The Overall Scale

Scale insects have been the subjects of posts here twice before: in the first, I described their remarkable development, and in the second, I referred to the unusual genetics of some species. An appropriate next subject would, I suppose, be some of the ecological connections between scales and other animals.

Cochineal insects Dactylopius coccus on prickly pear, photographed by Joan Mundani.

Which starts, of course, with connections between scales and ourselves. Many scales are known as agricultural and horticultural pests, such as the red scale Aonidiella aurantii that attacks citrus. However, some scale species are not only welcomed but even deliberately cultivated due to commercial usage of the resins that they secrete. The two most significant commercial scales are the cochineal insects of the genus Dactylopius and the lac insect Kerria lacca. Other scale insects have also been used to produce similar products to those extracted from these species. Cochineal insects live on prickly pears, and produce carminic acid to ward off insect predators (though one predator, the caterpillar Laetilia coccidovora, is not only immune to the acid but stores it up to regurtitate at its own predators: Grimaldi & Engel 2005). Humans, on the other hand, are undeterred by carminic acid. The insects are collected, crushed, and the carminic acid extracted to produce the red dye cochineal, used (among other things) to give colour to food, or to dye fabric. It was an ill-fated attempt to establish a cochineal industry in Queensland that lead to the introduction of prickly pears to Australia: the plague-proportion spread of the prickly pears and their subsequent control by the moth Cactoblastis cactorum has become one of the textbook examples of biological pest control.

Branch covered with sticklac, produced by lac insects Kerria lacca, photographed by Jeffrey W. Lotz.

Lac insects produce a hard resinous shell for protection that, again, is their undoing in the eyes of humans. Sticklac, the twigs of trees covered by lac bugs, is harvested, then heated in canvas tubes. The resin melts and runs out through the canvas, leaving the wood and remaining insect parts behind. The resin is then processed to make the lacquer shellac. As a varnish, shellac has been mostly superseded by synthetic products, though it still has its afficionados. It is also used in the food industry to produce a shiny coating for confectionary or fruit.

Mating pair of the ant Acropyga epedana, photographed by Alex Wild. The queen is carrying the mealybug which while found the stockline for her new colony.

The use of scale products by humans has a long history. The Indian epic Mahabharata, believed written about the 8th century BC, describes the Lakshagriha, a highly flammable palace built by the Kaurava family out of shellac, jute and ghee in which they hoped to trap their enemies of the Pandava family (the Pandavas escaped through a tunnel when the palace burnt after having been warned by their uncle, though one wonders if the smell of ghee in the walls might have also aroused their suspicions). However, ants have been exploiting scale products for at least 40 million years, and probably much longer. Ants (like many other animals) are interested in scales for their honeydew, the excreted sugary waste from their sap diet. Ants not only collect the honeydew, they protect the scales from other insects and may carry them to fresher growth or more protected sites. Ants of the genus Acropyga are so dependent on mealybugs, waxy scale insects of the family Pseudococcidae, that when a young queen leaves her parent nest to mate, she will carry a mealybug with her so that her new colony can maintain its own stock. She even mates while holding on to it, as seen in the photo above. Acropyga queens have even been found preserved in Dominican amber, still carrying their mealybugs (Grimaldi & Engel 2005).

REFERENCE

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

Gordians

Long-term followers of this site may recall this video, linked to over four years ago:

The animal emerging from the unfortunate cricket in the video is a Gordian or horsehair worm, Nematomorpha. Gordian worms spend most of their lives as internal parasites: either of insects (in the freshwater/terrestrial order Gordiida) or of shrimps and crabs (in the marine genus Nectonema). Of the two commonly used vernacular names for this group, 'Gordian worm' refers to the famed Gordian knot, and is derived from the appearance of mating tangles of these elongate animals. 'Horsehair worm' refers to the long-held belief (again, inspired by appearance) that the adult worms developed from horse hairs decaying in water. So persistent was this belief that Leidy felt compelled to report in 1870 on an attempt to generate horsehair worms by this method, explaining that, "I need hardly say that I looked at my horse-hairs for many months without having had the opportunity of seeing their vivification". He also scuttled the fear, which even Linnaeus had reported as fact, that a horsehair worm could inflict a nasty bite on anyone careless enough to handle one. In fact, Gordian worms (being internal parasites absorbing nutrients directly from the host when young and not feeding as adults) do not even possess a mouth. Instead, the males of many species possess a bifurcated tail end, used in copulation, that may have been mistaken for jaws. The complete absence of active feeding has the interesting side effect that adult Gordians may completely lack an internal bacterial flora (Hudson & Floate 2009).

