Field of Science

Star-Grass

Common star-grass Hypoxis hirsuta, photographed by Merel R. Black.


It does not require a great deal of insight to understand why the plant pictured above has acquired the vernacular names of 'star-grass' or 'gold-star'. This small native of North America is one of the few representatives in that region of the family Hypoxidaceae, a group of about 150 species of thin-leaved monocots that is most diverse in the Southern Hemisphere, particularly in Africa. Most Hypoxidaceae are small like the North American gold-star, though the Asian hill coconut Curculigo latifolia may be over a metre in height. All grow from underground corms or rhizomes. Their flowers have the typical monocot arrangement of three sepals and three petals, and are most commonly yellow to pink in colouration. They mostly produce little scent (some have a faint sweet scent), and usually attract pollinators by offering pollen as a reward. Some species are grown as ornamentals, but for the most part the Hypoxidaceae are not that significant economically. The tubers of the African potato Hypoxis hemerocallidea (which, despite its vernacular name, does not seem to be eaten as a vegetable per se) have been used to make a medicinal tea; it has become particularly widely used in recent years to supposedly alleviate the symptoms of HIV, but tests of its actual efficacy remain in progress.

Lemba or hill coconut Curculigo latifolia (previously Molineria latifola), photographed by Ahmad Fuad Morad.


Recent authors have recognised up to ten genera within the Hypoxidaceae, but a phylogenetic analysis of the family by Kocyan et al. (2011) lead them to suggest the reduction of that number to four or six, depending on how one might chose to deal with the position of Hypoxidia. This is a distinctive genus of two species found on the Seychelles. Flowers of Hypoxidia are a dark red-brown colour, and in contrast to the weak scent of other Hypoxidaceae they have a strong foetid odour that attracts flies as pollinators. Kocyan et al.'s phylogenetic analysis placed Hypoxidia as sister to the other species of Hypoxidaceae found on the Seychelles, Curculigo seychellensis, and the two together were placed as sister to a clade containing the remaining species of Curculigo (as well as species of Molineria, which Kocyan et al. suggested be synonymised with Curculigo). Curculigo species bear their flowers at the base of the plant; the ovary is actually found beneath the ground, with the corolla borne above the ground on an elongate tubular rostrum. This rostrum is particularly long in C. seychellensis, up to 12 cm. Curculigo seychellensis also has bifurcated leaves that make it look superficially like a palm seedling. It remains to be settled whether future authors will prefer to place C. seychellensis in its own new genus, or to sink Hypoxidia into Curculigo.

Pauridia capensis (previously Spiloxene capensis), photographed by Bob Rutemoeller.


The remaining Hypoxidaceae can be divided between the genera Hypoxis (containing Rhodohypoxis as a junior synonym), Empodium and Pauridia (containing Spiloxene and Saniella as synonyms, as well as some Australian species previously placed in Hypoxis). Members of the genera Empodium and Pauridia produce annual corms, while Hypoxis species have tuberous rhizomes. Empodium and Pauridia differ in features of the flowers and seeds (Kocyan et al. 2011). The two African Pauridia species previously classified as Saniella resemble Curculigo in having subterranean ovaries. It is perhaps unfortunate that the name Pauridia, previously restricted to two particularly small African species only a couple of centimetres in height, takes priority over Spiloxene, previously used for a larger group of about thirty species. But such are the vagaries of nomenclature, and that which we now call Pauridia capensis (Snijman & Kocyan 2013) will smell as... generally indifferent, actually, but it at least looks pretty specky.

REFERENCES

Kocyan, A., D. A. Snijman, F. Forest, D. S. Devey, J. V. Freudenstein, J. Wiland-Szymańska, M. W. Chase & P. J. Rudall. 2011. Molecular phylogenetics of Hypoxidaceae—evidence from plastid DNA data and inferences on morphology and biogeography. Molecular Phylogenetics and Evolution 60 (1): 122-136.

Snijman, D. A., & A. Kocyan. 2013. The genus Pauridia (Hypoxidaceae) amplified to include Hypoxis sect. Ianthe, Saniella and Spiloxene, with revised nomenclature and typification. Phytotaxa 116 (1): 19-33.

Caulerpa: Sea Grapes, Feather Algae and Other Variations on a Tube

Invasive Caulerpa taxifolia in the Mediterranean, from here.


