 and references therein). Its forests also included frost-intolerant palms. Green River Raphidioptera probably inhabited similar or warmer temperatures than Harraphidia at Tarifa, Spain, where MAT is 17.2° C and had mild coldest months as above ( Monserrat & Papenberg, 2006; AEMET, 2020). Although the Green River Formation was deposited in a lowland intermontane basin, the abundance of snakefly fossils indicates that they were part of the local community, as river transport from a distant mountain source would have been a rare event. There are numerous rocks with more than one specimen, in one case, four ( Fig. 14A ). Further, almost all specimens in the UCM collection examined have a body with wings, i.e., are complete or at least relatively so. The wings may be folded in some, but in general, these do not appear to have suffered mechanical degradation characteristic of long-distance river transport. We believe that these also lived near their depositional lake.

A generalised climatic profile has been estimated for the Kishenehn formation by a nearest living relative analysis of mollusks ( Pierce & Constenius, 2001). These include taxa associated with both megathermal and microthermal climates. Pierce and Constenius rejected the notion that these coexisted, assuming the mixing of populations from different elevations. Such a mixed community is, however, consistent with a temperate climate with mild winters as in the Okanagan Highlands (above).

Global MAT declined through the remainder of the Eocene ( Zachos et al., 2008), but by the late Priabonian, close to the end of the greenhouse world, MAT estimates for upland Florissant, indicate an upper microthermal climate ( Allen et al., 2020) like those for the Okanagan Highlands, apparently also with no frost days by the presence of e.g., palms and Pachymerina ( Manchester, 2001; Archibald et al., 2014). The youngest occurrence of the otherwise Cretaceous raphidiopteran family Baissopteridae is at Florissant.

Therefore, all known Raphidioptera before the Oligocene, including Inocelliidae and Raphidiidae , inhabited regions of mild winters without cold intervals whether in hot or temperate climates. An adaptation to winter cold must have evolved independently in both extant families sometime after the Eocene/Oligocene boundary in the modern icehouse world climatic regime, with some members retaining or subsequently reverting to the plesiomorphic adaptation to warm winters.

After the onset of cold extra-tropical icehouse world winters, biota of latitudes outside of the tropics either went extinct (perhaps accounting for the extinctions of, e.g., the Limaiinae and Baissopteridae ), moved to or restrict their ranges to lower latitudes and Southern Hemisphere regions of mild winters (e.g., Nymphidae , palm bruchids, Dinidoridae , Mastotermitidae , Mymeciinae, and Diplopterinae ), or remained and adapted to more severe winters (e.g., aphids, Panorpa , Tenthredinidae , Siricidae , Pamphiliidae , Cephidae , Panorpidae , shift to dominance of the Chrysopinae within Chrysopidae ). Raphidioptera appears to have mostly followed the latter path.

Figures

FIGURE 14. Archiinocellia protomaculata (Engel, 2011).A, UCM 61231, rock with four specimens of A. protomaculata (a–c) and one undeterminable Raphidioptera (d); B, UCM 62523. Scale bars = 1 mm.