When Mayflies Last for Millions of Years

Mayfly Paraleptophlebia prisca preserved in amber, from Penney & Jepsen (2014).

Of all the media available for the preservation of fossils, none approaches perfection anywhere near as close as amber. There is little structural that amber does not preserve: external apperance, soft tissues, even cellular structure may potentially be examined. Amber offers us a window into the past unlike any other. The first amber deposits to go on the record (going back as far as the ancient Greeks), and the largest deposit yet discovered, was the Baltic amber of northern Europe, formed in the Eocene or Oligocene epoch* from the sap of a relative of the modern pines.

*Calculating the age of amber deposits is not easy. The amber itself cannot be dated directly (it is too old to be carbon-dated, of course, and it usually contains no mineral sediments that can be dated by other means) so aging it depends on indirect methods such as comparison of enclosed fossils with other aged samples, or dating of the deposits in which the amber is buried. Fossil comparisons suffer from the fact that the types of organisms that tend to be found preserved in amber are usually different from those preserved by other means (so we may know that two amber deposits are of similar age as each other, but we may still not know what that age actually is). Dating of surrounding deposits may be more straightforward, but is complicated by the fact that amber's relative buoyancy makes it prone to reworking (when a geological specimen becomes eroded out of its original formation and reburied in a younger one, thus making it appear younger than it is). Of course, if an amber deposit was produced over a long period of time, it may be impossible to tell if a particular piece comes from early or late in that period. Current consensus seems to indicate an Eocene age for the Baltic amber, but older references may refer to it as Oligocene or even Miocene.

Most of the insects we find preserved in Baltic amber are similar to those we find today (though differences in climate between now and then mean that the amber contains a number of groups that we would not expect to find so far north today). The specimen shown at the top of this post is assigned to a genus of mayfly (of the diverse family Leptophlebiidae) that remains widespread in Europe and North America today, of which it represents the earliest record. Like other members of the genus, it is a relatively small mayfly with a wingspan of less than 15 millimetres (I came across this fly-fishing website referring to the frustration of anglers trying to handle such small flies). Several specimens of this species are known from the Baltic amber, representing the subimaginal and imaginal stages of both sexes (mayflies are unique among living insects in that their wings become functional before they are fully mature; when a mayfly first emerges from the water it is as a near-mature subimago, subsequently moulting to a fully mature imago). Presumably, like modern mayflies, Paraleptophlebia prisca emerged as synchronised swarms, many individuals of which may have found themselves landing on an unwisely chosen tree-trunk and trapped within weeping sap.

This species was first described in 1856 by F. J. Pictet-Baraban along with other species of 'Neuroptera' from Baltic amber (at the time, the order Neuroptera included a wide range of insects with relatively unspecialised wings such as mayflies, dragonflies, bark-lice and lacewings). Pictet assigned it to the genus Potamanthus, another Recent mayfly genus albeit one in a different family Potamanthidae. He did express some uncertainty about this placement, an uncertainty that was later borne out by Demoulin (1968) who re-identified it as Paraleptophlebia on the basis of wing venation and male genital characters. Subsequent authors have agreed with Demoulin's assessment, regarding the mayflies of today as little different from those you might have found flying about over 50 million years ago.

REFERENCES

Demoulin, G. 1968. Deuxième contribution à la connaissance des Éphéméroptères de l’ambre oligocène de la Baltique. Deutsche Entomologische Zeitschrift, N. F. 15 (1–3): 233–276.

Penney, D., & J. E. Jepson. 2014. Fossil Insects: An introduction to palaeoentomology. Siri Scientific Press: Manchester.

Pictet-Baraban, F. J., & H. Hagen. 1856. Die im Bernstein befindlichen Neuropteren der Vorwelt. In: Berendt, G. C. Die im Bernstein befindlichen organischen Reste der Vorwelt vol. 2 pp. 41–126. Nicolaischen Buchhandlung: Berlin.

Mayflies in their Spring

Armoured mayfly Baetisca obesa, photographed by Jason Neuswanger.

Mayflies have occasionally put in an appearance here at CoO, most notably in an earlier post where I explained how the one thing that everyone 'knows' about mayflies is simply not true. In this post, I thought that I'd look briefly at the fossil context of mayflies.

