Glassy fingers, orange goo and scrambled eggs: tree-stump slime-moulds

A definite departure from my usual taxonomic interests today as I look at some slime-moulds associated with tree-stumps. Traditionally, the slime-moulds (or Myxomycetes) have been included within the Fungi, but - with mitochondria and tubular cristae in their cells - are actually members of the Protozoa. However, as their habitats, spore dispersal and methods of study are similar to those associated with true Fungi, they are often still considered 'honorary' Fungi for everyday purposes.

High-level taxonomy aside, the slime-moulds are, to many people, an unfamiliar and mysterious group of organisms even though many are distinctive and, during their 'plasmodium' stage quite large - some, weighing 20kg or more (yes, 20kg) and covering/filling whole logs, may even be the largest single cells known. However, most are smaller but still have a plasmodium several centimetres across and often brightly coloured. Most species live in woodlands in the sorts of microhabitat often associated with Fungi - the focus here is on just one of these, tree-stumps. In fact, the first three species were all found on a single stump in the New Forest just a couple of days ago:

Tubifera ferruginosa (the patch is about 4-5cm long). This specimen is clearly orange; others may be more pinkish giving this species the not always accurate common name 'Raspberry Slime Mould'. It darkens as it ages, and sometimes looks rather like a strawberry. But not a raspberry. Hey ho - this is why we use scientific names... The individual 'pimples' are the tops of cylindrical sporangia (spore-bearing structures) and this species is common during late summer and early autumn throughout Britain.

The bright yellow plasmodium of Fuligo septica, somewhat reminiscent (to me at least) of scrambled egg. The main mass here is about 5cm across and you can see some of the thin threads produced by 'cytoplasmic streaming' as the organism stretches and flows across the surface. This species is found as far north as Yorkshire, but has a scattered distibution, mainly in south-east England.

The transparent, glassy/watery 'fingers' of Ceratiomyxa fruticulosa. Each finger contains the spore-bearing structures and is a few millimetres tall, to a maximum of maybe 1 cm. This is another common widespread species - in Britain found from early summer onwards.

A closer view of C. fruticulosa.

Earier in the year, in June, I found some other myxomycetes which exhibited quite a different structure.

Brown slime-mould structures, like eggs in a nest.

Here they are emerging and also showing orange and reddish colours.

More examples of orange structures.

A close-up of an orange-red structure emerging from a crack in a stump.

These four pictures are all of slime-moulds of the Lycogala epidendrum group, a complex recently split into two species, L. epidendrum and L. terrestre. Microscopic examination (e.g. of the spores) is needed to separate them, so for the purposes of this post, I shall simply call them Lycogala. In these species, the spore-bearing structures are effectively fused to form an 'aethelium' several millimetres across which can be greyish, brownish or pinkish - the 'eggs' seen above. Like F. septica, Lycogala can escape from dead wood via holes such as those made by beetles as well as through existing cracks and crevices. The plasmodium is reddish or pinkish as seen in the lower picture as well as near the right-hand edge of the second picture.

Now, I am no expert on slime-moulds, but I do find them intriguing - and it's quite a sight when several brightly coloured species inhabit the same piece of dead wood. However, I can recommend the excellent handbook by Ing (1999) which I am highly reliant on - it won't give you lots of colour photos (though a web search will), but it does provide simple line drawings of key structures, keys, spore measurements and so on covering the few hundred species in Britain. It's also the first comprehensive work on the British species since 1877! 

Enjoy the slime!

Reference

Ing, B. (1999). The Myxomycetes of Britain and Ireland: An Identification Handbook. Richmond, Slough.

The Sand Wasp cometh...

When it's sunny, a trip to the sandy areas of the New Forest can be a good way of finding some of the sand-dwelling invertebrates that specialise in using this type of heathland habitat. One example is the Sand Wasp Ammophila sabulosa. In Britain, this is a distinctive genus with a long abdominal petiole (the thin bit) and a blackish and orangey gaster (the widened bit of the abdomen). There are two species, A. sabulosa and A. pubescens - both are fairly common and can be separated by subtle wing-vein variation and by the colour of the tip of the gaster - black in A. pubescens and (as here) blue-black in A. sabulosa. In a global context, care needs to be taken to separate the genera Ammophila and Podalonia, but with so few species in Britain, this is not an issue here.

