Field of Science

Flies on Stilts

Flies deserve a much better rep than they're usually given. They are animals of grace and poise that step lightly through the world. And perhaps few flies have an appearance that conveys that grace better than the stilt-legged flies of the Micropezidae. For today's post, I wanted to look at one particular subfamily of micropezids, the Taeniapterinae.

Scipopus sp., copyright Gail Hampshire.


Stilt-legged flies are found in most parts of the world but are particularly diverse in tropical regions. As their name indicates, they are light-bodied flies with notably long legs, the middle and hind legs being much longer than the fore legs. This legginess perhaps reaches its peak in the Madagascan genus Stiltissima, males of which have the hind femora alone at least 2.5 times the length of their thorax (Barraclough 1991). The adults are predators of small insects but are also attracted to decaying fruit or dung. Larvae of the family are little known but indications are that they feed on the aforementioned ordure or other rotting vegetation. Many of them are mimics of wasps such as ichneumons or ants with their slender figure resembling the narrow-waisted appearance of a wasp. Because micropezids belong to the brachyceran lineage of flies, in which the antennae are few-segmented and usually short, the front pair of legs is instead held out in front to imitate the wasp's antennae.

Habitus of Stiltissima violacea, from Barraclough (1991).


The Taeniapterinae are the most diverse of three subfamilies recognised within the Micropezidae. Distinctive features of this subfamily include ocelli sitting relatively forward on the top of the head, a dense vertical fan of bristles on the sternopleuron (the sclerite on the side of the thorax just between the base of the fore and middle legs) and a vestigial subscutellum (Jackson et al. 2015). Though cosmopolitan in distribution, and the only micropezid subfamily known from sub-Saharan Africa (Barraclough 1991; the only non-taeniapterines known from the Afrotropical region are restricted to the Mascarene islands), taeniapterines are most diverse in the Neotropical region.

Mesoconius dianthus contrasted with its ichneumon model Cryptopteryx, from Marshall (2015).


The Taeniapterinae have been divided into two tribes based on the length of the cup cell near the base of the fore wing, the short-celled Rainieriini and the long-celled Taeniapterini (Jackson et al. 2015). All taeniapterines found outside the Neotropical region belong to the Rainieriini, as well as a number of Neotropical genera. The Taeniapterini are restricted to the New World. Genera of Taeniapterinae are often poorly distinguished with the relationships between species obscured by the evolution of features related to mimicking their wasp models. A phylogenetic analysis of selected Taeniapterinae by Jackson et al. (2015) indicated many recognised genera were non-monophyletic. It also cast doubt on the tribal classification with the Taeniapterini rendering the Rainieriini paraphyletic.

REFERENCES

Barraclough, D. A. 1991. Review of the Madagascan Taeniapterinae (Diptera: Micropezidae), with the description of a remarkably elongate-legged new genus and first record of Rainieria Rondani from the subregion. Annals of the Natal Museum 32: 1–11.

Jackson, M. D., S. A. Marshall & J. H. Skevington. 2015. Molecular phylogeny of the Taeniapterini (Diptera: Micropezidae) using nuclear and mitochondrial DNA, with a reclassification of the genus Taeniaptera Macquart. Insect Systematics and Evolution 46: 411–430.

The Origin of Hexapods

Insects have been described as the most evolutionarily successful group of animals in the modern world, and with good reason. Something like two-thirds of the currently known animal species are insects, and they are near-ubiquitous in the terrestrial and freshwater environments (for whatever reasons, they've never made that much of a go of it marine-wise). Nevertheless, the questions of how and when insects first came to be remains very much an open one.

The long-necked fungus beetle Diatelium wallacei, one of the countless weird oddballs in the insect world. Copyright Artour Anker.