Representative nematomorphs: Gordius (Gordiida) on the left and Nectonema on the right, from Biodidac.

The primary division within the Nematomorpha between the marine Nectonema and the terrestrial Gordiida is universally agreed upon. The two branches are ecologically, morphologically and molecularly divergent (Bleidorn et al. 2002). Adults of Nectonema have dorsal and ventral double rows of swimming bristles, while those of Gordiida lack bristles (except for, in some species, minute patches of bristles in front of the cloacal opening). Mature adults of Gordiida emerge from their insect host when the latter approaches or enters water. It has been suggested that the worm is able to cause its host to actively seek out water, but it seems more likely that the worm simply causes erratic but non-directional behaviour that may make the host more likely to come into contact with water than if it had remained in its preferred microhabitat (Thomas et al. 2002). Once the host does come close to water as a result of random movement, the worm may be able to induce a last suicidal jump; alternatively, it may simply be that the addled host does not recognise the water as dangerous and makes no attempt to avoid it.

Female Chordodes wrapped around a stick, laying a white egg string. Photo from the Hairworm Biodiversity Survey.

Once in the water, the adult Gordians will mate with any others present; when multiple adults emerge in close proximity, they may begin mating before they have even finished emerging from their host (Hanelt & Janovy 2004). The females lay their eggs in long strings: one female may lay nearly six million eggs, making them one of the potentially most fecund animals on the planet. The larvae that hatch from the eggs look nothing like their parents, being kind of sausage-shaped with an eversible, spiny proboscis. A larva will find itself an aquatic animal host such as an insect larva or mollusc to burrow into and form a cyst. If the aquatic secondary host is then eaten by a suitable terrestrial primary host (for instance, after an aquatic insect larva matures into a terrestrial adult), the cyst will hatch out and the Gordian will complete its development within the terrestrial host. The Gordian larva may also bypass the secondary host if a primary host drinks water containing Gordian larvae. The larva or mode of transmission of Nectonema remains unknown,but, as Nectonema adults live in the same habitat as their primary host, they probably do not require a secondary host.

Larva of Chordodes encased in a cyst, from the Hairworm Biodiversity Survey.

Phylogenetically, Gordians have usually been regarded as related to nematodes, with which they share a number of morphological features. However, a molecular analysis by Sørensen et al. (2008) suggested a relationship between Gordians and loriciferans (albeit with support that was not overwhelming). The Gordian larva (which has no equivalent in the direct-developing nematode life-cycle) does bear a vague resemblance to an adult loriciferan, though it is debatable whether the resemblance is more than superficial. Loriciferans have not appeared in many phylogenetic analyses to date, and further investigation is required to establish whether it is the adults or the larvae of the Gordians that hold the clues to their affinities.

REFERENCES

Bleidorn, C., A. Schmidt-Rhaesa & J. R. Garey. 2002. Systematic relationships of Nematomorpha based on molecular and morphological data. Invertebrate Biology 121 (4): 357-364.

Hanelt, B., & J. Janovy Jr. 2004. Untying a Gordian knot: the domestication and laboratory maintenance of a Gordian worm, Paragordius varius (Nematomorpha: Gordiida). Journal of Natural History 38: 939-950.

Hudson, A. J., & K. D. Floate. 2009. Further evidence for the absence of bacteria in horsehair worms (Nematomorpha: Gordiidae). Journal of Parasitology 95 (6): 1545-1547.

Leidy, J. 1870. The gordius, or hair-worm. The American Entomologist and Botanist 2 (7): 193-197.

Sørensen, M. V., M. B. Hebsgaard, I. Heiner, H. Glenner, E. Willerslev & R. M. Kristensen. 2008. New data from an enigmatic phylum: evidence from molecular sequence data supports a sister-group relationship between Loricifera and Nematomorpha. Journal of Zoological Systematics and Evolutionary Research 46 (3): 231-239.

Thomas, F., A. Schmidt-Rhaesa, G. Martin, C. Manu, P. Durand & F. Renaud. 2002. Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts? Journal of Evolutionary Biology 15 (3): 356-361.

Stars in the Pasture

Flowers of Stellaria graminea, photographed by Jiří Bohdal.