A. sabulosa at rest on sand, around 20mm long excluding appendages.

As well as being quite spectacular insects, the behaviour shown by females of this genus makes for a fascinating story. The female A. sabulosa makes a short burrow in sandy soil, ending in a single cell - the waste material is spread around to avoid crating a clear spoil-heap that might attract predators. She then hunts for prey some metres around the burrow. This takes the form of large smooth caterpillars, usually of noctuid moths (and rarely sawfly larvae), paralysing them with her sting and carrying them back to the burrow, having memorised its location using small local landmarks. The burrow will soon become a nest for her young, so she can be seen arranging it before dragging the live but immobile prey inside. Once two or three caterpillars have been collected, she lays an egg on one of them. The burrow is then filled in and the entrance obscured by making it appear the same as its surroundings - in North America, females have even been seen using small stones to tamp down the sand in the entrance - a remarkable example of tool use in invertebrates! The larvae, hatching in their hidden nest, are carnivorous and now have a ready food supply once they hatch in a few days' time. Development is rapid with larvae reaching full size in about 10 days, then pupating inside a spun cocoon.

Adults are active hunters and can be seen running quickly across sandy surfaces - making for some tricky photography, but an excellent excuse to look closely at some local heathlands, something well worth doing the next time to sun is shining...

A. sabulosa running on sand, its legs a blur of motion!

References

Baldock, D.W. (2010). Wasps of Surrey. Surrey Wildlife Trust, Woking.
Richards, O.W. (1980). Scolioidea, Vespoidea and Sphecoidea: Hymenoptera, Aculeata. Handbooks for the Identification of British Insects 6 (3b): 1-118.
Yeo, P.F. & Corbet, S.A. (1983). Solitary Wasps. Richmond, Slough.

Zahradnik, J. (1991). Bees, Wasps and Ants. Hamlyn, London.

Eat! Roost! Bathe! Nest! The role of humans in everyday bird-life

The last couple of months have seen my posts taking on a distinctly invertebrate-centric flavour, so to balance out the ecological spread a little, I thought I'd have a look at some vertebrates, in particular the everyday interactions between birds and people. I don't want to cover the broader areas of nature conservation for birds - there are many excellent websites already doing this, such as the RSPB's page on farmland bird conservation. In a previous post I also looked at the effects of urban noise on birds. Instead I would like to share some examples of simple actions having positive effects, and others where birds have begun to adapt to the human-dominated world in which they find themselves (and not just the use of roadkill by crows).

A species well-used to intereacting with humans - the mallard Anas platyrhynchos relaxing after a hard day of being fed by passers-by in Winchester, southern England.

One place where many of us experience wild birds, and often help them out, is in our gardens. This is something that hasn't gone unnoticed, hence the popularity and importance of projects such as the RSPB's (yes, them again) Big Garden Birdwatch which attracts hundreds of thousands of volunteer recorders and counts millions of birds in the UK alone. So, what do our gardens provide?

A male blackbird Turdus merula hunting for invertebrates in our garden. Digging often brings a bold female right to our feet as she looks for earthworms.

On the simplest level, gardens provide habitat i.e. a replacement for what was there prior to urban development. Since the photo above was taken, our garden has changed a lot - the boring lawn has been largely dug up to create flower and vegetable beds, with the remainder being allowed to form a small 'wild' meadow. However, species such as the blackbird are still frequent visitors and I suspect that the increased structural diversity is beneficial (more invertebrates = more food for insectivores). Of course, this isn't all we do - 'supplementary' feeding is very important to a range of species (which tends to increase during harsh weather) and has the added benefit of bringing birds closer to us, giving us the opportunity to watch them and their behaviour at close quarters.

L-R: siskin (Carduelis spinus), redpoll (Carduelis flammea) and goldfinch (Carduelis carduelis) - three congenerics together on a nyger seed feeder.

A blur of activity - a mob of long-tailed tits (Aegithalos caudatus) in a squirrel-proof feeder.

 Of course, it's not just food - with the loss of many natural sources of water such as ponds (something that can only get worse with predicted hotter, drier conditions as climate change bites), water is also key. We can create ponds in gardens and elsewhere (we're in the middle of digging ours) and, even more easily, ensure regularly topped-up bird baths are in place.

A blackbird taking the opportunity for a drink.

A wood pigeon (Columba palumbus) hogging the bird bath and having an enthusiastic splash! Smaller birds have been seen waiting in nearby shrubs for pigeons to finish their ablutions.

 Sometimes water isn't what's required for bathing - sometimes it needs to be dry and sunny...

A juvenile robin (Erithacus rubecula) sun-bathing in an old chimney-pot, originally put in the garden to grow tomatoes.

Afterwards, time to cool off...

...while a beady-eyed adult looks on.

So, we've covered food and water - but what about shelter? Bird-boxes are familiar so I won't cover them here, however birds sometimes manage to make their own good use of structures put in place by humans for entirely other purposes.

