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.

The Prostigmata: Endless Forms

Water mite, possibly Piona coccinea, photographed by Roger Key.

Some groups are just so diverse that it is difficult just to know where to start in introducing them. My topic for today, the mites of the Prostigmata, are definitely one of those groups. Even though few would doubt the coherence of the Prostigmata, their morphological diversity is such that it is difficult to identify features that characterise them all. The majority are small and/or poorly sclerotised mites, but some species are extremely large (by mite standards, at least) and others are heavily armoured. The name 'Prostigmata' refers to the presence in many species of tracheae with the spiracle openings between the cheliceral bases, but many lack tracheae. In one group, the Heterostigmatina, the males and juveniles usually lack tracheae but adult females have tracheae with the spiracles placed at the front of the sides of the body, outside the chelicerae. Prostigmatans include predators, plant-feeders and parasites; their chelicerae, accordingly, may be pincer-like like those of other mite groups, or they may be variously modified. Many groups have the chelicerae fused into a puncturing stylophore; others have the mobile finger adapted into a protruding blade or stylet. The plant-feeding spider mites of the Tetranychoidea have the mobile fingers modified into long thin whips that can be retracted right back into the body, or extended to form the two halves of a sap-sucking tube. Even such features as the number of legs can't always be relied upon: while most Prostigmata have eight legs as is usual for arachnids (though, as with other mites, the fourth pair of legs only develops in the post-larval instars), the hind pairs are reduced or lost in a number of parasitic groups. The gall-forming plant mites of the Eriophyoidea have only four legs at the very front of the body, with an anal sucker at the end of the body to hold them in place.

The labidostomatid Sellnickia, from Macromite.

The phylogeny of the Prostigmata remains poorly known. Six major groups ('cohorts' or 'supercohorts') were recognised in the Prostigmata by Walter et al. (2009), but Dabert et al. (2010) found in their (admittedly somewhat preliminary) molecular analyses that relationships between the groups were not stable with regard to analysis method. These groups are the Labidostomatidae, Eupodides, Anystina, Parasitengonina, Raphignathina and Heterostigmatina. The Labidostomatidae are a small group of heavily armoured predatory mites with chelate chelicerae, found living on or in soil or leaf litter. The other groups, in contrast, are all more diverse.

Bdellid mite feeding on a psocodean, photographed by John J. Kent.

The Eupodides are mostly soft-bodied forms with striated integument. Most have a pair of specialised sensory setae called bothridia on the prodorsum, but these are missing in the Eriphyoidea and the marine Halacaroidea. The snout mites of the Bdelloidea are predatory mites with the chelicerae extended into an elongate proboscis; other members of the Eupodides include plant-feeders, fungivores and parasites.

Rake-legged mite Microcaeculus, photographed by Walter P. Pfiegler.

The Anystina are mostly predatory mites; some species of the families Caeculidae and Anystidae are relatively large, over a millimetre in length. Most Anystina, as well as members of the Parasitengonina and Raphignathina, have the pedipalp developed into what is called a 'thumb-claw process': the tarsus of the pedipalp is offset on the tibia, which has a terminal claw-like seta (sometimes more than one). The tibial 'claw' and the tarsus work together for grasping prey. The Caeculidae, rake-legged mites, are currently particular favourites of mine as my colleagues and I are currently in the process of preparing a description of a new species of one. These heavily sclerotised mites have a double ventral row of large spine-like setae on the forelegs; they sit in place with the forelegs raised until a springtail or some other small animal walks underneath them, at which point they drop the legs like a cage.

Trombidiid velvet mite taking down a micro-wasp, photographed by Jason Green.
The Parasitengonina are most notable for their complex life cycles, with parasitic larvae and free-living predatory adults. The group includes both terrestrial and aquatic species; the aquatic Hydrachnidiae are particularly diverse and often heavily armoured. Differences between larvae and adults are so great that taxonomists have often had no choice but to establish separate classifications for both, with relatively few larval 'species' as yet connected to their corresponding adults. Some of the terrestrial species are particularly large: velvet mites of the Trombidiidae may be over a centimetre in length.

Peacock mite Tuckerella, photographed by Christopher Pooley.
The Raphignathina are another ecologically diverse group: the Tetranychoidea are plant parasites, while other species are animal parasites or free-living predators. Raphignathina may be armoured or soft-bodied; the prodorsum lacks bothridial setae. Vertebrate-associated members of the Raphignathina include the Demodex mites that many people have peacefully living in their hair follicles. Other members of the Raphignathina include the Syringophilidae, bird parasites that live inside the quills of feathers, and the Cloacaridae that can be found in the mucous membranes of a turtle's cloaca.

