Ants as farmers, aphids in the slow lane

If you grow soft fruit such as currants or gooseberries, you are probably familiar with the sight of numerous aphids densely clustered around the stems, making the leaves wrinkled and deformed. You may also be aware that some ant species 'farm' aphids in order to consume the sugar-rich 'honeydew' that they excrete. However, this is a complex behaviour and so is worth a closer look.

A colony of Aphis schneideri on black-currant Ribes nigrum, attended by black garden ants Lasius niger

First of all, there are numerous aphid species that are found on cultivated fruits, and they can be difficult to tell apart. Those in the photo above are on black-currant Ribes nigrum - within the Ribes-feeding group, only one species has long, acute, erect to semi-erect hairs on the antennae (especially near the base), and that is what is seen here, Aphis schneideri (I couldn't get a clear photo of the antennal hairs, so you'll have to trust me on that). A. schneideri is also commonly associated with R. nigrum, causing clusters of terminal leaves to fold and distort, and the aphids are grey-green, with paler legs and often with a scattered coating of small pale waxy particles.

The ants seen attending the aphids here are the common black garden ant Lasius niger. Not all colonies of L. niger do this, although when it occurs, there is mutual benefit - the ants obtain sugar-rich honeydew and in turn protect the aphids from predators. So far, all straightforward, but exactly how the ants do this was not understood until a few years ago. It has been known for some time that, in order to ensure a dense crop of aphids, ants limit their dispersal (aphids otherwise disperse to new plants when overcrowded, the production of winged forms being triggered by the frequency of contacts between individuals) by biting off their wings (Kunkel 1973) or by secretions from the ants' mandibular glands interfering with the development of winged aphids (Kleinjan & Mittler 1975). Conversely, ants have been seen carrying aphids to suitable, good-quality host plants within their nest's home range. However, aphids also disperse by wandering (which may be more prevalent than dispersal by flying) and some decades ago, both Banks & Nixon (1958) and El Ziady (1960) noticed that the presence of ants seemed to slow aphids down, but no further research was undertaken until Oliver et al. (2007).

Ants are known to lay 'semiochemical' trails by touching glands onto the surface of their substrate, and these chemical marks can lead others from their nest sources of food (Hölldobler & Wilson 1990). Such chemicals can also be applied passively through shedding of cuticular hydrocarbons and this is important recognising nest-mates and, possibly, home-range territories (Devigne & Detrain 2002). Moreover, other insects, including herbivores and aphid predators, have  been found to respond to chemicals that indicate the presence of ants (Offenberg 2004). However, Oliver et al. (2007) found that these ant chemicals had a tranquilising effect on aphids, causing them to walk more slowly and thus - as with the other effects noted above - maintain higher densities which provide the ants with more honeydew.

Although ants have been seen deterring aphid predators such as ladybirds, this does not mean that the mutualistic arrangement is entirely positive for the aphids. Their manipulation by ants means that their dispersal is reduced (or at least delayed) which may have impacts on the wider population (or 'metapopulation') and increase competition between closely packed individuals. Also, dense aggregations may be repeatedly attacked, and sometimes wiped out, by specialist parasitoids which are able to evade ants, such as the small parasitic wasp Lysiphlebus cardui (Weisser & Völkl 1997). Lastly, ants are occasionally seen eating aphids and it appears that this primarily occurs when individual aphids are encountered away from the aggregation. Not only would this prevent aphids being used for the benefit of rival neighbouring ant nests, but presumably produces an evolutionary pressure to 'obey' ant chemical cues rather than be able to avoid the tranquilising effect.

A Lasius niger individual at a colony of Aphis schneideri

So, definitely a more complex story than it initially seemed (isn't it always!), but also a more interesting one, and something you may be able to witness in your own garden. If you are interested in identifying aphids, a standard work covering British Aphidini (the 'tribe' including the genus Aphis) is Stroyan (1984), but be warned - this is not an easy group to identify to species level, and the separation of some species can be uncertain.