A female house sparrow (Passer domesticus) just beneath a telephone junction box outside our house. Males and females have been seen taking nest materials and food into this box.

Three juvenile swallows (Hirundo rustica) in a nest built under the roof of a visitors' gantry at Monkey World Ape Rescue Centre in Dorset, southern England. Adults made frequent feeding visits to the nest, even flying between people as they came in through the open sides of the walkway. Apologies for the poor photo - I didn't want to disturb the nestlings by using the flash.

It looks like a model, but it was definitely real - one of the adult swallows at Monkey World, unusually seen perching.

 Sometimes of course, all that is needed is a quiet place away from too much human disturbance.

A family of mute swans (Cygnus olor) with juveniles developing after having nested in the middle of a rural riverside footpath. Unsurprisingly the adults were aggressive if approached at the nest, but a little understanding (i.e. a willingness to take a minor detour) was all that was needed.

A dipper (Cinclus cinclus) in its familiar position, looking for aquatic invertebrates from a rock in a stony stream. This looks like it could be an isolated moorland stream, but is in fact in the town of Lyme Regis, Dorset, southern England. There's a footpath alongside the river, but the dipper has become used to people who of course don't tend to wade into the water.

So, we have seen some brief snapshots of some of the ways in which birds benefit (or at least don't suffer) from the presence of thoughtful humans. This has not always been the case - until 1832, the world's heaviest flying bird, the Great Bustard Otis tarda nested in Britain, but subsequently became extinct here, probably due to a combination of hunting, pressure from increased human population and changes in land use (Waters & Waters 2006). 

A mounted specimen of the Great Bustard, a male with a splendid 'tache.

This is an unusual example, but one which shows a more positive recent trend as there is now a well-known reintroduction programme. The Great Bustard Group (which can be visited for a small fee) manage this through introductions from Russia and in 2009, after a gap of 177 years, Britain had wild Great Bustard chicks! Currently the effects of humans on birds vary - there have been huge declines in farmland and trans-Saharan migratory species (though even this has shown an improvement where there has been improved management in Africa), while many woodland species in Britain have increased, not to mention some complex trends relating to garden bird feeding. The conclusion - aside from getting involved in wider-scale conservation efforts, we can all act as individuals by making the places we live (and work?) in more attractive to birds and other wildlife. 'Nuff said!

Reference

Waters, E. & Waters, D. (2006). The Great Bustard (2nd ed.). Great Bustard Group, Salisbury.

I've got my hooks in you: damselflies and the wheel of life

Instead of one of my usual forays into the morphology and taxonomy of some of our more obscure invertebrate friends, today I've decided to look at the reproduction of a species in one of the more popular and familiar groups, the damselflies (Odonata: Zygoptera), namely the Blue-tailed Damselfly Ischnura elegans, often known as the Common Bluetail outside the UK.

A typical I. elegans showing the black abdomen with blue 'tail-light'

This is one of the commonest damselfly species in Britain and can be found throughout the mainland. It tends to colonise new ponds quickly and is tolerant of pollution and brackish conditions to some extent. Given how some of our freshwater habitats are treated, this may be one reason why it is doing well. However, the identification, distribution and ecology of this species are well documented in the references given below as well as online. Instead, I would like to focus on just one aspect of this species' biology, namely reproduction, and in particular the behaviour and reproductive structures of the adults.

In both males and females, the primary genitalia are located beneath the 8th abdominal segment (near the end of the abdomen). However, males are unique among insect groups in also having secondary genitalia - these are found under the 2nd abdominal segment and consist of a pair of lateral hooks (called 'hamules') plus, usually hidden away in an opening, a flexible penis - more of that later. The presence of these secondary genitalia is the reason why the Odonata are the only insect group to adopt the characteristic 'wheel' position during copulation.

A pair of I. elegans in the wheel position - the male is on top.

Before mating, the male passes a spermatophore (sperm-packet) from his primary to his secondary genitalia. Once this is done, and a suitable female has been located, he grasps her on the pronotum (the top of the thorax just behind the head) using claspers at the end of his abdomen. These claspers have tiny teeth to ensure a tight grip and are shaped to fit onto contours, grooves and notches on the female's head and pronotum. This is not always a gentle process as the hooked appendages have been known to leave females with scarred eyes (Brooks 2004). Once the female has been grasped, the male tries to induce copulation by swinging his abdomen (with the female) forwards to bring her genitalia in contact with his secondary set where the spermatophore is now located. Once this occurs, the wheel position is formed.