Last week, I described a situation where low morphological diversity disguised high chromosomal and species diversity. So it is fitting that, this week, I should be focusing on an almost opposite situation: high morphological and chromosomal diversity, but low species diversity.

Stellaria is a genus of weedy herbaceous plants native primarily to Northern Hemisphere temperate regions, though some species have been spread around the world by humans. Members of the genus are mostly characterised by deeply divided petals, so that at a glance a Stellaria flower may look to have twice as many petals as it really does. (It is notable in this regard, however, that Chinnappa & Morton (1984) recorded raising a single seedling of Stellaria longipes during breeding tests that produced flowers with entire rather than divided petals, a feature generally characteristic of other related genera.) Up to about 200 species have been assigned to the genus (Bittrich 1993) but, considering what has been established with the American S. longipes complex (read on), I would not be at all surprised if that number is somewhat inflated. A review of Chinese Stellaria by Wu (1991), for instance, reduced a third of the species treated to synonyms.

Stellaria longipes, photographed by Michael Shepard.

The issue, as already alluded, is that some of the widespread Stellaria species are extremely variable. A case study of variability in Stellaria has been conducted with members of the Holarctic S. longipes complex (Macdonald et al. 1988). Four taxa are currently recognised within this group: Stellaria longipes ssp. longipes and S. longifolia both have circumpolar distributions in North America and Eurasia, but S. longipes ssp. arenicola and S. porsildii are each known from only small ranges in North America (Chinnappa 1992). Stellaria longifolia and S. porsildii are diploid species with a chromosome number of 26; S. longipes is a polyploid species, possibly derived from the hybridisation of the two diploids.

Specimens attributed to Stellaria longipes show a huge variation in morphological characters: stem pubescence, growth habit, floral and fruit characteristics, etc. They also show a wide variation in chromosome numbers: individuals have been recorded with as few as 51 or as many as 107 chromosomes. You may be thinking that such variation must indicate that a number of species have been conflated under the name of 'S. longipes', and, indeed, a number of names have been based on variant forms of this complex. However, breeding tests conducted by Chinnapa & Morton (1984) failed to find any reproductive isolation between different morphs: all were fully interfertile. What is more, different morphs are not found in different ranges, supporting the cultivation results as a valid representation of what is happening in the wild. Despite their variability in morphology, the representatives of S. longipes form a single, rampantly outbreeding gene pool. Even the apparent question of chromosome variability becomes less significant when one considers that the majority of individuals have chromosome numbers of 52, 78 or 104: all multiples of 26. If you compare this to what we find in Rhogeessa, you can see that it is not chromosome number alone that is the controlling factor for interfertility. The important thing is the ability or otherwise of the chromosomes to form associations during meiosis allowing their appropriate segregation to form viable gametes.

A slightly different form of Stellaria longipes, from here.

The only segregate of Stellaria longipes that appears to be truly distinct is the form known as S. longipes ssp. arenicola. This is a restricted range form known only from an area of sand dunes south of Lake Athabasca at the northern end of the border between Alberta and Saskatchewan (true S. longipes ssp. longipes is also found in the same area). Distinction appears to have arisen between S. l. ssp. longipes and S. l. ssp. arenicola as a result of a change in fertilisation regime for the latter. Stellaria species (including S. longipes) are mostly protandrous: the male parts of the flower mature before the female parts are ready to receive pollen. This means that a flower generally cannot fertilise itself, and individuals are generally outcrossing. Stellaria longipes ssp. arenicola has reversed this pattern: protandry is reduced, and individuals are more likely to be fertilised by themselves than by others. The nature of their sand dune habitat also means that plants are more likely to germinate successfully alongside another individual than in an isolated position. This results in a clumped and patchy distribution of individuals that also promotes fertilisation within a limited pool of individuals, as fertilising insects forage within a single patch rather than travelling between patches. Genetic analysis indicates that a limited amount of gene flow probably does still occur between sympatric S. longipes ssp. arenicola and S. l. ssp. longipes (Purdy et al. 1994); nevertheless, this outcrossing is on the order of the occasional dalliance rather than the shameless harlotry of their conspecifics elsewhere.

REFERENCES

Bittrich, V. 1993. Caryophyllaceae. In: Kubitzki, K., J. G. Rohwer & V. Bittrich (eds) The Families and Genera of Flowering Plants vol. 2. Flowering Plants: Dicotyledons: Magnoliid, hamamelid and caryophyllid families pp. 206-236. Springer.