Broad mite Polyphagotarsonemus latus, from here. Note the oddly stick-like modified hind legs, which are used by the male to carry larval females until they moult to maturity, as also done by the Scutacaridae.

The unusual tracheal system of the Heterostigmatina has already been referred to; this group also includes both free-living and parasitic species, with many species found in association with insects. Most species have a dorsal covering of sclerotised plates, and the palps are often greatly reduced. Heterostigmate mites described in previous posts are the Pygmephoroidea and Acarophenax.

REFERENCES

Dabert, M., W. Witalinski, A. Kazmierski, Z. Olszanowski & J. Dabert. 2010. Molecular phylogeny of acariform mites (Acari, Arachnida): strong conflict between phylogenetic signal and long-branch attraction artifacts. Molecular Phylogenetics and Evolution 56: 222-241.

Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 233-420. Texas Tech University Press.

Prototaxites Revisited

Reconstruction of Prototaxites by Richard Bizley, used with permission.

Richard Bizley has been kind enough to allow me to reproduce the above painting, which he produced in response to the discussion arising from an earlier post at this site. It shows a 'forest' (for want of a better word) of the enigmatic Silurian-Devonian organism Prototaxites reconstructed as a giant fungus. Richard has asked if anyone has any comments to make on the final product. Is this environment plausible? Could Prototaxites have grown in clusters like this, or would nutrient restrictions been such as to prevent such large organisms from persisting in close proximity to each other?

Since I produced my earlier post on the possible re-interpretation of Prototaxites as representing rolled ground-cover mats (Graham et al. 2010a), the proposal has been criticised in print by Boyce and Hotton (2010) and Taylor et al. (2010), and defended by Graham et al. (2010b). Boyce and Hotton regard it as taphonomically implausible that such rolls could form, while Taylor et al. also point out that the major tubes making up Prototaxites are arranged longitudinally down the 'trunk', not radiating outwards. Graham et al. have pointed out how they feel this is not incompatible with their liverwort mat hypothesis.

Colour-enhanced cross-section of Prototaxites specimen, from Graham et al. (2010b). Note that the 'growth rings' are not regularly concentric.

Prototaxites, it should be pointed out, was just one of a number of Silurian-Devonian organisms called nematophytes. Nematophytes are united by their similar internal structure, composed of hypha-like tubes. However, other nematophytes did not have the gigantic columnar form of Prototaxites: Nematothallus, for instance, was an encrusting lichen-like form, while Nematasketum fossils are only a couple of centimetres in size. Edwards and Axe (2012) have recently published a study on Nematasketum and supported comparisons between nematophytes and fungi. In particular, they compare Nematasketum to root-like anchoring and foraging structures called rhizomorphs produced by some large modern basidiomycetes. Hillier et al. (2008) nominated Prototaxites as potentially connected to root-like casts found in the Anglo-Welsh Old Red Sandstone, but admitted that the grounds for connection were slight.

REFERENCES

Boyce, C. K., & C. L. Hotton. 2010. Prototaxites was not a taphonomic artifact. American Journal of Botany 97 (7): 1073.

Edwards, D., & L. Axe. 2012. Evidence for a fungal affinity for Nematasketum, a close ally of Prototaxites. Botanical Journal of the Linnean Society 168: 1-18.

Graham, L. E., M. E. Cook, D. T. Hanson, K. B. Pigg & J. M. Graham. 2010a. Structural, physiological, and stable carbon isotopic evidence that the enigmatic Paleozoic fossil Prototaxites formed from rolled liverwort mats. American Journal of Botany 97 (2): 268-275.

Graham, L. E., M. E. Cook, D. T. Hanson, K. B. Pigg & J. M. Graham. 2010b. Rolled liverwort mats explain major Prototaxites features: response to commentaries. American Journal of Botany 97 (7): 1079-1086.

Hillier, R. D., D. Edwards & L. B. Morrissey. 2008. Sedimentological evidence for rooting structures in the Early Devonian Anglo-Welsh Basin (UK), with speculation on their producers. Palaeogeography, Palaeoclimatology, Palaeoecology 270 (3-4): 366-380.

Taylor, T. N., E. L. Taylor, A.-L. Decombeix, A. Schwendemann, R. Serbet, I. Escapa & M. Krings. 2010. The enigmatic Devonian fossil Prototaxites is not a rolled-up liverwort mat: comment on the paper by Graham et al. (AJB 97: 268–275). American Journal of Botany 97 (7): 1074-1078.