This is the starting point for what can be a long copulation - an average of 324 minutes according to a study of I. elegans in southern France (Miller 1987) which is much longer than for most other Odonata. It is unclear exactly why this is the case, although it may be a form of mate-guarding, preventing other males from mating with the same female (Corbet & Brooks 2008).This is important because there is another form of competition between males - sperm competition. This occurs when, during much of the early part of copulation, the penis (which is flexible and tipped with hooked appendages) is used to remove sperm from any male that previously mated with the same female. Only then does the male introduce his own spermatophore. I have seen mating wheels on may occasions in a range of species, however I have not often seen the structures involved in any detail.

The same pair immediately post-copulation - note the male's white secondary genitalia near the base of his abdomen.

The secondary genitalia in close-up. Although rather blurred at this magnification, the hooked appendages are visible.

In the above photo, little detail can be seen, although it is still a rarely captured view. However, for some amazing hi-res detail in a Scanning Electron Microscope image, have a look at the picture in the Science Photo Library here. This is also of I. elegans and clearly shows the hooks used in sperm competition. It also makes me jealous of their microscope.

And so, as far as copulation goes, that's about it... except of course there is a lot more to damselfly reproduction than simply the wheel position. Males have to recognise females even though some have a male (andromorph) colouration which may be a male-mimicing strategy to reduce interference from other males during egg-laying (oviposition), although it risks failure to reproduce as recognition is visual (and not fully understood) and males may not find such females, and may also lead to greater predation risk due to the bright colour. Afterwards there is of course oviposition itself with eggs being laid in a slit cut into aquatic vegetation - unlike most species, this is done by lone females late in the day once male activity has ceased (Brooks 2004). Then, once the eggs hatch, there is the development of the predatory nymphs prior to emergence as a new generation of adults. The flight season ranges from May (sometimes April) to September, peaking between June and August in the UK.

Lastly, these photos were all taken at Highbridge Farm in Hampshire, site of our community farm project, and the location of a new but rapidly developing farm pond - just the sort of habitat favoured by I. elegans!


References

Brooks, S. (2004). Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland. (revised ed.). BWP, Gillingham.
Corbet, P. & Brooks, S. (2008). Dragonflies. Collins (New Naturalist), London.
Dijkstra, K.-D. B. (2006). Field Guide to the Dragonflies of Britain and Europe. BWP, Gillingham.
Miller, P.L. (1987). An examination of the prolonged copulations of Ischnura elegans (Vander Londen) (Zygoptera: Coenagrionidae). Odonatologica 16: 37-56.

Faking stinkers: Do bug nymphs mimic harvestmen?

A fairly short (well, short-ish) post for once, but one that asks a question that's been niggling at me for a while - do bug nymphs mimic harvestmen? At first glance this is not a question with a clear foundation, and certainly falls within the realm of 'speculative biology' (a place I don't visit that often) - bug nymphs don't look anything like harvestmen which have long spindly legs attached to a central round-oval body. However, by 'bugs' I'm only thinking of the Coreidae ('squashbugs') and Pentatomoidea ('shieldbugs'), and only one aspect of harvestmen (Opiliones) - the ocularium (eye-bearing structure) on top of the body. Let's look a little more closely...

A 'typical' harvestman of the genus Mitopus.

A North American harvestman, courtesy of 'bugman' at What's That Bug?

Both of these show the oval body on top of which is the small ocularium - a raised structure with two laterally oriented eyes which incidentally, despite appearing large like spider eyes, are ocelli and do little more than register light-intensity - touch is a far more important sense in harvestmen (Hillyard 2005). The top picture also shows the central dark band or 'saddle'. OK, so we know what harvestmen look like - but what's this got to do with bug nymphs? Well, let's see...

Late-instar nymph of the coreid Coriomerus denticulatus

Late-instar nymph of the coreid Arenocoris falleni

Final-instar nymph of the Juniper Shieldbug Cyphostethus tristriatus

[All three bug photos are copyright Tristan Bantock at the excellent British Bugs guide to British Hemiptera which has an extensive gallery including other nymphs showing these markings, though not all species do. For example, nymphs of the familiar Dock Bug Coreus marginatus do not show these 'ocularium' markings.]

Looking at these three nymphs, I can't help but wonder if the structures on the top of the abdomen are mimicing the ocalarium of harvestmen. To me, they certainly appear similar, especially in the coreids which share similar colouration and segmentation (superficially at least) with the harvestmen, though of course my poor human brain is wired to look for patterns, so simple appearance may not tell me much. I can do little more than speculate at present, and am unaware of anyone else who has looked at this, but the phrase 'Batesian mimicry' leaps to mind. This is the type of mimicry where a harmless species mimics one that is dangerous or unpalatable (for example, there are many species which mimic the colours of wasps). How might this suggested example be Batesian?