Chinnappa, C. C. 1992. Stellaria porsildii, sp. nov., a new member of the S. longipes complex (Caryophyllaceae). Systematic Botany 17 (1): 29-32.

Chinnappa, C. C., & J. K. Morton. 1984. Studies on the Stellaria longipes complex (Caryophyllaceae)-biosystematics. Systematic Botany 9 (1): 60-73.

Macdonald, S. E., C. C. Chinnappa & D. M. Reid. 1988. Evolution of phenotypic plasticity in the Stellaria longipes complex: comparisons among cytotypes and habitats. Evolution 42 (5): 1036-1046.

Purdy, B. G., R. J. Bayer & S. E. Macdonald. 1994. Genetic variation, breeding system evolution, and conservation of the narrow sand dune endemic Stellaria arenicola and the widespread S. longipes (Caryophyllaceae). American Journal of Botany 81 (7): 904-911.

Wu Z.-Y. 1991. Some problems on the taxonomy of Chinese Stellaria. Acta Botanica Yunnanica 13 (4): 351-368.

From Three to Two

(I've been waiting three and a half years to use Neil's icon.)

The mysterious anabaritids of the Lower Cambrian have been referred to on this site before. In the earlier, somewhat brief post, I referred to their triradial structure and uncertain, though probably coelenterate-grade, relationships. In the time since that post appeared, the anabaritids have been the subject of a review by Kouchinsky et al. (2009) that brought together a lot of the previously scattered information on these animals.

The image of an anabaritid in the previous post showed the best known species, Anabarites trisulcatus. However, this was not the only species in the group. The image just above, from Kouchinsky et al. (2009), shows another species, Anabarites biplicatus, recorded from the Siberian Platform. This species differs from A. trisulcatus in that it started out life triradial (albeit with the internal dividing ridges between the lobes only weak), but as it grew it lost its triradiality and became more bilateral (cross-section below from Kouchinsky et al.):

Some of my readers may remember that Cambrian problematica were something of a cause célèbre during the mid-90s when a lot of journals and magazines ran features on them (probably inspired to a certain degree by Stephen Jay Gould's somewhat dreadful book Wonderful Life). In the more academic corners of this pageant, the triradiality of anabaritids (as well as some other early animals such as Tribrachidium) garnered them a certain degree of attention. It was suggested by some that they might represent a unique animal lineage that was eventually superseded by our own bilateral dynasty. However, the changing symmetry of Anabarites biplicatus serves as a reminder that we should not be too hasty to assign great significance to such features. Indeed, in the modern fauna, nematodes are partially triradial (they have a triradial head structure, with one upper and two lower lips around the mouth). Though the affinities of anabaritids are somewhat debatable, the most popular scenario is that they are related to cnidarians: their tubes bear a certain resemblance to the polyps of some medusozoans. Cnidarians also exhibit a wide variety of symmetries, such as the tetraradial organisation of scyphozoans and the hexaradial organisation of hexacorals. Without preserved soft tissue to inform us what the organisation of the inhabitant animal may have been, it is difficult to say just how much weight the triradiality of the anabaritid tube should be given.

REFERENCE

Kouchinsky, A., S. Bengtson, W. Feng, R. Kutygin & A. Val'kov. 2009. The Lower Cambrian fossil anabaritids: affinities, occurrences and systematics. Journal of Systematic Palaeontology 7 (3): 241-298.

Little Yellow Bats

Specimen of an unidentified Rhogeessa, photographed by Kate Comis.
The Neotropical members of the genus Rhogeessa go by the incredibly imaginative vernacular name of 'little yellow bats'. On the face of it, this would seem to sum up the salient features of these animals pretty succinctly: they're small, they're yellow, and they're bats. Eleven species of Rhogeessa were recognised by Baird et al. (2008a), found from northern Mexico to southern Brazil. The species of Rhogeessa are rather difficult to distinguish from each other; in particular, the six species that referred by Baird et al. (2008a) to the 'Rhogeessa tumida complex' are all but indistinguishable. Nevertheless, recent authors have regarded them as good species, and it is because of the reasons why this is that the genus has attracted the most interest.

The only really reliable way to distinguish the species of the R. tumida complex is to take a look at their chromosomes. Despite their external similarity, the species have different chromosome numbers and arrangements from each other. This forms an interesting contrast to other bat genera, which may have more morphological variability but little chromosome variation. Comparison between Rhogeessa chromosomes has lead to the suggestion that the various species may have diverged as a result of a process called Robertsonian translocation.