References

Banks, C.J. & Nixon, H.L. (1958). Effects of the ant, Lasius niger L., on the feeding and excretion of the bean aphid, Aphis fabae Scop. Journal of Experimental Biology 35: 703-711.
Collins, C.M. & Leather, S.R. (200). Ant-mediated dispersal of the black willow aphid Pterocomma salicis L.; does the ant Lasius niger L. judge aphid-host quality? Ecological Entomology 27: 238-241.  
Devigne, C. & Detrain, C. (2002). Collective exploration and area marking in the ant Lasius niger. Insectes Sociaux 49: 357-362.
El Ziady, S. (1960). Further effects of Lasius niger L. on Aphis fabae Scopoli. Proceedings of the Royal Entomological Society A 35: 30-38.
Hölldobler, B. & Wilson, E.O. (1990). The Ants. Harvard University Press, Cambridge, MA.
Kleinjan, J.E. & Mittler, T.E. (1975). A chemical influence of ants in wing development in aphids. Entomologia Experimentalis et Applicata 18: 384-388. 
Kunkel, H. (1973). Die Kotagabe der Aphiden (Aphidina, Hemiptera) unter Einfluss von Ameisen. Bonner Zoologische Beiträge 24: 105-121.
Offenberg, J. (2004). Evidence that insect herbivores are deterred by ant pheromones. Proceedings of the Royal Society B 271: S433-S435.
Oliver, T.H., Mashanova, A., Cook, J.M., Leather, S.R & Jansen, V.A.A. (2007). Ant semiochemicals limit apterous aphid dispersal. Proceedings of the Royal Society B: Biological Sciences 274: 3127-3131.
Stroyan, H.L.G. (1984). Aphids - Pterocommatinae and Aphidinae (Aphidini). Homoptera, Aphididae. Handbooks for the Identification of British Insects 2(6): 1-232.
Weisser, W.W. & Völkl, W. (1997). Dispersal in the aphid parasitoid, Lysiphlebus cardui (Marshall) (Hym., Aphidiidae). Journal of Applied Entomology 121: 23-28.

Banking on bees

Much has been written about the importance of sunny banks for invertebrates - they are often included (sometimes as 'beetle banks') in habitat management plans and as sometimes appear as recommended features related to mitigation of various types of site development. There is much about this and other aspects of habitat management for invertebrates in e.g. the excellent Kirby (2001), but I want to focus on a local example of a feature that has just this function, albeit unintentionally - a south-facing bank alongside the footpath leading to the front of a nearby church in Hampshire, southern England.

The south-facing bank in a nearby churchyard

This is not a huge feature - it's around 10 metres long and no more the around a metre high at most (as the path rises, the bank gets lower until it merges with the rest of the churchyard). However, there are nearby flowers as pollen sources, plus small bare patches of light soil, just right for burrowing. It is also south-facing and hence sunny and warm - this is key as the north-facing slope on the other side of the path does not support the range of pollinators described below. A couple of days ago, these were found and photographed during less than an an hour on a sunny day in what is currently proving to be an unusually warm spring (I'll leave out discussion of climate change on this occasion). So, what did I find?

The bee-fly Bombylius major

The commonest bee-fly in the UK, B. major can be identified by its broad, dark wing-bands (not to mention the long proboscis characteristc of this gorup) and parasitises a number of solitary bee species - so, as well as being a herald of spring, it's also an indicator that there are bees around somewhere. However, in this case, a indicator of bee activity wasn't needed - I just needed to look at the grassy bank to see a flurry of activity.

Andrena cineraria - an attractive black and white ground-nesting species

The most obvious bee species was A. cineraria - I spotted around 15 at one time (it's difficult to be more precise as they rarely stop flying around), including some using entrance holes to burrows, plus brief 'tussles' between pairs of individuals. In some places, this species nests in huge aggregations of thousands of females and attendant males (Baldock, 2008) and I wonder if these 'tussles' are aggressive encounters related to access to nest-burrows, or are failed mating attempts - they are too fast to be certain about the sex of the participants. Of course, where one species is found, its parasites often follow - in this case the cleptoparasite Nomada lathburiana which can be very numerous in the large aggregations mentioned above. Cleptoparasites are also known as 'cuckoo bees' as they lay their eggs in nests of other bee species, their  larvae feeding on the food provided for the host larvae. Needless to say, I was on the lookout for N. lathburiana and it wasn't long before I saw one investigating the entrances to A. cineraria nests.

Nomada lathburiana showing the yellow spots and bands seen in several species of this genus.

However, although this was an interesting find, there were other bee species, including a small (aroung 6mm long) fairly dark bee that was extremely difficult to photograph due to its rapid darting flight and tendency to re-launch whenever I got near. However, I did manage to get a shot and it turned out to be another cleptoparasite, N. flavoguttata.