There is an observation (it has been dubbed 'Gorton's Law') that any discussion of any marine organism will invariably lead to someone asking whether it can be served with chips. In most cases, the answer will be some variation on 'yes', 'no', 'eww' or 'that's just stupid'. Sometimes, however, the answer will be 'it depends'.

Caulerpa is a genus of marine green algae found in tropical and subtropical waters around the world. Over seventy species of Caulerpa are currently recognised, with many further divided into bewildering arrays of subspecies, varieties and formae. At least two of these species, C. taxifolia and C. racemosa, have become notorious as invasives in the Mediterranean and nearby parts of the Atlantic. Apart from the closely related genus Caulerpella, it cannot really be mistaken for anything else (at least if you have a microscope). The thallus of Caulerpa takes the form of a long branching, creeping tube (the stolon) from which arise numerous tubular, flattened or globular fronds. A Caulerpa thallus can grow reasonably large: the stolon may be over three metres in length, with fronds several centimetres high. Despite this size, the thallus is not divided into individual cells: the multinucleate cytoplasm is freely connected throughout. Instead of cell divisions, branching ingrowths of the cell wall called trabeculae provide strengthening for the thallus. Large Caulerpa individuals are therefore one of the leading contenders for the title of 'world's largest single-celled organism', though I've noted many times that the concept of 'largest' doesn't mean much when talking about multinucleate structures that are indeterminate in size.

Caulerpa racemosa, photographed by Guillermo Diaz-Pulido.


The two genera Caulerpa and Caulerpella that share this unique cell structure (there are other tubular, non-cellular algae, but they lack the trabeculae) are distinguished by their reproductive morphology. Caulerpa usually reproduces vegetatively: older pieces of a thallus die, separating the growing tips, or pieces of the thallus break off and settle separately. When sexual reproduction does occur, Caulerpa fronds do not produce dedicated reproductive structures. Instead, the cytoplasm within an entire frond becomes divided into gametes that are released through slender papillae that grow on the frond surface. In the single species of Caulerpella, C. ambigua, fronds may bear specialised reproductive structures formed from a compound whorl of tightly-branched lobes lacking trabeculae (Price 2011).

Caulerpa nummularia, photographed by D. S. Littler.


Species and varieties are distinguished by the morphology of the thallus, using features such as the shape of the fronds (which may be simple tubes, or branched and feathery, or disc-shaped, or broad and flat, or globular and looking like clusters of grapes, or any number of further variations) and their manner of branching. However, many species may vary considerably in habit, and in some species forms described as separate varieties may later be found growing as separate sections of a single thallus. In a detailed study of variation in four species of Caulerpa found in the Philippines, involving comparisons of specimens collected in the wild, thalli cultured in the laboratory, and molecular data, de Senerpont Domis et al. (2003) found that three of the four species showed morphological variation consistent with their previous classification, but the fourth species (C. racemosa) varied to the extent that previously recognised 'varieties' could not be reliably distinguished.

Caulerpa prolifera, photographed by Elizabeth Lacey.


But to get back to the important question: can it be served with chips? It depends. Many Caulerpa are widely eaten, particularly in eastern Asia, and particularly those with globular fronds (which are referred to as 'sea grapes'). Nevertheless, Caulerpa species have also been referred to as toxic, particularly in relation to their noxious weed status in the Mediterranean (where their invasiveness has been attributed to the absence of suitable grazers). Toxicity of Caulerpa, it appears, can vary between taxa and possibly between seasons (the taste of edible varieties in the Philippines becomes more peppery during the rainy season). The identity of the toxic compound(s) in Caulerpa has been subject to debate: a number of candidates have been identified, but studies have disagreed on their effects and severity, and it remains uncertain whether Caulerpa poisoning poses a serious risk to humans (Higa & Kuniyoshi 2000). One of these candidates, caulerpicin, was tested by Doty & Aguilar-Santos (1966) using the straightforward method of feeding it to 'volunteers'. They recorded the results as follows:

Different people respond differently to caulerpicin. Some merely obtain a mild anaesthetizing sensation which is not immediate but is delayed for a minute or two. Others also obtain a numbness of the tongue or lips. In one subject exposed to the substance at various times truly toxic symptoms have become stronger and stronger following each contact. Almost immediately on chewing the raw dried Caulerpa material, the subject felt a numbness at the tip of the tongue. This has developed to a point at which the reaction is one of numbness of the extremities coupled with a cold sensation in the feet and fingers, rapid and difficult breathing, slight depression and, finally, loss of balance requiring the subject to lie down. The symptoms wear off, depending on the dosage, in a few hours to a day. Coupled with these reactions to the impure and pure substance, the same subject has developed a sensitivity to oysters and crabs and eating them produces the same symptoms.