Insects are usually recognised as including three main subgroups: the winged insects, silverfish and bristletails. They are readily united into a group known as the hexapods with a few less speciose assemblages: the springtails, the proturans and the diplurans. All living hexapods have the body divided into a head, thorax and abdomen, with three pairs of walking legs on the thorax and none on the abdomen. Though monophyly of the hexapods has been questioned in the past (which is why the springtails and the like are usually excluded from our concept of 'insect' these days despite having been included previously), the majority view is now firmly in favour of regarding them as a single, coherent lineage. How hexapods are related to other arthropods has been more vigorously debated. Earlier authors commonly associated them with the myriapods, the lineage including centipedes and millipedes. In more recent years, an increasing number of studies have instead associated insects with crustaceans. This realignment has primarily been pushed by molecular studies but there are also a number of interesting morphological features such as eye and brain structure that are more crustacean- than myriapod-like in insects. Indeed, it seems not unlikely that insects are not merely related to but are nested within crustaceans: for instance, a few recent studies have supported a relationship between hexapods and a rare group of crustaceans known as remipedes (Schwentner et al. 2017). The features previously seen as shared between insects and myriapods, such as tracheae and uniramous (unbranched) limbs, are then held to probably be convergent adaptations to a terrestrial lifestyle.

Whatever its relationships, it seems most likely that the immediate ancestor of the living hexapods was indeed terrestrial. Of the six basal hexapod lineages referred to above, five (all except winged insects) are almost exclusively terrestrial and were probably ancestrally so. The winged insects include a number of basal subgroups (such as mayflies and dragonflies) that are aquatic for at least the early part of their life cycle, but a terrestrial origin for winged insects as a whole remains credible.

Head of Rhyniella praecursor, from Dunlop & Garwood (2017).


From the perspective of the fossil record, the evidence related to hexapod origins is incredibly slight. The earliest fossil species that have been directly proposed as hexapod relatives are known from the Early Devonian and less than half a dozen such species have been mooted as such in recent years. The only named Devonian fossil whose status as a hexapod seems unimpeachable is Rhyniella praecursor, a springtail from the Rhynie chert of Scotland (Dunlop & Garwood 2017). The same deposit provided Rhyniognatha hirsti, a fragmentary fossil comprising a pair of mandibles and surrounding parts of the head capsule. Rhyniognatha has long been thought to be an insect, possibly even an early member of the winged insect lineage, but Haug & Haug (2017) recently argued that it could just as easily be the head of a centipede (a group already known from other fossils in the Rhynie chert).

Rhyniognatha hirsti, from the University of Aberdeen. Scale bar = 200 µm; m = mandible.


The Windyfield chert, a deposit of similar age and location to the Rhynie chert, has provided Leverhulmia mariae, originally described as a myriapod but reinterpreted as a hexapod relative by Fayers & Trewin (2005). Leverhulmia is a difficult beast to know what and how much to make of it. The original specimen is, speaking charitably, a bit of a mess: a flattened smear looking a bit like a sausage burst open after cooking for too long on the pan. The front and back ends of the animal both appear to be missing and the only features really distinguishable are a series of small jointed legs. Other specimens associated with this species by Fayers & Trewin (2005) are simply more legs detached from their original body. These legs, though, do preserve a reasonable amount of detail, including the presence of paired lateral claws at the ends of the tarsi like those of most insects (Leverhulmia also possesses a smaller median claw between the lateral claws, a feature not found in winged insects but present in silverfish and bristletails). In contrast, the legs of myriapods (as well as those of springtails and proturans) end in a single terminal claw.

Holotype specimen of Leverhulmia mariae, from Dunlop & Garwood (2017); the size of the scale bar was not specified but the entire specimen is about 12 mm long.


The overall appearance of Leverhulmia's legs might therefore be seen a suggestive of a relationship specifically to insects and not just to hexapods in general, but their number provides something of a barrier to accepting Leverhulmia as a bona fide insect. The train-wreck nature of Leverhulmia's preservation means we can't state confidently how many legs it had but there were at least five pairs: a couple more than the hexapods' standard-issue three. A number of structures on the abdomens of some living hexapods are potentially derived from modified legs, such as the springing furca of springtails and the ventral styli in hexapods other than springtails and winged insects, so some parallelism in appendage reduction is not out of the question. Nevertheless, unjointed styli are one thing; fully-jointed, functional walking legs are another. Supposed early members of the bristletail and silverfish lineages with jointed abdominal legs have been described from the Carboniferous by Kukalová-Peck (1987) but (as I've noted before) many of the more outlandish reconstructions of early insects by Kukalová-Peck have failed to stand up to subsequent scrutiny.

Similar interpretative difficulties surround Strudiella devonica, described as an early relative of the winged insects from the Late Devonian of Belgium. Though I was not unfavourable to this specimen when it was first described, Hörnschemeyer et al. (2013) would later argue against recognising it as an insect. The latter authors professed to be simply unable to discern many of the features cited by its original describers as evidence of insect affinity, and saw Strudiella as closer to a Rorschach blot than a dragonfly. Strudiella's status was defended by its original authors (Garrouste et al. 2013) but a number of subsequent authors seem to have taken Hörnschemeyer et al.'s caution to heart.