Nomada flavoguttata - note the reddish-brown bands on the abdomen, including a pair of tiny yellowish dots (the species name means 'yellow spots').

Although possibly not that familiar, this is a fairly common species which parasitises several small Andrena species. This is important because at this point I hadn't seen any small Andrena, only A. cineraria, so I continued looking. Then, among the flurry of black & white bee bodies, I noticed a likely candidate - a small bee crawling around under the leaf litter. Again, photography was tricky as, when on the ground, it crawled underneath leaves, and flew rapidly away as soon as it emerged - definitely camera-shy! However, I did get the following picture, and (from other views, including while feeding on dandelion Taraxacum flowers) enough detail to identify it as A. minutula, the commonest of several similar species and a known host of N. flavoguttata.

Andrena minutula crawling underneath leaves.

Two host-parasite pairs was good for an hour's work, but there was more bee-biodiversity to be found as I saw another Andrena species exiting a burrow.

Andrena flavipes flying away from its burrow

A. flavipes is identified by the pale abdominal hair-bands and orange-yellow pollen-hairs in the hind tibiae, and like A. cineraria, can be found in large nesting aggregations. It also has its own cleptoparasite, N. fucata although I could not find this although it is present at almost all of its host's nest sites - more hunting will be required.

So, I hope these observations have reinforced the importance of sunny banks as invertebrate habitat - including for our often-declining, yet essential, populations of pollinators. If you have a little spare space in your garden or another piece of land, why not add a little bank - even a small one will attract invertebrates.

An insect's-eye view of the bank with A. cineraria flying overhead.

References

Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Pirbright.
Kirby, P. (2001). Habitat Management for Invertebrates: a Practical Handbook (revised reprint). RSPB, Sandy.

Antscape: ghosts in the graveyard

Despite the title, it's early March and so couldn't be much further from Halloween, but wildlife respects its own calendar, not ours... Anyhow, having gone for a wander in the woods a few days ago, I took my usual shortcut through the nearby churchyard and noticed something intriguing. On many of the graves, especially on the corner-stones, tussocks of soil and grass had developed. I must have seen this dozens of times before but it had never really grabbed my attention - this time however, I stopped to take a closer look...

Tussocks on the corners of a grave.

From a distance, these tussocks just look like long grass that the mower has missed. However, close up it is clear that they are actually formed of fairly loose soil with grass and moss growing on it. Like any good ecologist, I began to delve...

Yellow Meadow Ants (Lasius flavus) in a churchyard 'tussock'

As soon as pulled open one of these tussocks, it was clear how they had formed - they are actually anthills of the Yellow Meadow Ant (Lasius flavus). L. flavus is a common species, associated with the formation of 'antscapes' comprised of hundreds of closely spaced anthills on undisturbed grasslands (although sometimes they don't form anthills). In Britain it is the only species building long-term nests of the 'unthatched mound' type - a couple of others such as L. niger produce occasional small, temporary mounds, while the Wood Ant Formica rufa produces much bigger mounds covered ('thatched') with bits of twig etc. (Skinner & Allen 1996). L. flavus nests are up to about 30 cm tall, the mound effectively being the spoil heap from soil dug out when the colony builds its underground nest of tunnels and chambers. Being subterranean, L. flavus is rarely seen unless sought out, despite probably being Britain's commonest ant - the underground habit has led to reduced eye development and even soil is brought to the surface at night (Pontin 2005), so daytime activity is unlikely to be seen in passing - certainly I saw no ants on the surface. This also keeps different species apart - in this case, L. flavus lives in the same areas as L. niger, but the latter lives on the surface forming much larger territories (active L. flavus mounds can be as little as 2 m apart, below which inter-colony competition is too intense and queens are attacked prior to establishment of a new colony) (Pontin 2005). As the ants cannot make mounds in mowed (or heavily grazed) areas, it is clear that their use of the graves as a focus for their anthills allows them to avoid disturbance by mowers. In meadows, I have seen newly forming nests being built around plant stems - possibly to provide an initial scaffolding for the loose soil particles.