It's the 'following each contact' bit that worries me. "Feeling better? Good. Have some more!"

REFERENCES

Doty, M. S., & G. Aguilar-Santos. 1966. Caulerpicin, a toxic constituent of Caulerpa. Nature 211: 990.

Higa, H., & M. Kuniyoshi. 2000. Toxins associated with medicinal and edible seaweeds. Toxin Reviews 19 (2): 119-137.

Price, I. R. 2011. A taxonomic revision of the marine green algal genera Caulerpa and Caulerpella (Chlorophyta, Caulerpaceae) in northern (tropical and subtropical) Australia. Australian Systematic Botany 24: 137-213.

de Senerpont Domis, L. N., P. Famà, A. J. Bartlett, W. F. Prud’homme van Reine, C. A. Espinosa & G. C. Trono Jr. 2003. Defining taxon boundaries in members of the morphologically and genetically plastic genus Caulerpa (Caulerpales, Chlorophyta). Journal of Phycology 39: 1019-1037.

Dragons in a Desolate Land

Ring-tailed dragon Ctenophorus caudicinctus, from here.


The comb-bearing dragons of the genus Ctenophorus are an assemblage of 28 (and counting!) species of medium-sized lizards found around Australia. Darren Naish has recently been giving an overview of the Australian dragons; you can read what he's already said about Ctenophorus here. I'd suggest reading that first, then coming back here.

Species of Ctenophorus are distinguished from other dragons by the presence of a row of tectiform (roof-shaped) scales running from behind the nostrils under the eyes, though in some species this row is only weakly pronounced (Melville et al. 2008). In most species, the tympanum (ear-drum) is exposed, though a few species have it covered over. I'm personally familiar with one species of Ctenophorus, the ring-tailed dragon C. caudicinctus. Where we've been doing fieldwork on Barrow Island, ring-tailed dragons are a common site perched on termite mounds or larger rocks, invariably just one dragon to a rock, monitoring the surrounding territory for food or mates. Not all Ctenophorus species engage in such behaviour: the species have been divided between three groups depending on whether they prefer rocky habitats, whether they prefer sandy habitats and use tufts of spinifex and other vegetation for cover, or whether they shelter in burrows. Phylogenetic analysis suggests that the burrowing habit was ancestral for the genus; rock-dwelling or scrub-dwelling habits may have each evolved more than once within Ctenophorus, though the possibility cannot be entirely ruled out that they may characterise monophyletic groups (Melville et al. 2001). These differences in ecology also correlate with morphological differences: rock-dwelling species have dorsoventrally flattened heads, while the scrub-dwelling species are long-legged and cursorial.

Military dragon Ctenophorus isolepis, a sand-dwelling species associated with spinifex, photographed by Stewart Macdonald.


While some species of Ctenophorus are widespread, others are far more restricted in range. Ctenophorus caudicinctus, for instance, is found across most of northern Western Australia and the Northern Territory, but Butler's dragon* C. butleri is restricted to coastal sand dunes between Shark Bay and Kalbarri in Western Australia (Cogger 2014). The most recently described species to date, the Barrier Range dragon C. mirrityana, is known from two locations about 100 km apart in western New South Wales (McLean et al. 2013). And it possibly does say something that new species continue to be described even in this not inconspicuous genus.

*Or should that be 'Butlers' dragon', as it was apparently named after both Harry and Margaret Butler?

Lake Eyre dragon Ctenophorus maculosus, photographed by Rune Midtgaard.