Specimen of the Yucatan yellow bat Rhogeessa aeneus photographed by Alex Borisenko.

To explain Robertsonian translocation, I have to indulge in a bit of background terminology of chromosomes (skip this if you know all this stuff). Think of the classic picture of an X-shaped chromosome (this is actually a doubled chromosome that develops during cell division: two copies, called chromatids, have been produced of the chromosome that will be separated when the cell divides). The point where the two chromatids are joined is a region of the chromosome called the centromere: it provides an attachment point for the spindle fibres that will draw each individual chromatid apart. The centromere is not always positioned at the midpoint of the chromosome: those chromosomes in which it is are called metacentric, while other acrocentric chromosomes have the centromere close to one end so the conjoined chromatids look closer to V-shaped than X-shaped.

Translocation is a process where a piece of genetic material breaks off one chromosome and becomes attached to another. Robertsonian translocation is a particular type of translocation where two acrocentric chromosomes, by breaking at the centromeres, effectively become fused to form a single metacentric chromosome (as shown in the diagram below from here):

Robertsonian translocation has been observed in many species; it even happens occasionally in humans. Where it becomes interesting for evolutionary studies is that the resulting metacentric chromosome continues to function in the same manner as the original acrocentric chromosomes, with little or no negative effects (the short bit from each acrocentric chromosome that is lost rarely contains any functioning genes). The individual carrying the fused chromosome even remains fertile, because when meiosis occurs in any individual with both the fused and unfused chromosomes, the two unfused chromosomes will each line up with their matching arm on the fused chromosome. However, imagine a situation where one individual in a population experiences a Robertsonian translocation between two chromosomes (call them 1 and 2), but another individual has a translocation between one of those chromosomes and another chromosome (say, 2 and 3). The individual that carries chromosomes 1-2 and 3 will produce fertile offspring if mating with an unfused individual, as will that carrying 1 and 2-3. However, if the 1-2 individual mates with the 2-3, their offspring will carry both fused chromosomes. Because these chromosomes and the unfused 1 and 3 cannot easily match up in a way that allows them to be separated effectively during meiosis, the hybrid offspring will have significantly reduced fertility. This has been practically shown to be the case between Robertsonian races of mice (Capanna et al. 1977). If the two fused chromosomes each become more predominant in a population than the original unfused chromosomes (either by drift or hitchhiking), then gene flow will be slowed or stopped between individuals carrying one or the other. Hey presto, speciation!

Speciation as a result of Robertsonian translocation also provides a counter-example to those who, when objecting to the taxonomic recognition of 'cryptic' species, raise the Biological Species Concept to defend their viewpoint. Contrary to popular assumption, there is no essential correlation between speciation and morphological divergence. Even under the Biological Species Concept, two populations may be good species (i.e. non-interfertile) and yet morphologically indistinguishable.

Allen's yellow bat Rhogeessa alleni, photographed by Merlin Tuttle. Rhogeessa alleni is the most morphologically distinct species of Rhogeessa, and has been included in a separate genus or subgenus Baeodon. Baird et al. (2008a), on the basis of molecular phylogeny, recognised the subgenus Baeodon but also including R. alleni's sister species, R. gracilis.

Anyway, I have a vague memory that somewhere along the line I was talking about bats. The known karyotype numbers for Rhogeessa vary from 30 (in three species: R. alleni, R. gracilis and R. io) to 52 (in a specimen from Suriname that Baker et al. 1985 assigned to R. tumida but which almost certainly represents an undescribed species). The phylogeny for the genus that was recovered by Baird et al. (2008a, b) could be consistent with both Robertsonian fusions and fissions taking place during the genus' history. In an attempt to test whether the different karyotypes truly function as isolating mechanisms (and hence whether the chromosomal 'species' are actually species), Baird et al. (2008b) could only find genetic indicators of possible recent hybridisation between the two apparently least divergent species, R. tumida (34 chromosomes) and R. aeneus (32 chromosomes); all other species maintained reciprocal monophyly in each of the three gene types (mitochondrial, Y-chromosome and somatic chromosome) tested. Baker et al. (1985) referred to another 32-chromosome karyotype ('32N') that differed from R. aeneus (assuming, on the basis of geography, that R. aeneus corresponds to Baker et al.'s '32B') in terms of exactly which chromosomes had been fused and so would be reproductively incompatible with R. aeneus. However, this 32N form appears to be assigned by Roots & Baker (2007) to R. io, otherwise with 30 chromosomes. As 30-chromosome R. io and the 32N form differ only in a single pair fusion in the former (Baker et al. 1985), they would probably remain interfertile by the principles described earlier. In contrast, despite their apparent difference in chromosome number of only two, R. tumida and R. aeneus actually differ in five chromosome fusions (three on one side, two on the other), meaning their interfertility should be considerably lower.