Like many ant species, L. flavus feeds on the sugary 'honeydew' produced by aphids that are tended by the colony. However, the ant-aphid relationship is more complex than this as L. flavus has 'collected' a number of subterranean aphid species (e.g. of the genera Forda, Geoica, Aphis, Tetraneura and Baizongia) which, in Britain, have lost their sexual generation associated with woody plants and are now entirely asexual on the roots and stolons of grasses. Others such as Sappaphis bonomii lay over-wintering eggs on plants above ground, and L. flavus tends them as if they were ant eggs (at the wrong time of year). Not only that, but as well as using aphids as sources of honeydew, L. flavus also uses them as prey to feed its larvae (Donisthorpe 1927, Pontin 2005). The aphids rarely disperse openly above ground level, suggesting that despite the possibility of being eaten by ant larvae, the protection of subterranean ants outweighs this risk. Further, this may effectively form a type of 'culling', keeping aphid density below levels where they tend to start producing winged forms for dispersal.

So, although this is a common and widespread species, it hides a symbiotic lifestyle that is rarely seen despite taking place beneath our feet as we walk across many an old pasture or other undisturbed grassland. I'm certainly tempted to ask permission to investigate these churchyard mounds more closely and maybe learn more about the aphids and other 'guest' invertebrates that can be found within. As the church (St. Mary's, Bishopstoke) was consecrated in 1891, there has been plenty of time for the ant community to develop. Lastly, I want to look briefly at the landscape, or 'antscape', effects of L. flavus. In an old pasture, the anthills are close together (active ones may abut or overlap old inactive ones), but their arrangement is effectively patchy and random. In the churchyard, the association with graves means that this is not the case and the anthills are arranged more or less in a grid matching the positions of graves - even where these have since disappeared - in such cases the pattern of anthills marks the outlines of old graves - ghosts in the graveyard!

Pattern of L. flavus anthills following the locations of current and missing graves.

References

Donisthorpe, H.St.J.K. (1927). Guests of British Ants. Routledge, London. [An old classic; can be a bit expensive, and a lot of the taxonomy has changed, but still worth having].
Pontin, J. (2005). Ants of Surrey. Surrey Wildlife Trust, Woking. [Maps of ant distribution in Surrey, but plenty of other more widely applicable information about British ants].
Skinner, G.J. & Allen, G.W. (1996). Ants. Richmond, Slough. [An excellent little book in the Naturalists' Handbooks series. Includes species-level keys to the British ant fauna. If you want just one book on British ants, I recommend this].

Further reading

Agosti, D., Majer, J.D., Alonso, L.E. & Schultz, T.R. (2000). Ants. Standard Methods for Measuring and Monitoring Biodiversity. Smithsonian, Washington DC. [Takes a global approach with particular emphasis on the tropics and Americas, but covers a range of widely applicable techniques and ideas].

Observations of Macleay's Spectres I: early stages

Last summer, I was given an unusual birthday present - a 35mm film canister full of eggs of the Macleay's Spectre stick-insect Extatosoma tiaratum, sometimes known as the Giant Prickly stick-insect. These are found mainly in the Australian forests of Queensland and New South Wales where they feed on the foliage of eucalyptus trees, although in captivity they eat a range of unrelated species - one popular food is bramble (Rubus fruticosus agg.) which is what I use as it is readily available for free. However, young bramble leaves can contain noxious chemicals that make them unpalatable or even toxic, especially to young insects - I tend to remove these, and when some have been included by accident with more palatable older leaves, the insects ignored them.There is plenty of information about this species (e.g. from the Phasmid Study Group) as they are popular pets for those who like their companion animals to be six-legged, but I have made some observations which may be of interest. So, let's start at the beginning - with eggs...

Egg of E. tiaratum with the lid (operculum) open.

Close-up of the operculum showing the translucent sealing membrane around the edge. There is another membrane below this which seals the opening to the egg and which is broken to allow emergence by the first-instar nymph.

The eggs are a few mm long, oval with a sculptured ridge along one long axis, and a round lid (operculum). They are speckled with various shades of yellow and brown, camouflaging them to look like seeds when they fall to the forest floor in the wild. In those I have at home, the eggs are a pale yellow-brown when freshly laid and dry, but when placed on damp tissue to (eventually) hatchm they become a dark brown - presumably this aids camouflage with dry soil conditions also being paler and darkening when there is rain. When females lay eggs, they often flick them several feet by twitching their abdomens, presumably to aid dispersal  - the function of this might be to prevent a cluster all being eaten in one go by a predator. The eggs have a coating (made of lipids and other organic materials) which is edible to ants and which induces them to take the eggs to their colony and eat this coating. The eggs remain otherwise intact and the ants dump them on the colony's waste pile where they hatch. Thus the eggs are likely to become clustered following the initial dispersal during laying but will have spent much of their time in an ant colony where the chance of predation is much reduced. Luckily, the eggs can hatch without the attention of ants which means that those missed by foraging ants and of course those in captivity remain viable.