Perhaps the most hard-core of the comb-bearing dragons is the Lake Eyre dragon Ctenophorus maculosus, a specialised inhabitant of dry salt lakes in South Australia. This is a spectacularly harsh environment: searing hot sun, often at temperatures above 40°, beating down on a crust of crystallised salt. Few other animals can survive there without spontaneously combusting. The dragons protect themselves from the head by burrowing into the layer of unconsolidated sand beneath the salt-crust; Pedler & Neilly (2010) discovered one female with its head protruding from a burrow with an entrance too small for its body, and suggested that she must have gotten there by 'swimming' through the sand. The Lake Eyre dragons feed on ants such as Melophorus (themselves no slouch in the hard-core stakes) or other insects that have become stranded on the salt-pan. When the lake becomes filled with water (as it does about once a decade or so), the dragons are forced to flee into the habitats surrounding the lake shores and wait for the flood to clear. Two Western Australian species, the claypan dragon Ctenophorus salinarum and the Lake Disappointment dragon C. nguyarna, are also associated with salt-pans, but they do not have quite the level of specialisation of the Lake Eyre dragon.

REFERENCES

Cogger, H. G. 2014. Reptiles and Amphibians of Australia, 7th ed. CSIRO Publishing: Collingwood.

McLean, C. A., A. Moussalli, S. Sass & D. Stuart-Fox. 2013. Taxonomic assessment of the Ctenophorus decresii complex (Reptilia: Agamidae) reveals a new species of dragon lizard from western New South Wales. Records of the Australian Museum 65 (3): 51-63.

Melville, J., L. P. Shoo & P. Doughty. 2008. Phylogenetic relationships of the heath dragons (Rankinia adelaidensis and R. parviceps) from the south-western Australian biodiversity hotspot. Australian Journal of Zoology 56: 159–171.

Melville, J., J. A. Shulte II & A. Larson. 2001. A molecular phylogenetic study of ecological diversification in the Australian lizard genus Ctenophorus. Journal of Experimental Zoology 291: 339-353.

Pedler, R. & H. Neilly. 2010. A re-evaluation of the distribution and status of the Lake Eyre dragon (Ctenophorus maculosus): an endemic South Australian salt lake specialist. South Australian Naturalist 84 (1): 15-29.

Shock Me like an Electric Eel

Electric eel Electrophorus electricus, photographed by Stefan Köder.


The electric eel Electrophorus electricus is one of those animals that seem to border on the mythical. Most people will have come across some sort of reference to their existence, and may even have seen some sort of intended depiction of one in cartoon form. However, said depiction will probably bear little if any resemblance to a real-life electric eel. Most commonly, it will look more like a standard Anguilla eel, to which true electric eels are not close relatives. Instead, electric eels belong to a uniquely South and Central American group of fish, the Gymnotiformes.

The Gymnotiformes, commonly known as the Neotropical knife-fishes, are more closely related to catfish than they are to anguillid eels. They are characterised by an elongate body form, lacking the dorsal fin of other fish. The anus has been moved forward relative to other fish: in some gymnotiforms, the anus is actually in front of the pectoral fins, just behind the head. The anal fin that runs behind the anus has become greatly elongated, and instead of swimming by undulating the body from side to side like other fish, gymnotiforms swim by undulating the anal fin alone while the main body remains more or less rigid. This unusual swimming style is directly related to another distinctive feature of the gymnotiforms: their production of an electrical field. Many fish are able to passively sense electrical fields in the water: gymnotiforms take the next step and generate their own electrical field, which they use to sense their surrounding environment (Albert & Crampton 2005). As a result, they can live and hunt effectively at night and in turbid waters with poor visibility. They can also use their electrical fields for communication, signalling their moods and identities to other fish. The connection between electricity generation and swimming style is that, if gymnotiforms swam in the manner of other fish, their changes in body aspect would create changes in the shape of their electrical field. Holding the body more or less rigid means that the electrical field also remains constant, and any distortions must be caused by something external. Another group of fishes found in Africa and Asia that also navigates by electricity, the Notopteridae, has evolved a very similar appearance and swimming style to the gymnotiforms (and are also known as knife-fishes), but are entirely unrelated phylogenetically.

Tiger knife-fish Gymnotus tigre, from Trix.


The electric eel is something of an outlier among gymnotiforms. For a start, it's a monster: electric eels can be over two metres in length, while other gymnotiforms are all much smaller. The electric eel has also had a Susan Storm-style upgrade, and weaponised its electrosensory system. Electric eels can produce up to 600 volts of electricity, allowing them to stun reasonably large prey. The closest relatives of the electric eel are the banded knife-fishes of the genus Gymnotus; both are predators of fish and other aquatic animals. Males of at least some Gymnotus species and the electric eel build nests that the females lay their eggs into; males of Gymnotus carapo have been recorded to mouth-brood larvae.