Close-up of the head of Strudiella devonica from Hörnschemeyer et al. (2013); the asterisk marks the base of a structure originally interpreted as an antenna.


The final candidate for stem-hexapod status worthy of consideration here is Wingertshellicus backesi from the Lower Devonian Hunsrück Slate of Germany. This marine fossil was interpreted as a stem-hexapod under the name Devonohexapodus bocksbergensis, with a thorax bearing three pairs of legs and an elongate abdomen with uniramous appendages. However, it was reinterpreted by Kühl & Rust (2009) who synonymised Devonohexapodus with the previously described Wingertshellicus, regarded the previously described 'thoracic legs' as appendages of the head, and did not accept the presence of differentiated thorax and abdomen. The appendages of the trunk (previously seen as the abdomen) were biramous rather than uniramous with a small endopod and a large flap-like exopod adapted for swimming, and the end of the body bore a pair of fluke-like appendages (comparable to the tail of a crayfish). Wingertshellicus thus lacked any resemblance to a hexapod, and Kühl & Rust doubted that it even belonged to the crown group of arthropods.

Laterally preserved specimen of Wingertshellicus backesi, from Kühl & Rust (2009); scale bar = 10 mm.


An attempt to estimate the age of divergence of hexapods from other arthropods using a molecular clock analysis by Schwentner et al. (2017) suggested that hexapods and remipedes went their separate ways in the late Cambrian or early Ordovician. This is up to 100 million years earlier than the fossils described above but we should be careful how much to read into this discrepancy. If most of the features associated with hexapods are related to adoption of a terrestrial lifestyle, then it might be difficult to recognise any early marine relatives if found. Conversely, while it is uncertain how much if any terrestrial vegetation was present prior to the Devonian, the only potential cover would have been low lichens, non-vascular plants or micro-algae. If stem-hexapods emerged onto land during this time, the environment would not be conducive to their preservation in the fossil record. Finally, not only are hexapods other than winged insects not found in the fossil record before the Devonian, they are barely found after it: after Rhyniella, none are known until the appearance of amber-producing trees during the Cretaceous. So if we can't find any sign of them for some 300 milion years that we know that they are around, then we obviously can't say too much about not finding them over the previous hundred million years. The stem-hexapods may have been around in this time but they remain in hiding.

REFERENCES

Dunlop, J. A., & R. J. Garwood. 2017. Terrestrial invertebrates in the Rhynie chert ecosystem. Philosophical Transactions of the Royal Society of London Series B—Biological Sciences 373: 20160493.

Fayers, S. R., & N. H. Trewin. 2005. A hexapod from the Early Devonian Windyfield Chert, Rhynie, Scotland. Palaeontology 48 (5): 1117-1130.

Garrouste, R., G. Clément, P. Nel, M. S. Engel, P. Grandcolas, C. D'Haese, L. Lagebro, J. Denayer, P. Gueriau, P. Lafaite, S. Olive, C. Prestianni & A. Nel. 2013. Is Strudiella a Devonian insect? Garrouste et al. reply. Nature 494: E4–E5.

Haug, C., & J. T. Haug. 2017. The presumed oldest flying insect: more likely a myriapod? PeerJ 5: e3402.

Hörnschemeyer, T., J. T. Haug, O. Bethoux, R. G. Beutel, S. Charbonnier, T. A. Hegna, M. Koch, J. Rust, S. Wedmann, S. Bradler & R. Willmann. 2013. Is Strudiella a Devonian insect? Nature 494: E3–E4.

Kühl, G., & J. Rust. 2009. Devonohexapodus bocksbergensis is a synonym of Wingertshellicus backesi (Euarthropoda)—no evidence for marine hexapods living in the Devonian Hunsrück Sea. Organisms, Diversity & Evolution 9: 215–231.

Schwentner, M., D. J. Combosch, J. P. Nelson & G. Giribet. 2017. A phylogenomic solution to the origin of insects by resolving crustacean-hexapod relationships. Current Biology 27: 1818–1824.

The Psitteuteles Lorikeets

Varied lorikeet Psitteuteles versicolor, copyright Joshua Robertson.