Well, bugs are likely to be potential prey of many insectivores, and during their nymphal stages cannot fly - so, mimicry might be useful, but why harvestmen? Although predatory, mainly on small soft-bodies invertebrates, harvestmen have no venom and are unlikely to be able to inflict much damage on potential predators such as birds, spiders, beetles, centipedes, fish, frogs and shrews. However, they do possess odoriferous glands (also called 'repugnatorial' or 'stink' glands) which are found on the sides of the carapace, approximately level with the ocularium. These glands produce a spray or droplet than the harvestman can spread on itself or an attacker. Though the chemicals (various alcohols, ketones and naphthoquinones) are not always easily detected by humans' sense of smell, there have been various observations made of potential predators avoiding (e.g. ants and spiders) or expelling (e.g. frogs) harvestmen because of their distastefulness (Hillyard 2005). Might this form the basis of mimicry by bug nymphs?

At present this is mere speculation derived from observation - something that might be formulated as a hypothesis - and some initial questions are raised such as whether the bug nymphs themselves are distasteful, in which case this would be an example of Müllerian, rather than Batesian, mimicry (as both groups have anti-predation characteristics). It is something that I will have to dig into further and as ever I welcome thoughts and suggestions.

Reference

Hillyard, P.D. (2005). Harvestmen (3rd ed.). Field Studies Council, Preston Montford. A small but excellent book, and the current standard work on British harvestmen.

Further reading

If you are interested in squash- and shieldbugs in Britain, this is excellent:

Evans, M. & Edmondson, R. (2005). A Photographic Guide to the Shieldbugs and Squashbugs of the British Isles. WGUK.

Just skimming the surface: dragonflies of the genus Orthetrum

Following my post about the Hornet Robberfly, I thought I'd focus on one of the groups that Hatchet Pond is justly famous for - dragonflies and damselflies (Odonata). The site consists of a large central pond surrounded by grassland, scrub and heathland, plus a series of much smaller ponds scattered around the area. Although there was an unseasonally stiff, cool breeze, the scrub does provide some shelter and a range of species were seen, especially in the afternoon when the sun came out and the breeze subsided a little (it's true, we British do talk about the weather a lot...). Anyhow, to the insects - here I will focus on the genus Orthetrum (skimmers), starting with one individual that was particularly obliging - a male Black-tailed Skimmer Orthetrum cancellatum.

O. cancellatum - a male basking

This male clearly had a favourite basking spot - a small curved indent in the pond edge which presumably formed an effective heat-trap. The green eyes, brownish thorax and blue abdomen with a black tip are unmistakeable even though the yellow spots on the abdomen of mature males have not developed in this specimen. Aside from being an impressive beast (the abdomen here is about 30-35mm long), it's a good indication of how important habitat (and micro-habitat) structural diversity is when trying to ensure species diversity. Males of this species (as is often the case with Odonata) are highly territorial and patrol regularly, sometimes occupying 50m or more of bank, while females spend most of their time feeding away from water. They hunt from perches and often prefer larger prey such as butterflies, grasshoppers and damselflies - they are able to swivel their head to fix on targets visually, and often return to the same perch to feed.

A male O. coerulescens

Superficially similar, but smaller with darker eyes and thorax, and less black at the tip of the abdomen, the Keeled Skimmer Orthetrum coerulescens maintains a smaller territory - around 5m diameter - and there may be about 15 territories along a 100m stretch of bank.

In both species, females entering a male's territory may be quickly seized - in O. cancellatum, mating usually takes no more than 30 seconds (and may occur without landing), while in O. coerulescens, the process (involving a mating wheel) may take anywhere from a few minutes to over an hour. In both species, egg-laying (oviposition) behaviour depends to some extent on the density of males and the resulting levels of 'harassment' the female may experience (females can be seen waiting for an opportunity to lay eggs unmolested!).

O. cancellatum is found throughout much of the southern half of England and has extended its range northwards in recent decades (at least as far as Durham) following creation of wetlands from flooded mineral and peat extraction sites. O. coerulescens has a more south-westerly distribution being most widespread in Ireland, west Wales and Cornwall/Devon, but is found at a scattered range of sites elsewhere in the country and can colonise new sites quickly - a useful ability given the losses of some areas of its wet heathland habitat due to development and peat extraction.

So, only a brief taster of the species found at Hatchet Pond - more to come, including patrolling Emperors and a few rather more uncommon species...