The apteronotid Sternarchorhynchus mesensis, from here.


The remaining gymnotiforms were placed by Albert (2001) in a clade called the Sternopygoidei; these taxa have a smaller gape and feed on correspondingly smaller prey (some are planktivores). Two families, the Hypopomidae and Rhamphichthyidae, are united by the lack of teeth in the oral jaws; rhamphichthyids also have a very long and tubular snout. The other sternopygoids are placed in the families Sternopygidae and Apteronotidae; a distinctive feature uniting these two families is that they produce a wave- or tone-type electrical field instead of the pulse-type electrical field of other gymnotiforms. Pulse-type species produce discrete pulses of electricity at a lower frequency, while wave-type species produce a continuous series of electrical discharges at a much higher frequency (Albert 2001). While Albert (2001) regarded the pulse-type electrical field as ancestral for the gymnotiforms and the wave-type field as derived, other authors have preferred the opposite scenario. Sternopygids retain well developed eyes, in contrast to the reduced eyes of other gymnotiforms, while apteronotids are the only gymnotiforms to retain a caudal (tail) fin. If the wave-type families form a derived clade, then either these features were lost independently in the other families, or they represent reversals to an ancestral type.

One final thing to note is that the gymnotiforms have been going through something of a taxonomic boom, with many new species described in recent years. Albert & Crampton (2005) estimated that the total number of species out there could be nearly twice the 135 that had been named so far. In South America, it turns out, the streams are alive with the buzz of electricity.

REFERENCES

Albert, J. S. 2001. Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei). Miscellaneous Publications, Museum of Zoology, University of Michigan 190: 1-129.

Albert, J. S., & W. G. R. Crampton. 2005. Diversity and phylogeny of Neotropical electric fishes (Gymnotiformes). In: Bullock, T. H., C. D. Hopkins, A. N. Popper & R. R. Fay (eds) Electroreception, pp. 360-409. Springer: New York.

Stunning Sea Slugs

Chromodoris willani, photographed by Samantha Craven.


The name 'sea slug' does not sound immediately enticing to those not in the know, but in fact they include some of the world's most fabulous animals. The Chromodorididae are a family of more than 300 species of ludicrously colourful sea slugs, with the greater number of species found in the Indo-West Pacific region. They are specialist feeders on sponges, preferring those sponges that do not contain mineralised spicules.

Cadlinella ornatissima, photographed by Barb Makohin.


Their bright colours, of course, are a signal to any would-be predators that they are highly toxic and distasteful, in the manner of monarch butterflies, poison-arrow frogs, or raspberry cordial. As noted by sea-slug researcher Bill Rudman in this forum post, toxic compounds are often concentrated in the mantle fringe that runs around the sides of the chromodorids. They will often flap this mantle, and Rudman speculates that this may make an attacker more likely to bite off a piece of the distasteful (and easily regrown) mantle rather than the main body of the animal.

Pair of Hypselodoris bennetti, photographed by Richard Ling.


Until the classification of the Indo-West Pacific species was reviewed by Rudman (1984), most chromodorid species were lumped into a single genus Chromodoris under the belief that there were few relaible characters available to distinguish higher taxa within the family. Rudman (1984) identified a number of features, including characters of the reproductive and digestive tracts, that could be used to separate genera. A molecular analysis of the family by Johnson & Gosliner (2012) led them to recognise 17 genera of chromodorids, with genera recognised by Rudman (1984) as pan-tropical proving to be polyphyletic.

Doriprismatica kulonba (previously Digidentis kulonba), photographed by Bill Rudman).


Though sea slugs are mostly thought of as tropical animals, there are some that make there homes in cooler waters. Doriprismatica kulonba, for instance, is found around south-east Australia.

Ceratosoma tenue, photographed by Steve Childs.


Long-term readers may have noticed that this post contains a somewhat lower text-to-imagery ratio than usual. For this, I make no apology.

Diversidoris flava (previously Noumea flava), photographed by Chad Ordelheide.