Few groups of birds have been the object of human interest as much as parrots, with their striking coloration and intelligence inviting comment at least as far back as ancient Greek times. This interest has continued into recent times and scientific research into all aspects of parrot life has been extensive. Nevertheless, the classification of parrots has long been problematic. As a group, parrots combine a high degree of superficial disparity in features such as colour pattern with an underlying overall morphological conservatism (a not uncommon issue with birds). As such, though recognition of distinct species may be fairly straightforward, establishing the relationships between those species may be less so. Prior to the advent of molecular studies, few higher groups of parrots could be considered widely accepted. One such group was the lories, found in Australasia and the Pacific Islands (smaller members of this group are known as 'lorikeets' but, as with 'parrots' vs 'parakeets', the difference between the two is a question of size and shape rather than affinities). Members of this group evolved a long, narrow, brush-tipped tongue that allowed them to pursue a diet of nectar and pollen (Schweizer et al. 2015). About a dozen genera of lories are currently recognised: one such genus, Psitteuteles, is the subject of the current post.

Goldie's lorikeets Psitteuteles goldiei, copyright Ltshears.


Psitteuteles is commonly recognised to include three species of smaller lory: the varied lorikeet P. versicolor, the iris lorikeet P. iris and Goldie's lorikeet P. goldiei. In general, these are primarily green species with a red forehead and with varying amounts of blue across the back of the head and/or behind the eyes. The plumage is longitudinally streaked in the varied lorikeet and Goldie's lorikeet. Goldie's lorikeet has mauve cheeks whereas those of the varied lorikeet are partially yellow. The varied lorikeet is also mauve across the upper breast whereas the other two species are more evenly green. All three species are separated geographically: the varied lorikeet is widespread in northern Australia, Goldie's lorikeet is found in New Guinea and the iris lorikeet is found on the islands of Timor and Wetar in Indonesia. The varied lorikeet is particularly common in association with paperbarks and eucalypts around streams and waterholes, migrating as required to find trees in flower. Similar wandering habits are characteristic of Goldie's lorikeet which is mostly found in montane forest. The more sedentary iris lorikeet is mostly found in lowland monsoon forest. The varied and Goldie's lorikeets are not currently regarded as being of conservation concern but the iris lorikeet is more threatened by habitat loss and collection for the pet trade.

Iris lorikeet Psitteuteles iris, copyright Dick Daniels.


Not all authors have recognised Psitteuteles as a distinct group: some have included its species in the related genus Trichoglossus with the rainbow and scaly-breasted lorikeets. Recent phylogenetic studies suggest that suspicion of Psitteuteles' status may not be unwarranted. Molecular studies by Schweizer et al. (2015) and Provost et al. (2018) both fail to identify the three Psitteuteles species as forming a single clade. Instead, P. iris is placed close to Trichoglossus species whereas P. versicolor and P. goldiei are both placed outside a clade including Trichoglossus and related genera such as Eos, the red lories, and the musk lorikeet Glossopsitta concinna. A case could probably be made for restricting Psitteuteles to the varied lorikeet as type species while including the iris lorikeet in Trichoglossus. The fate of P. goldiei is more uncertain: though neither of the aforementioned studies identified P. versicolor and P. goldiei as sister species, it might be too early to exclude the possibility. Alternatively, should P. goldiei prove too phylogenetically isolated to include in any pre-existing genus, I am not aware of any available genus name for it. As seems to be one of my standard sign-offs on this site, further study is required.

REFERENCES

Provost, K. L., L. Joseph & B. T. Smith. 2018. Resolving a phylogenetic hypothesis for parrots: implications from systematics to conservation. Emu 118 (1): 7–21.

Schweizer, M., T. F. Wright, J. V. Peñalba, E. E. Schirtzinger & L. Joseph. 2015. Molecular phylogenetics suggests a New Guinean origin and frequent episodes of founder-event speciation in the nectarivorous lories and lorikeets (Aves: Psittaciformes). Molecular Phylogenetics and Evolution 90: 34–48.

Echinoids: Regularly Irregular

In manufacturing, one of the most desired qualities is regularity. Success is achieved by ensuring that each unit matches the last, that its qualities remain predictable and reliable. In evolution, by contrast, the opposite is often true: embracing irregularity may allow a lineage to expand in directions not previously available. For evidence, just look at the success of the irregular echinoids.