The basalmost relationships among insects have been subject to some discussion over the years, but the current majority view is probably that mayflies were the first of the living winged insect lineages to diverge from the rest. Evidence for this is their retention of some plesiomorphic features such as the presence of three caudal filaments at the end of the abdomen, and a sliding rather than fixed inner mandibular articulation in the nymphs (adult mayflies don't have functional mouthparts). Mayfly nymphs, offhand, are known as naiads. Naiads were originally supposed to be nymphs that inhabited freshwater springs, so at some point the term 'naiad' was transferred from this:

Hylas and the Nymphs, by John William Waterhouse, in which our hero is fatally tempted by a septet of skinnydipping broads.
to this:

Drunella cornuta, photographed by Jonas Insinga.
Which I'm sure came as something of a disappointment to Hylas (though, of course, had Hylas been more disappointed, he may have also been less dead).

As discussed in an earlier post on stoneflies, there is some uncertainty whether aquatic nymphs are ancestral or derived for winged insects. However, mayflies were spending the first part of their lives in water by at least the Permian (Kluge & Sinitshenkova 2002; Grimaldi & Engel 2005). Representatives of the mayfly crown group (i.e. the group stemming from the most recent common ancestor of all living mayflies) are not known until the Jurassic; earlier species all belong to the stem group. The Carboniferous Syntonopterodea may also be stem-mayflies, but in superficial appearance the large, broad-winged syntonopterodeans may have looked more like the contemporary palaeodictyopteroids.

Reconstruction of Protereisma permianum, one of the best known of the Permian stem-mayflies, via here.

The Permian and Jurassic Ephemeroptera themselves had some notable differences from crown-group mayflies. Modern mayflies have heteronomous wings, with the fore- and hind wings differing in size (in some mayflies, the hind wings have almost disappeared entirely). Permian mayflies, in contrast, had homonomous wings, with the two pairs more or less identical; the hind wings became shortened in Triassic stem-mayflies (Grimaldi & Engel 2005). At least some stem-mayflies also retained well-developed mouthparts as adults; this suggests that they may well have lived longer as adults than modern mayflies. While Grimaldi & Engel (2005) included Permian and Triassic species in the Ephemeroptera, Staniczek et al. (2011) restricted that name to the crown group and its nearest and dearest, placing most of Grimaldi & Engel's stem-group 'Ephemeroptera' into an extinct clade Permoplectoptera.

REFERENCES

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Kluge, N. Yu., & N. D. Sinitshenkova. 2002. Order Ephemerida Latreille, 1810. The true mayflies (=Ephemeroptera Hyatt et Arms, 1891 (s. l.); =Euephemeroptera Kluge, 2000. In History of Insects (A. P. Rasnitsyn & D. L. J. Quicke, eds) pp. 89-97. Kluwer Academic Publishers: Dordrecht.

Staniczek, A. H., G. Bechly & R. J. Godunko. 2011. Coxoplectoptera, a new fossil order of Palaeoptera (Arthropoda: Insecta), with comments on the phylogeny of the stem group of mayflies (Ephemeroptera). Insect Systematics and Evolution 42: 101-138.

Everything You Knew About Mayflies is Wrong (Taxon of the Week: Pisciforma)

Mayfly nymph of the pisciform genus Baetis (Baetidae). Photo by Jan Benda.

Mayflies are one of the most basal, if not the most basal groups of winged insects alive today. The one thing that everyone knows about mayflies is that they only live for a day. This is reflected in their order name Ephemeroptera - "fleeting wing". But like so many other things that "everyone knows", this is complete twaddle. Mayflies do not live for a day. Indeed, by insect standards mayflies often have very long lives - many will live for an entire year, and some will live for as long as three years. The "fleeting" part of their lives is not their life as a whole, but only their time as adults.