REFERENCES

Johnson, R. F., & T. M. Gosliner. 2012. Traditional taxonomic groupings mask evolutionary history: a molecular phylogeny and new classification of the chromodorid nudibranchs. PLoS ONE 7 (4): e33479. doi:10.1371/journal.pone.0033479.

Rudman, W. B. 1984. The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the genera. Zoological Journal of the Linnean Society 81 (2-3): 115-273.

Bark Beetles and their Hidden Harems

Galleries dug in a grand fir Abies grandis by fir bark beetles Pityophthorus pityographus, photographed by Louis-Michel Nageleisen.


For producers of commercial timber, the above picture would not be a pretty sight. Bark beetles are named after what they feed on: they chew galleries under the bark of trees. In some species that attack otherwise healthy trees, these borings may result in stunted growth or death. The beetles may spread fungal diseases as they move from one tree to another (Dutch elm disease is one example of a well-known disease spread by bark beetles). But on the other hand, many bark beetles play a vital row in nutrient recycling, feeding on already dead and dying trees and breaking down the wood.

The fir bark beetle Pityophthorus pityographus itself, from PaDIL.


The bark beetles belong to a group called the Scolytinae. The scolytines include over 6000 species worldwide (only a relatively small percentage of which, it should be noted, are recognised as significant pests). Oddly enough, they are actually a kind of weevil. The most characteristic feature of most weevils is their elongate snouts, but in scolytines these snouts have been lost (they would probably not be ideal for burrowing through wood). The fir bark beetle belongs to a subgroup of the scolytines called the Corthylini, distinguished from other scolytines by their elytra, which lock down so that a panel on the side of the body called the metepisternum is hidden when the elytra is closed (in other scolytines, it remains at least partially visible), and by the flattened round clubs on their antennae (Wood 1986). The Corthylini are themselves divided into two subgroups, the Corthylina and the Pityophthorina. The two groups are not that easily separated by their morphological appearance, but they are very different in their ecology. The Corthylina don't live directly under the bark, but deeper in the tree amongst the xylem (the central water-conducting tissue). Corthylinans and ecologically similar beetles, known as ambrosia beetles, live in association with a fungus that grows on the xylem. The beetles, which cannot directly digest the xylem themselves, feed instead on the fungus.

The Pityophthorina, on the other hand, are true bark beetles, with most species feeding directly on the tree's phloem (the sugar-conducting tissue around the outer part of the tree). Other species in this group burrow in the tree's seeds, or feed on the pith inside slender stems. The main diversity of Pityophthorina (and of Corthylini in general) is in the Americas, particularly in cooler temperate or tropical highland environments, with over 500 known species in North and South America. Two species, Pityodendron madagascarensis and Sauroptilius sauropterus, are found in Madagascar, while the genus Mimiocurus includes ten species found in Africa and Asia. The largest genus in the Pityophthorina, Pityophthorus, includes about sixty species in Africa and Eurasia in addition to over 300 in the Americas. Wood (1986) suggested that the Eurasian species of Pityopthorus were probably descended from relatively recent migrations from North America, but African and Madagascan species of Pityophthorina may represent more basal lineages.

Female Dendroterus decipiens, photographed by T. H. Atkinson.


As well as their economic and ecological significance, scolytines have attracted attention for the range of breeding behaviours they exhibit (Kirkendall 1983). Bark beetle galleries are not just feeding structures, they are also breeding structures. Females mate and lay their eggs within the galleries, and their larvae hatch and continue to feed there. The Pityophthorina are described as including both monogynous and polygynous species, but these terms refer to the number of females in a gallery, not necessarily the mating habits of the males. In monogynous species, a gallery will be home to only a single female. Construction of this gallery may have been started by the female herself, or it may have been started by a male who was then joined by the female. In most monogynous Pityophthorina, the latter is the case (and the mating system is monogamous as well as monogynous), but the female is the one to start the gallery in the genus Conophthorus (Conophthorus species feed in pine cones, and may be restricted to monogyny by the small spaces available for gallery construction). Conopthorus males that mate with the female may not remain in the gallery, but may leave directly after mating. Males that don't stay in the gallery can mate with more females, of course, but males who do stay will be able to prevent their mate from mating with another male herself before laying her eggs. Also, by helping to maintain the gallery (or by constructing the gallery himself to begin with), the male may encourage the female to oviposit faster, or improve conditions for the larvae when they hatch.