Echinoneus cyclostomus, one of the few living holectypoid urchins, copyright Philippe Bourjon.
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The Echinoidea, sea urchins, are commonly divided between regular and irregular forms. In regular echinoids, representing the ancestral type for the class, the mouth and anus are positioned at opposite points on the test. The mouth sits squarely in the centre of the animal's underside (the oral surface) while the anus sits at the centre of the upper (aboral) surface. The five ambulacra, the lines of small plates in the test from which the tube feet emerge, are more or less evenly arranged around the superficially radially symmetrical test. Irregular echinoids, in contrast, have the anus more or less displaced from the midpoint of the test. In the earliest irregular echinoids, this displacement might be relatively slight: the periproct (the membrane through which the anus opens, usually covered in echinoids with an array of small plates) was still found at the centre of the aboral surface but was enlarged and/or stretched towards one end of the test (Saucède et al. 2007). In more derived forms, the periproct has moved more significantly, potentially being found on the side of the test or even on the oral surface near the mouth.

Front view of heart urchin Spatangus purpureus, copyright Roberto Pillon.


This displacement of the anus indicates a directionality to the test that isn't found in regular echinoids. A number of other changes have associated it in the evolution of echinoids, such as reduction of the size of the spines covering the test and an increased directionality in their axes of movement. The mouth may also become displaced towards the front of the test, and the test as a whole may become more bilateral in its overall shape. The jaws become modified or, in a couple of groups, lost entirely. All these alterations add up to indicate a distinct change in lifestyle between regular and irregular echinoids. Whereas regular echinoids roam the surface of sea bottom, using their powerful jaws to graze directly on algae or scavenge on animal carcasses, irregular echinoids are deposit feeders that tend to live at least partially buried in the sidement. They may swallow large amounts of sediment and digest organic matter mixed therein, or gather up organic particles with their tube feet and/or by means of mucous strands transported in ciliary grooves. Burrowing is achieved by movement of the spines or by using the tube feet to pass sand grains above the aboral surface. In the shallow-burrowing heart urchin Spatangus purpureus, an array of longer spines on the aboral surface are used to keep a funnel open between the buried urchin and the surface, allowing water to carry oxygen to it. Echinocardium cordatum, which burrows as deep as 18 cm beneath the substrate surface, maintains an opening to the surface by means of elongate tube feet (Durham 1966).

One of the most irregular of irregular echinoids, the deep-sea Pourtalesia miranda, from Oliver (2016). The enlarged insert shows a symbiotic bivalve Syssitomya pourtalesiana.


The change in lifestyle was certainly a successful one: nearly 60% of living echinoids are irregular. The earliest irregular echinoids appeared in the early Jurassic, with recent analyses agreeing that they represent a monophyletic group (Saucède et al. 2007; Kroh & Smith 2010). Nevertheless, a certain degree of parallelism in adaptations appears to have been occurred. Living irregular echinoids can be divided between two clades: one is relictual, containing only two genera in the order Holectypoida, whereas the remaining species belong to the larger clade Microstomata. The earliest known members of the holectypoid lineage retained strong jaws even after they evolved the ability to burrow in sediment. In contrast, the earliest known member of the Microstomata retained large spines, indicating a non-burrowing lifestyle, but already possessed the adaptations for a particulate diet (Saucède et al. 2007). With time, both lineages developed the feature that they lacked, adding them together for a winning combination.

REFERENCES

Durham, J. W. 1966. Echinoids—ecology and paleoecology. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U257–U265. The Geological Society of America, Inc., and The University of Kansas Press.

Kroh, A., & A. B. Smith. 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Palaeontology 8 (2): 147–212.

Saucède, T., R. Mooi & B. David. 2007. Phylogeny and origin of Jurassic irregular echinoids (Echinodermata: Echinoidea). Geological Magazine 144 (2): 333–359.

A Slipper of the Lip

The world of flowering plants includes many unusual and eye-catching examples but even among all this variety the orchids often stand out. Their remarkable array of colours and forms have long fascinated people around the world. One of the more distinctive of orchid subgroups is the Cypripedioideae, commonly known as the slipper orchids.

Pink slipper orchids Cypripedium acaule, copyright Sasata.