And if you've gotten this far and you're still not sick of bats, Darren Naish covered Rhogeessa and its relatives as part of his mammoth series on vesper bats earlier this year.

REFERENCES

Baird, A. B., D. M. Hillis, J. C. Patton & J. W. Bickham. 2008a. Evolutionary history of the genus Rhogeessa (Chiroptera: Vespertilionidae) as revealed by mitochondrial gene sequences. Journal of Mammalogy 89 (3): 744-754.

Baird, A. B., D. M. Hillis, J. C. Patton & J. W. Bickham. 2008b. Speciation by monobrachial centric fusions: A test of the model using nuclear DNA sequences from the bat genus Rhogeessa. Molecular Phylogenetics and Evolution 50 (2): 256-267.

Baker, R. J., J. W. Bickham & M. L. Arnold. 1985. Chromosomal evolution in Rhogeessa (Chiroptera: Vespertilionidae): possible speciation by centric fusions. Evolution 39 (2): 233-243.

Capanna, E., M. V. Civitelli & M. Cristaldi. 1977. Chromosomal rearrangement, reproductive isolation and speciation in mammals. The case of Mus musculus. Bolletino di Zoologia 44 (3): 213-246.

Roots, E. H., & R. J. Baker. 2007. Rhogeessa parvula. Mammalian Species 804: 1-4.

The Beak-Shells' Legacy

Specimen of Conocardium japonicum, from here.

The Rostroconchia were a group of molluscs that lived during the Palaeozoic, being definitely found from the Late Cambrian to the latest Permian. The name means, roughly, 'beak-shell', and refers to the shape of the shell. Similar to bivalves (with which rostroconchs were classified prior to the 1970s), rostroconchs had a shell divided into left- and right-hand valves, which in many species were elongate in one or both directions forewards and backwards. The shells of different species gaped to varying degrees at the posterior and anterior ends. Rostroconchs differed from bivalves, however, in that the original larval shell was not divided. Instead, the shell of a rostroconch was initially cap-shaped and entire. As the animal developed past the larval stage, the shell developed lateral lobes that would eventually become the two valves. As a result, rostroconchs did not have a toothed hinge connecting the valves like that of bivalves. Nevertheless, in many species, the pressure of the valves growing outwards eventually caused the larval shell to break through (Pojeta & Runnegar 1976). Like many bivalves, rostroconchs would have been infaunal, living buried in the sediment; however, the absence of a toothed hinge or the adductor muscles that connect the valves in bivalves means that the shell of rostroconchs would have been less flexible.

The Ordovician scaphopod Rhytiodentalium (topmost image) and the scaphopod-like rostroconch Pinnocaris (lower two images), from here.

The largest work to date on rostroconchs was that of Pojeta & Runnegar (1976), who suggested that rostroconchs included the ancestors of two living classes of molluscs, the bivalves and the scaphopods. Scaphopods, tusk-shells, are a group of tubular molluscs with an opening at each end of the tube that also live buried in sediment. According to Pojeta & Runnegar's suggestion, the bivalves evolved through the evolution of two calcification centres in the larval stage. The scaphopods would have evolved through the fusion of the valves along the ventral margin (as embryological studies have shown the scaphopod shell does develop), together with the restriction of growth to the anterior direction only.