References

Brooks, S. (2004). Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland (revised edition). BWP, Gillingham.

Smallshire, D. & Swash, A. (2004). Britain's Dragonflies. WILDGuides, Old Basing.

Is it a hornet? No, it's Britain's biggest fly

At first glance, Britain may not stand out as the home of impressively large insects - the largest beetles are tropical, as are the stick insects, large water bugs and largest butterflies. We do have some large dragonflies such as the Emperor Dragonfly Anax imperator, and a male Stag Beetle Lucanus cervus is always striking, but Britain is not home to the 'island gigantism' seen in, for example, the Giant Wetas of New Zealand, particularly the Little Barrier Island Giant Weta or Wetapunga Deinacrida heteracantha, the world's largest orthopteran (grasshoppers & crickets). The world's largest fly is Gauromydas heros, a member of the family Mydidae found in Brazil. It can be up to 60 mm long (excluding appendages) and is a wasp mimic, but relatively little is known about it with few specimens being collected as the adult lifespan appears short.


However, Britain is not without its own sizeable flies as noted during a visit to Hatchet Pond yesterday. Although used heavily for recreation, this site is home to many uncommon plant and invertebrate species, and one of these was this striking insect seen shortly after arriving.

The Hornet Robberfly Asilus crabroniformis, Britain's largest fly.

This is of course the Hornet Robberfly Asilus crabroniformis, Britain's largest species of fly, seen here resting on a dry stick in damp grassland. Although it can't rival G. heros, large specimens are about half its size with the body length varying from 18-28 mm (this individual was at the upper end of the range).


Despite the large size and bright yellow abdominal section (it is a hornet mimic), it is remarkably well camouflaged when the brown wings cover this area. As it is a 'darting' hunter (capturing prey via short darting flights, usually of no more than 50cm, but sometimes up to 3m), this cryptic colouration is presumably important, while defense may also be involved as it has been seen hanging upside-down when resting at night, a position that displays only the cryptic brown colours of the underside - in this position it has the appearance of a dead curled-up leaf (Clements & Skidmore 1998). The hunting flights are launched from low-lying perches such as the stick in the photo above; prey items are large insects such as grasshoppers, larger beetles and flies, butterflies, moths, bees and wasps (WBP 2008) and take 10-30 minutes to drain using the fly's sharp piercing mouthparts (Stubbs & Drake 2001).

A. crabroniformis is associated with a range of open habitats such as heath and various types of grassland with rough shrubby vegetation, and is associated with old dung with females laying their eggs on or under the crust, particularly cow dung, though also horse and (if in mounds) rabbit. Despite this fairly broad habitat requirement, it is a local species rarely found in large numbers. The map below (courtesy of the National Biodiversity Network) suggests a fairly widespread distribution in southern Britain; however, some of these records date from before the loss of much of its habitat (especially in East Anglia) - most recent records are from the heaths of Dorset, Hampshire (as here) and Surrey, plus chalk grassland in Hampshire and Wiltshire, with scattered records elsewhere.

Distribution map of Asilus crabroniformis in Britain (as of 08/08/11)

Although it has no specific legal protection, it was listed as Notable (scarce) by Falk (1992) who noted key threats as:

• Habitat loss/degradation due to conversion to intensive agriculture or forestry.
• Loss of dung and associated beetles due to removal of appropriate grazing animals e.g. replacement of cattle by sheep in much of its historical Hampshire habitat (Stubbs & Drake 2001) and removal of dung during paddock management (WBP 2008). Sheep dung appears to be unsuitable for A. crabroniformis and the shift from cattle to sheep accelerated during the BSE outbreak of the 1990s (Clements & Skidmore 2002).

Since then, Stubbs & Drake (2001) have also noted a new threat in the form of avermectins used as a veterinary treatment to remove parasites from the guts of livestock. In some cases, this makes dung harmful to the dung beetles which appear to be important in the fly's life cycle (see below). Further, WBP (2008) suggest a possible effect of climate change as adult activity appears to be temperature-regulated (i.e. reliant on high ambient air temperatures), though it is unclear whether this would simply increase levels of adult activity and if so, whether this would be beneficial or harmful.


The life cycle is not fully understood, but the later-instar larvae are free-living in the soil and thought to be predatory on dung beetle larvae (geotrupines such as Minotaur Beetle Typhaeus typhoeus) which are associated with herbivore dung. The diet of early-instar larvae is unknown as eggs are laid in dry dung which has no large insect larvae, only small invertebrates such as nematodes and springtails. Territorial behaviour has been suggested, centred on dung-pats (e.g. by Stubbs & Drake 2001) but this has never been clearly demonstrated with Clements & Skidmore (2002) indicating no strong territoriality following their mark-recapture study - indeed individuals could be seen sharing parches and previous records of agression may be attempts at copulation. Clements & Skidmore (2002) found the mean adult lifespan to be 15.9 days in the field with peak emergence in late July and August following a larval lifespan thought to be 2-3 years, although this is not well documented (WBP 2008).