Slipper orchids get their name from their most easily recognisable feature, a flower with a deeply saccate labellum or lip (the lower of the three petals) that is supposed to resemble a slipper (an analogy presumably settled on because the alternative of 'scrotum orchid' doesn't have the same ring to it). Like many other orchids, slipper orchids attract pollinators through deception rather than offering a genuine reward. Pollinators are enticed into entering the lip through its large central opening but find themselves unable to exit the same way (presumably because of the way that the rim of the opening curls inwards). Instead, they are forced to make their exit through one of two smaller openings at the base of the lip where it joins the flower's central column. As the pollinator exits this way, it must crawl past the stigma and stamens, removing any pollen it might already be carrying and depositing a new load.

Dwarf slipper orchid Cypripedium fargesii, copyright Steve Garvie.


The exact manner in which the pollinator is lured in varies by species and target (Pemberton 2013). Many produce odours that mimic legitimate nectar-producing flowers or potential food sources such as carrion. A group of species in the genus Cypripedium that are pollinated by bumble bees have low-growing flowers with a purple lip whose main opening appears black. They therefore resemble the opening of a mouse-hole of the type bumble bees use as nest sites. The North American Cypripedium fasciculatum produces a mushroom-like smell that attracts diapriid wasps that parasitise fungus gnats. Some species of the genus Paphiopedilum have light-coloured spots or warts on the flower that are mistaken for a colony of fat, healthy aphids by egg-laying hover flies seeking a food source for their larvae. Perhaps one of the oddest known set-ups is found in the species Cypripedium fargesii whose hover fly pollinator normally feeds on fungal spores. The orchid lures the fly in with patches of hairs on its leave that resemble a fungal infection. A few slipper orchid species are known to be habitually self-pollinating without the intervention of a pollinator; one such species, the South American Phragmipedium lindenii, has lost the slipper-shaped labellum and instead has a lip resembling the other petals.

Selenipedium dodsonii, a species only described as recently as 2015, copyright Andreas Kay.


Slipper orchids have been recognised as a distinct group from other orchids since at least 1840. A number of features isolate them from other orchids, such as their possession of two functional stamens (most other orchids have flowers with only a single stamen). More recent phylogenetic studies have corroborated their position as one of the earliest-diverging orchid lineages. Over 170 species of slipper orchid are currently known, divided by most authors between five genera; most of these genera have widely separated geographic ranges. The genera Selenipedium and Cypripedium have plicate leaves (that is, leaves that are folded within the bud several times longitudinally, in the manner of a fan) that are widely spaced along a well-developed stem, and a prominent rhizome (Rosso 1966). Selenipedium is a small genus found in northern South America that may reach heights of five metres. It differs from the more diverse Cypripedium in having trilocular ovaries and a commonly branching stem; Cypripedium, with over fifty species found across the Holarctic region, has unilocular ovaries and never branches. Cypripedium is the most widely distributed of the slipper orchid genera; the North American C. passerinum may even be found growing in tundra.

Paphiopedilum Leeanum, a cultivated hybrid originally developed in Britain in the 1880s, copyright David Eickhoff.


Phylogenetic analysis of the slipper orchids places Selenipedium as the sister group of the other genera with Cypripedium the next to diverge (Cox et al. 1997). The remaining three genera likely form a single clade united by the possession of a condensed rhizome and conduplicate leaves (folded once in the bud along the midline) arranged in a basal rosette. Paphiopedilum is the most speciose genus of slipper orchids with over ninety species found in India and southeastern Asia; it is also the genus most commonly found in cultivation. Phragmipedium includes over 25 species found in Central and South America; one of these, the Peruvian P. kovachii, has the largest known flowers of any slipper orchid, reaching twelve centimetres in diameter. The third genus Mexipedium, includes a single species M. xerophyticum found in Oaxaca state in Mexico. The three conduplicate-leaved genera are less distinct than the other two genera (one notable distinction is that Phragmipedium has trilocular ovaries whereas those of Paphiopedilum and Mexipedium are unilocular) and it has been suggested that they should be merged into a single genus. Nevertheless, not only are they all geographically distinct, they are supported as monophyletic by molecular analysis (Cox et al. 1997).

Phragmipedium caudatum, copyright Eric Hunt.


Their dramatic appearance has made slipper orchids highly prized in cultivation or by flower collectors. Unfortunately, many species have been subject to over-collection as a result. Many of the temperate Cypripedium species now require intensive conservation management, and populations of some Paphiopedilum species have been driven close to extinction. Once again, it would be a tragedy if such a fascinating group of plants was to vanish from the world.