However, more recent studies of molluscan phylogeny (Wilson et al. 2010) have mostly indicated that bivalves and scaphopods are not closely related relative to other molluscs. Bivalves are probably a relatively basally derived group, while scaphopods are closer to cephalopods. The upshot of this for rostroconchs is that they may be related to bivalves or scaphopods, but not to both. Of the two, it seems more likely that rostroconchs are related to scaphopods (or, as some authors have put it more bluntly, scaphopods are living rostroconchs). As well as the anatomical arguments that have been made in favour of such a relationships, a scaphopod connection has the advantage over a bivalve one of stratigraphy. The oldest definite rostroconchs, as previously noted, are known from the Late Cambrian. However, the oldest definite bivalves come from the Early Cambrian—that is, some 40 or 50 million years earlier than the first known rostroconchs. Scaphopods, on the other hand, do not appear until the Ordovician or Devonian (depending on whether the earlier forms are accepted as scaphopods), well after the known appearance of their supposed ancestors. Pojeta & Runnegar (1976) did not recognise a stratigraphic conflict in deriving bivalves from rostroconchs as they had identified the Early Cambrian Heraultipegma as a rostroconch. This identification, however, was disputed by MacKinnon (1985), who held that the characters cited by Pojeta & Runnegar in support of their assignation had been misinterpreted and were not truly present in Heraultipegma.

The conocardioid rostroconch Arceodomus longirostris, from here.

Pojeta & Runnegar (1976) recognised three orders of rostroconch: the Ribeirioida, Ischyrinioida and Conocardioida. This division has been followed by all subsequent authors, though it should be noted that Pojeta & Runnegar recognised the Ribeirioida as explicitly paraphyletic with regard to the other orders. Pojeta & Runnegar (1976) regarded the scaphopods as derived from ribeirioids (specifically related to the somewhat scaphopod-like Pinnocaris), but Peel (2004) stated that protoconch characters indicated a derivation from conocardioids. The ribeirioids and ischyrinioids both became extinct at the end of the Ordovician, leaving only the conocardioids and their descendent scaphopods until the former also became extinct at the end of the Palaeozoic.

REFERENCES

MacKinnon, D. I. 1985. New Zealand late Middle Cambrian molluscs and the origin of Rostroconchia and Bivalvia. Alcheringa 9 (1): 65-81.

Peel, J. S. 2004. Pinnocaris and the origin of scaphopods. Acta Palaeontologica Polonica 49 (4): 543-550.

Pojeta, J., Jr & B. Runnegar. 1976. The paleontology of rostroconch molluscs and the early history of the phylum Mollusca. U.S. Geological Survey Professional Paper 968: 1–88.

Wilson, N. G., G. W. Rouse & G. Giribet. 2010. Assessing the molluscan hypothesis Serialia (Monoplacophora + Polyplacophora) using novel molecular data. Molecular Phylogenetics and Evolution 54 (1): 187-193.

The Red-lined Wings of South America

So far as I've been able to find, what you see above is the only published illustration of the South American lacewing Belonopteryx arteriosa, from its original description by Gerstaecker (1863) (or very nearly from there: somewhat confusingly, the plate illustrating this animal was published an issue earlier than the article describing it). It is something of a pity that the drawing is not in colour, as Gerstaecker's description indicates a quite handsome animal: about 16 mm long with a ca 20 mm wingspan* with a mostly orange head, golden yellow body and blood red veins on the wings. The first segment of the antenna was yellow, the second darker, and the remainder black. Along the medial and secondary radial veins, the red colour of the veins extended to part of the cells on either side, producing two longitudinal red stripes on each wing.

*Gerstaecker gives the body length as 8 lines, with the front wings 9.5 lines long. A 'line' is a unit of measurement used by a number of 18th and 19th century biologists. The exact length of a line seems to have varied somewhat between countries (see this page for explanations), though it seems to have generally been a little more than 2 mm. Linnaeus apparently defined a line in the introduction to Philosophia Botanica as the length of a lunule (the white half-moon at the base of a fingernail) on any finger other than the thumb.

Despite being the type of the tribe Belonopterygini, Belonopteryx arteriosa seems to have received little attention since its description. Gerstaecker (1863) held only a single female specimen, reporting its collection locality (with a precision not uncommon for his time) as "Brazil". Freitas & Penny (2001) indicated that this species was known only from three specimens from Argentina (whether this indicates that Gerstaecker's locality was mistaken, or whether this is in addition to the original location, I couldn't say). The larva of B. arteriosa has never been identified, though larvae of related genera are associated with ant nests.

REFERENCES

Freitas, S. de, & N. D. Penny. 2001. The green lacewings (Neuroptera: Chrysopidae) of Brazilian agro-ecosystems. Proceedings of the California Academy of Sciences 52 (19): 245-395.

Gerstaecker, A. 1863. Ueber einige neue Planipennien aus den familien der Hemerobiiden und Panorpiden. Entomologische Zeitung 24 (4-6): 168-188 (plate in issue 1-3).