Regarding conservation, it seems clear that this is a scarce species which has declined over recent decades - it is therefore of key importance to understand the combination of habitat and stock-related factors which may benefit it, especially as even historical records appear to show populations as being somewhat erratic with local ones becoming extinct as others appear. However, this shifting distribution may be threatened with collapse if suitable areas of habitat become isolated, especially as research suggests dispersal rates may be limited with individuals rarely moving more than a few hundred metres, and often less (Clements & Skidmore 2002). This, a range of conservation measures need to be considered:

• Maintain rotational grazing management by suitable species to ensure a continuous dung supply.
• Prevent excessive scrub invasion of suitable habitat while ensuring a range of vegetation type/structure.
• Reconsideration of the widespread use of avermectins.

These are simple enough in concept, but of course not so straightforward to implement. Asilus-friendly habitat management should be quite easy to achieve (at least in principle, assuming the time and resources to control scrub etc), though beyond the bounds of the nature reserve it is less clear how the loss of cattle dung can be reversed. However, education of land-owners is important and simple measures can ensure a continuous supply of horse dung, although again it will require a considerable change in behaviour to reduce the use of avermectins which are popular broad-spectrum treatments. In the end, this is a charismatic species which, with a little effort and creativity, could be more widely known and popular as 'Britain's biggest fly'.




References

Clements, D.K. & Skidmore, P. (1998). The autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997. Countryside Council for Wales Contract Science Report No. 263. Bangor.


Clements, D.K. & Skidmore, P. (2002). The autecology of the Hornet Robberfly Asilus crabroniformis L. in Wales, 1997-1999. Countryside Council for Wales Contract Science Report No. 525. Bangor.

Falk, S. (1992). A Review of the Scarce and Threatened Flies of Great Britain (Part 1). NCC, Peterborough.

Stubbs, A. & Drake, M. (2001). British Soldierflies and their Allies. BENHS, Reading. 

Worcestershire Biodiversity Partnership (2008). Hornet Robberfly Asilus crabroniformis Species Action Plan. [accessed 08/08/11]

Now for something really tricky...

Over recent months I've been posting occasional forays into the world of taxonomic morphology i.e. how an understanding of structure (mainly in invertebrates) allows us to identify them. So far all my postings have covered species I'm pretty confident about - however I thought this was the time and place to have a go, sort of publicly - at something where I'm definitely not so sure - the Ichneumonidae.

As a group, the ichneumons are fairly familar - elongate parasitic wasp-like insects, often black with a bit of red, orange or yellow, plus the long spike at the back of females' abdomens - the ovipositor for laying eggs which is long in those species that lay eggs in host larvae hidden deep inside wood or similar. However, their broad familiarity masks the difficulty in identifying them - in Britain alone there are around 3,200 species and few are readily identifiable by single characteristics, even at subfamily level, let alone genus or species (Broad 2011). Instead, varying combinations of features are often needed, plus there is a lack of readily available literature and some of what has been published can be difficult to use (e.g. Perkins 1959; Townes 1969), though some more recent works are a great improvement if not perfect (e.g. Wahl 1993). Here I will be relying on Broad (2011). This doesn't give an overview of the terminology and anatomy used with ichneumons, but useful information can be found in Fitton et al. (1988) which covers a single subfamily, the Pimplinae but gives the names of wing veins and other body parts.Anyhow, enough background, so what have I found in my garden this time?

A female ichneumon, length 7mm (not including antennae and ovipositor)

So, I have a female ichneumon - black with orange legs and a fairly long ovipositor. So, time to consult Broad (2011) to see if I can at least determine the subfamily. The starting point is easy enough - the wings are not reduced, and vein 2m-cu is present.

Forewing with vein 2m-cu indicated.

The top of the thorax is shiny and unwrinkled and the spiracle (air-hole) of the 1st metasomal tergite (the 1st abdominal segment) is found about halfway along it, with the hardened part of this segment extending more than halfway along it. Some of this can be tricky to see, so experiment with lighting and when you get it right, the spiracle shows up as a tiny but sharp-edged round hole...