Like dragonflies, mayflies spend the juvenile part of their life cycles as aquatic nymphs. They are easily distinguished from the aquatic nymphs of other insect orders by their three long caudal filaments and the leaf-like lateral gills that run down each side of their abdomens. Most mayfly nymphs are herbivorous or detritivorous, but a few are predatory. In contrast, adult mayflies have almost non-existent mouthparts, and do not feed - hence their "brief" lives. Mayflies are also unique in being the only living winged insects to undergo a moult after their wings develop. They first emerge from the water in a form known as the subimago, which soon moults into the final, fully mature imago. Mating usually happens in the imago stage, but there are some species that find themselves unable to wait that long, and begin mating as subimagoes (Grimaldi & Engel, 2005).


Siphlonurus quebecensis (Siphlonuridae), another pisciform nymph. Photo from Troutnut.com.

The contrast between mayflies' long juvenile lives and brief adult lives highlights a common prejudice to which almost all of us are prone - we tend to think of most animals in terms of their adult forms rather than their juvenile ones. There are a number of reasons for this. One is that we ourselves spend more of our life cycle (if we're lucky) as adults than juveniles. Another is that the adults are usually larger and more visible than the often reclusive juveniles, so we're more likely to notice them. Even working biologists may be prone to this trap - many animals can only be fully identified once they reach adulthood, which leads to the subconcious tendency to dismiss the unidentifiable juveniles as unimportant. But from an ecological perspective, it is the long-term juvenile mayflies that are far more significant than the brief adults. Adulthood in mayflies is simply a coda, a brief interlude to prepare for the next movement. Similarly, one could argue about how appropriate it is to characterise mayflies as flying insects when for so much of their lives they are not, with flight for them having no other purpose than to facilitate the reproductive process.


More Baetis nymphs. Photo by Surly Ghillie.

An influential classification of recent mayflies by McCafferty (1991) divided them into three suborders, Rectracheata, Setisura and Pisciforma, largely on the basis of their tracheal anatomy. Unfortunately, McCafferty did not explicitly refer to the characters on which he based the Pisciforma, other than noting that the name referred to the "minnow-like bodies and actions of the larvae". McCafferty (1991) regarded the Pisciforma and Setisura as forming a clade to the exclusion of the Rectracheata. In contrast, Kluge (2004) used an alternative classification* that more or less united McCafferty's Setisura and Rectracheata into a clade Bidentisetata, with two tooth-like setae on the maxilla (I say more or less because the families involved were not exactly the same), separate from the "Tridentisetata" (effectively McCafferty's original Pisciforma) with three tooth-like setae. Kluge explicitly stated, however, that his Tridentisetata was a taxon of convenience united by plesiomorphies only and probably paraphyletic with regard to the Bidentisetata. Recent molecular analyses have confirmed that the "Pisciforma" are paraphyletic or polyphyletic. Ogden & Whiting (2005) found both Rectracheata and Setisura nested within Pisciforma, and suggested that the fish-like nymphal form was plesiomorphic for all living mayflies and lost on numerous subsequent occasions.

*Very alternative, in fact. Kluge (2004) introduced a new system of nomenclature that attempts to provide an alternative to the rank-based system. Without wanting to go into too many details (for a start, that would require me to actually follow what's going on there, and frankly I haven't got a clue), Kluge's system involves a combination of a type genus plus a suffix indicating a taxon's position in the taxonomic hierarchy. So Arthropoda, for instance, is referred to by Kluge as Araneus/fg7. Seriously.


A male baetid moulting into an imago. One of the easiest differences to spot between the submature subimagoes and the fully mature imagoes is that in the subimagoes the wings are generally opaque, while the imagoes have transparent wings. Photo stolen from Mick Hall.

There is one little detail that I have to mention before ending this post. One characteristic of the wings of living mayflies as a whole is a significant difference in size between the fore- and hindwings, with the forewings significantly larger. This is taken to its extreme in one of the pisciform families, the Baetidae, which are one of the few groups of insects in which the hindwings have been almost entirely lost, reduced to small pair of buds like those of flies.

REFERENCES

Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

Kluge, N. 2004. The Phylogenetic System of Ephemeroptera. Springer.

McCafferty, W. P. 1991. Toward a phylogenetic classification of the Ephemeroptera (Insecta): a commentary on systematics. Annals of the Entomological Society of America 84 (4): 343-360.

Ogden, T. H., & M. F. Whiting. 2005. Phylogeny of Ephemeroptera (mayXies) based on molecular evidence. Molecular Phylogenetics and Evolution 37: 625-643.