REFERENCES

Cox, A. V., A. M. Pridgeon, V. A. Albert & M. W. Chase. 1997. Phylogenetics of the slipper orchids (Cypripedioideae, Orchidaceae): nuclear rDNA ITS sequences. Plant Systematics and Evolution 208: 197–223.

Pemberton, R. W. 2013. Pollination of slipper orchids (Cypripedioideae): a review. Lankesteriana 13 (1–2): 65–73.

Rosso, S. W. 1966. The vegetative anatomy of the Cypripedioideae (Orchidaceae). Journal of the Linnean Society, Botany 59: 309–341.

Hypsogastropods: Gastropods on High

Historically, the classification of molluscs has been a challenging prospect. Early researchers focused almost entirely on the shell which provided a somewhat limited range of characters with a definite possibility for convergence. Over time, more attention came to be paid to features of the soft anatomy but that required access to freshly collected material that might be difficult or impossible to obtain. As such, it has only been in the last few decades that a well-structured classification for many molluscan groups has begun to develop, and even now many significant uncertainties remain.

Common periwinkles Littorina littorea, a pretty typical hypsogastropod, copyright Fritz Geller-Grimm.


Until maybe the late 1990s, gastropods were primarily classified using a heavily grade-based system that was established in the 1930s. Gastropods were divided between three subclasses: the torted, gill-breathing prosobranchs, the untorted opisthobranchs, and the lung-breathing pulmonates. Prosobranchs were in turn divided into three main groups whose names directly reflected the 'level' of evolution at which they were supposed to sit: the archaeogastropods, the mesogastropods and the neogastropods. Many of these subdivisions were implicitly assumed to be ancestral to others. As the philosophical underpinnings of biological classification came to favour recognition of monophyletic taxa, it was obvious that such a system had to change. The prosobranchs and archaeogastropods both faded away as formal taxa. A major clade uniting the neogastropods and most of the mesogastropods came to be recognised as the caenogastropods. And while many questions still remain about relationships within the caenogastropods, most recent analyses have agreed in supporting a clade that was dubbed the Hypsogastropoda by Ponder & Lindberg (1997).

False cowrie Dentiovula dosruosa, copyright Nick Hobgood.


The prefix 'hypso-' means 'high' and was chosen because this clade corresponded to a group that had previously been known as the 'higher' caenogastropods (including the neogastropods and a fair chunk of the 'mesogastropods'). Hypsogastropods include many of the best known marine gastropods, such as whelks, periwinkles, moon snails, cones, cowries, conches and doubtless a ton of other things beginning with C (they also include freshwater and terrestrial forms but these are mostly minute and lack the public image of their marine relatives). They are ecologically diverse, including grazers, detritivores, filter feeders, predators and even parasites. The violet snails of the genus Janthina are planktonic, using a raft of bubbles to float on the water's surface so they can feed on Portuguese men-of-war. The similarly pelagic heteropods of the superfamily Pterotracheoidea have the foot extended and flattened to form a fin for active swimming.

Paraspermatozoon of violet snail Janthina, from Buckland-Nicks (1998). The arrow indicates the much smaller euspermatozoa attached to the tail.


Among the characters originally cited by Ponder & Lindberg (1997) as uniting the hypsogastropods were features of the spermatozoa. Most hypsogastropods have vermiform paraspermatozoa, sterile sperm cells that are released by the male together with the functioning euspermatozoa. The function of the paraspermatozoa seems to warrant further study. In some cases they may actively assist in the transport of the euspermatozoa; for instance, in violet snails a large number of euspermatozoa will be attached to a single super-sized paraspermatozoon able to swim harder and faster than any of the smaller cells could do on their own. In others, however, the two sperm cell types are not directly associated. It is possible that the paraspermatozoa act as a nuptial gift, providing nutrients to the female as a reward for mating, or that they somehow function to suppress sperm cells from any other males the female might made with (Buckland-Nicks 1998). Other synapomorphies of the clade include an external penis located behind the right cephalic tentacle, and statocysts (balance organs) each containing a single large statolith (Simone 2011).