1st metasomal tergite with spiracle marked. The hardening is not that clear in side view, but is present along the top (there is a tiny ridge just visible here which is the edge of this feature). The underside of this and the next segment is membranous - a feature of the Ichneumonidae.

This specimen is female but does not have eggs conspicuously hanging from the ovipositor, and the antennae have numerous (more than 16) segments. The next bit is trickier however - the hind and mid tibiae of the legs have spurs and these need to be counted.

Mid and hind tibiae with spurs indicated.

At first glance this looks straightforward - there's one spur on each tibia. However, following this route through the key soon arrived at couplets that simply didn't work - I'd clearly gone the wrong way. So, I went back to this point and looked again. When I did so, by rotating the specimen and looking more carefully, I saw that there were in fact two spurs on each tibia - in the photo the first hides the second in each case because of the angle of view when the specimen is on its back and the legs are tucked in (which is often the case). I should know better, but this is a good example of a feature that really needs care...

Moving on, I looked at the mesopleuron (the side of the large thoracic segment) and could see no sternaulus (a groove running across it on the lower half). From here, the large ovipositor precluded the possibility of this being in the genus Euceros (subfamiliy Eucerotinae) as this has a tiny ovipositor, while the areolet (the tiny cell in the wing, near the top of vein 2m-cu) is triangular.Here another tricky point is reached. Are the ovipositor sheaths stiff or 'flexible-looking'?

The bristly ovipositor sheaths.

Side (and upside-down!) view of the sharply pointed ovipositor plus sheaths.

To me they look stiff and this would indicate the genus Cidaphus - however, Cidaphus does not look like this, being orange with a shorter ovipositor as far as I am aware. So, moving on again, the next couplet requires a close look at the face.

The ichneumon's face. Please ignore the bits of household fluff/dust...

The key here asks whether the clypeus is separated from the rest of the face by a groove. Personally I find this hard to see, even trying different lighting under the microscope, but having learned from the tibial spurs, I did try again...

Another view of the face, this time with different lighting and removal of sone of the fluff...

There is a groove here (running across a little above the lower edges of the eyes), and although the face itself is somewhat convex, it is not smoothly bulging as suggested by the key; rather it is slightly angular. Taking this route, there is no triangular pattern of grooves on top of the abdominal segments, while the hardened part of the 1st metasomal tergite does not extend far past the spiracle (at least not past about three-quarters back); also this segment broadens towards the rear.

An abdominal segment - rather nice puncturation, but no triangular groove pattern.

From here, the hindwing venation and the presence of an 'epicnemial carina' (a narrow ridge along the edge of the mesopleuron) return us to the ovipositor which is clearly not strongly down-curved. The mandibles are not twisted as far as I can tell and the ovipositor has small teeth on the underside at the tip. Also, vein 2m-cu has two bullae (tiny faint patches) although these are very faint in the photo above. And so, if all the above is correct, this is a member of the subfamily Pimplinae.

This is handy as it falls within the remit of Fitton et al. (1988). I won't go through the same process here, but the key takes me towards the genus Scambus (my initial thought was Pimpla but the ovipositor is too long) via various features such as an ovipositor 3-4 times the length of the hind tibia (this ration is important in pimplines). Continuing to the species level, features such as colour become important (such the lack of yellow markings on the pronotum, plus orange-red coxae) and the specimen readily keys out as S. buolianae. This is reasonably common in England and, unusually for a pimpline, has a wide range of hosts, including some present in or near our garden.

So, to conclude, a tricky identification, but one that is possible with care (and a willingness to back-track occasionally) and the right literature - and the result is a record for the local biological records centre covering a group that is likely to be under-recorded.


References

Broad, G. (2011). Identification Key to the Subfamilies of Ichneumonidae (Hymenoptera). Test key available for download here. [accessed 05/08/2011]

Fitton, M.G., Shaw, M.R. & Gauld, I.D. (1988). Pimpline Ichneumon-flies. Hymenoptera, Ichneumonidae (Pimplinae). Handbooks for the Identification of British Insects 7(1): 1-110.

Perkins, J.F. (1959). Hymenoptera. Ichneumonoidea. Ichneumonidae, Key to Subfamilies and Ichneumoninae - 1. Handbooks for the Identification of British Insects 7(2ai): 1-116.

Townes, H.K. (1969). The genera of Ichneumonidae, Part 1. Memoirs of the American Entomological Institute 11: 1-300.

Wahl, D. B. 1993. Key to subfamilies of Holarctic and Neotropical Ichneumonidae. Pp. 396-509 in: Goulet, H. and J. T. Huber, eds. Hymenoptera of the world: An identification guide to families. Agriculture Canada, Ottawa.