What is the Sound of One Mayfly Fossilising?

Actually, two mayflies. That is, unless they're not mayflies.

Krzeminski, W. & C. Lombardo. 2001. New fossil Ephemeroptera and Coleoptera from the Ladinian (Middle Triassic) of Canton Ticino (Switzerland). Rivista Italiana de Paleontologia e Stratigrafia 107 (1): 69-78.

The Triassic is apparently not a fantastic time, insect-wise. Fossil insects from the Triassic are fairly few and far between. Needless to say, this is really annoying, because it was probably a fairly significant time in insect evolution. The giant insects of the Palaeozoic were no more - instead, it was about this time that many of the modern insect orders stepped in to take their place (Grimaldi & Engel, 2005). This unfortunately brief paper by Krzeminski & Lombardo (2001) gives us just a couple of pieces of the puzzle, but it's debatable just what we can do with them.

First, the maybe-mayfly. Krzeminski & Lombardo described Tintorina from two specimens (unfortunately, while the name Tintorina triassica appears in the abstract, the name used in the body of the article is Tintorina meridensis - I'm not sure, but I think the latter would be the correct name). The holotype retains most of the body (the head is missing), two of the wings and a few bits of leg. The paratype is just a pair of wings and a fragment of body. Though fragmentary, this collection does not put us too badly off. A large percentage of insect fossil species are only known from the wings, and the pattern of venation therein is hence the most commonly used suite of characters for distinguishing taxa. Krzeminski & Lombardo assign Tintorina to a new family of Ephemeroptera. They cite the wing venation and the general body shape as their reason for doing so, but unfortunately do not note exactly which features of the venation they refer to. Features such as the absence of a humeral vein (a small vein near the base of the wing) indicate that, if Tintorina is related to Ephemeroptera, it must lie outside the crown group. The only author to specifically comment on Tintorina since seems to be Kluge (2004), who concurred in the overall similarity of venation with Ephemeroptera, but also noted a couple of significant differences. As a result, Kluge moved Tintorina to Pterygota incertae sedis.

Next, the beetle. Coleoptera fossils are known since the Lower Permian, but carry problems all of their own. Almost the entire early fossil record of beetles is composed of isolated elytra, which offer few diagnostic characters and which may be suspected of rampant homoplasy (Ponomarenko, 2002). While representatives of recent families or their close relatives have been described from early on, a lot of doubt must hang over the accuracy of these identifications. A classic example is the Triassic family Obrieniidae, originally identified as the earliest representatives of the weevils (Curculionoidea), but now regarded as merely convergent (Kuschel, 2003). In the case of Krzeminski & Lombardo (2001), they assign a single elytron to the genus Notocupes in the family Cupedidae. The Cupedidae survive to this day - an example of a living species (Tenomerga mucida) is shown at the top of the post in a photo from Wikipedia. Generally found in rotten wood, the are one of the few survivors of the basal (paraphyletic?) beetle suborder Archostemata, making them very interesting in understanding beetle evolution. The genus Notocupes (recently regarded as a synonym of Zygadenia by Ponomarenko, 2006) is a fossil genus known from Triassic to the Palaeocene - a really quite spectacular length of time, and, in light of the problems I've just mentioned, really worth a further look.

REFERENCES

Kluge, N. 2004. The Phylogenetic System of Ephemeroptera. Springer.

Kuschel, G. 2003. Nemonychidae, Belidae, Brentidae (Insecta: Coleoptera: Curculionoidea). Fauna of New Zealand 45.

Ponomarenko, A. G. 2002. Superorder Scarabaeidea Laicharting, 1781. Order Coleoptera Linné, 1758. The beetles. In History of Insects (A. P. Rasnitsyn & D. L. J. Quicke, eds.) pp. 164-176. Kluwer Academic Publishers: Dordrecht.

Ponomarenko, A. G. 2006. [On the types of Mesozoic archostematan beetles (Insecta, Coleoptera, Archostemata) in the Natural History Museum, London]. Paleontologicheskii Zhurnal 2006 (1): 86-94 (transl. Paleontological Journal 40 (1): 90-99).