Relationships within the Hypsogastropoda remain more poorly supported. Most researchers have agreed that the traditionally recognised neogastropods represent a clade united by numerous features, many of them related to the digestive system. The 'mesogastropods' included in the Hypsogastropoda mostly possess a taenioglossan radula with seven teeth in each row. In neogastropods, the number of teeth becomes more varied and the teeth themselves become modified so that the lateral teeth are strongly distinct in form from the central tooth. Some of these neogastropod modifications have been discussed in earlier posts on this site. A number of recent analyses have further associated the neogastropods with 'mesogastropod' taxa such as cowries and tun shells that they resemble in possessing an inhalent siphon forming a groove at the front of the shell (Simone 2011). A number of the remaining 'mesogastropods', such as the periwinkles of the Littorinidae and the Rissoidae, have been united by molecular analyses into a group that has been labelled the 'asiphonate clade' or the 'GC group' (the latter name chosen by Colgan et al., 2007, in reference to a particular genetic sequence motif). This clade is less universally recovered, however, and the scope for further investigation certainly remains.

REFERENCES

Buckland-Nicks, J. 1998. Prosobranch parasperm: sterile germ cells that promote paternity? Micron 29 (4): 267–280.

Colgan, D. J., W. F. Ponder, E. Beacham & J. Macaranas. 2007. Molecular phylogenetics of Caenogastropoda (Gastropoda: Mollusca). Molecular Phylogenetics and Evolution 42: 717–737.

Ponder, W. F., & D. R. Lindberg. 1997. Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. Zoological Journal of the Linnean Society 119: 83–265.

Simone, L. R. L. 2011. Phylogeny of the Caenogastropoda (Mollusca), based on comparative morphology. Arquivos de Zoologia 42 (4): 161–323.

Leptocaris: Living on the Edge

Some of the most remarkable faunal diversity in the marine environment is to be found in the interstitial spaces between grains of sand. Grazers, predators and scavengers can be found creating entire food webs at scales of less than one millimetre. The minute crustaceans known as copepods are among the more abundant inhabitants of the interstitial, and today's subject, Leptocaris, is among those interstitial copepods.

Dorsal habitus of female (left) and male Leptocaris ryukyuensis, from Song et al. (2012).


Leptocaris contains more than twenty-five species of extremely slender, cylindrical harpacticoid copepods growing to a bit over half a millimetre in length. Characteristic features of the genus include having the maxillipeds (one of the pairs of appendages making up the mouthparts) reduced or lost, and the proximal part of the endopod of the first swimming leg bearing a special anteriorly directed seta with a terminal comb (Song et al. 2012). Representatives of this genus have been collected from localities around the world though mostly in the Northern Hemisphere. Nevertheless, one can't help wondering how much of the genus' apparent rarity in the Southern Hemisphere is an artefact of low collection effort. This possibility should also be kept in mind when considering differences in the ranges of individual species: whereas many have only been collected from single localities (Song et al. 2012), the species L. trisetosus has been found from Finland to the Bahamas to South Africa, as well as in Korea with the last population being treated as a distinct subspecies (Lee & Chang 2008).

The majority of collections of Leptocaris have been from among sand but the genus has also been found in other microhabitats. In general, they are found in sediments with a high organic content. They are found in euryhaline and eurythermal habitats: that is, locations subject to wide variations in salinity and temperature. These may include beaches and brackish pools. They have been found among decomposing leaves in mangrove swamps (offhand, I haven't found if the diet of Leptocaris has been firmly established but I suspect they are probably detritivores). One species, L. kunzi, was described from an estuarine lake in Louisiana; another, L. stromatolicolus, is known from among stromatolites in Mexico. Two species, L. brevicornis and L. sibiricus, have even been found in continental fresh waters in Europe as well as in coastal brackish waters (Song et al. 2012). Overall, Leptocaris species seem to be most abundant in marginal habitats that may be too harsh and unstable for other copepods, making them fronteir harpacticoids.

REFERENCES

Lee, J. M., & C. Y. Chang. 2008. Copepods of the genus Leptocaris (Harpacticoida: Darcythompsoniidae) from salt marshes in South Korea. Korean Journal of Systematic Zoology 24 (1): 89–98.

Song, S. J., H.-U. Dahms & J. S. Khim. 2012. A review of Leptocaris including a description of L. ryukyuensis sp. nov. (Copepoda: Harpacticoida: Darcythompsoniidae). Journal of the Marine Biological Association of the United Kingdom 92 (5): 1073–1081.