In polygynous species, such as most Pityophthorus species, a single gallery will be home to multiple females. In most polygynous Pityophthorina, a single male will co-habit with a harem of females. A few pityophthorinans of the genus Araptus are inbreeding polygynes: males do not leave their parent gallery, but instead mate with their sisters before the latter leave the gallery. Inbreeding species seem to show remarkable control over sex ratios in the population, with many more female larvae produced than males. Interestingly, monogynous and polygynous galleries tend to differ in physical structure: monogynous galleries tend to be simple and direct, with only one or two arms extending along or across the host plant from the central nuptial chamber. Polygynous galleries, on the other hand, may have several arms radiating from the nuptial chamber, with each arm probably being built by a separate female.

REFERENCES

Kirkendall, L. R. 1983. The evolution of mating systems in bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae). Zoological Journal of the Linnean Society 77: 293-352.

Wood, S. L. 1986. A reclassification of the genera of Scolytidae (Coleoptera). Great Basin Naturalist Memoirs 10: 1-126.

Thrips Wars!

Two males of Elaphrothrips tuberculatus fight it out on the left, while the object of their desire guards her egg-mass on the right. Figure from Crespi (1986).


All around, little dramas are taking place every day, conflicts as intense as the plot of any daytime soap opera. And like most daytime soap operas, the main focus of these dramas often comes down to who is shagging whom. Most people only known thrips as small annoying insects that damage garden plants and crops, but some thrips may engage in remarkable behaviours.

Elaphrothrips is a genus of thrips found almost throughout the tropics (though it is absent from Australasia). They are found on dead leaves, where they feed on fungal spores. Well over a hundred species have been named in Elaphrothrips, though Mound & Palmer (1983) pointed out that many of these may be turn out to be synonymous as individual species can vary significantly in appearance. Males may have thick forelegs with strong tubercles on the femora, while the forelegs of females are usually slender and lack tubercles. Indeed, the sexes are different enough that at one point they have been mistaken for separate genera. The males themselves may vary significantly in size, with larger males having correspondingly larger legs and spines.

A lot of these differences are related to the Elaphrothrips' mating behaviour. The best-studied of the Elaphrothrips species is E. tuberculatus, a widespread species in eastern North America and the largest North American thrips species. Elaphrothrips tuberculatus prefer dead oak leaves that are still hanging in clusters from the tree, where females lay eggs in clusters on the leaves and then stand guard over them. The females are themselves guarded by males, but the males may be challenged by others who want to take the female for themselves. Battles between male Elaphrothrips most commonly take the form of the two males lining up alongside each other, as in the drawing at the top of this post, and then one or each begins batting at the other with his elongate abdomen. Alternatively, one male may attempt to reach under his opponent's abdomen with his own, and then try to flip his opponent over. Crespi (1986) noted that challenging males were more likely to try to flip their opponent than defending males, perhaps because the success rate of flipping attempts was very low, making this tactic more of a gamble. Flipping could also act as a defense against a third attack strategy, in which one male would climb up onto the back of his opponent and use the tubercles on his forelegs to stab at his opponent's thorax. Larger males were more likely to stab their opponents than smaller males, which of course have less developed leg spines. However, a smaller male may also get around larger male through sneaking behaviour, mating with the female before her guarding male realises he is there.

Whichever male mates with the female, one thing is certain: he will only have daughters. Thrips have a haplodiploid sex determination system like that of ants and bees, with males developing from unfertilised ova and females from fertilised ones. Elaphrothrips tuberculatus adds another wrinkle to the system that only females hatch from eggs. Male offspring, on the other hand, develop inside their mother and are born live (Crespi 1989). Nevertheless, an individual female may have both male and female offspring, as she may change her reproductive mode between broods to be a live-bearer or an egg-layer!

REFERENCES

Crespi, B. J. 1986. Size assessment and alternative fighting tactics in Elaphrothrips tuberculatus (Insecta: Thysanoptera). Animal Behaviour 34: 1324-1335.

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Mound, L. A., & J. M. Palmer. 1983. The generic and tribal classification of spore-feeding Thysanoptera (Phlaeothripidae: Idolothripinae). Bulletin of the British Museum (Natural History): Entomology 46 (1): 1-174.