Unsurprisingly, given their birthplace, E. tiaratum hatchlings (forst-instar nymphs) are ant-mimics, specifically of the large, long-legged 'spider ants' in the genus Leptomyrmex.

Recently hatched nymph showing the orange head and black body providing its mimicry of Leptomyrmex ants.

The orange head and black body, as well as overall shape closely mimics that of ant species such as L. darlingtoni, L. erythrocephalus and L. rufipes and so benefit from appearing similar to these toxic species (the sting has degenerated in this group of ants and they instead secrete a chemical repellant).

When my specimens hatched, I couldn't help but notice that the nymphs were highly active (a well-known characteristic) and that their preferred direction was definitely up. This makes sense as they hatch on the ground but are foliage feeders and so need to climb quickly into trees to find food. After their first moult, they lose their ant-mimic coloration and develop one of a range of camouflage colours - oranges, yellows, browns, greys and greens - and they also become much less active, more like the typical behaviour of a stick-insect. They do make swaying movements to mimic the motion of leaves, especially when disturbed, but otherwise spend much of their time stationary or moving slowly as they feed, though being nocturnal much of their activity goes unobserved (or would if I wasn't so nosy). The image below shows three nymphs at different stages and with different colours. In subsequent parts of this series, I'll be looking at changes in colour and behaviour (in males and females, highlighting sexual dimorphism) plus the process of moulting.

Three nymphs showing different colours and stages of development. The two smaller ones are 2nd instar nymphs, the larger one a 3rd instar.

Feathers, flowers and feelers - more on wildlife-friendly gardening

Back in late March, I posted about bee-friendly gardening and the range of species found in our little terraced plot in southern England. Three months on, the early summer has turned into autumn, back to spring, and may again become summer soon, so I thought I'd provide another informal update about what's been happening in the garden. Also, Blogger's not playing nicely today, so apologies for the peculiar formatting 'choices' below!


As I sit here, I can see our contribution to the (hopeful) recovery of the UK's beleagured House Sparrow (Passer domesticus) population flitting about in the garden near the seed feeder - including fledged young still fluttering for food near their parents. There is no evidence of sparrows nesting in the garden, but just outside the front of our house, they can be seen being particularly resourceful...

Female sparrow sitting outside a nest entrance - inside a telephone-line junction box. The male has also been guarding and there has been much activity with food and nest materials being brought. This box was also used last year, but the box on the other (south-facing) side of the pole is much less popular - maybe due to the temperature difference?

Of course, sparrows are not the only birds using the garden - the bath has been very popular with bickering, and occasionally even English-style queueing.

Blackbird (Turdus merula) drinking from the bird-bath

Wood Pigeon (Columba palumbus) hogging the bird-bath - the passerines can be heard shouting nearby...

Now, our garden isn't huge - it's a long thin Edwardian terrace; there's hard-standing at either end (sheds, patio etc) but in the middle it's split into flower & veg beds, plus a wild 'meadow' area. It may not be huge but it does have diversity of structure, plus the planting is either native or known to be wildlife-friendly (especially for pollinators). Also, even the hard surfaces have their purpose - plants are allowed to grow over the sheds, and on the patio, some isolated slabs have been removed to allow low-growing, mat-forming species to be planted.

Meadow Clary (Salvia pratensis) - extremely rare in Britain, we now have one plant in our tiny wild meadow! Hoping to collect seed...

An ant's-eye view of the burnet-like Acaena planted where a patio slab has been removed.

There's more I could add about birds and plants, but as you may know, my favourite group is the invertebrates. Now, I do understand that many gardeners may be suspicious about non-pollinating insects and consider many to be pests. However, apart from a very few (we do apply nematodes as a biological slug control), we tend to leave the plants and beasties to fight it out among themselves and have had no major problems so far (the pigeons are more troublesome). It also provides opportunities to watch some interesting inter-species interactions.

Partly added as a pretty picture, here is a spider (Metellina sp. I think) building a web inside a foxglove (Digitalis purpurea) flower, hoping to catch those flighty nectar-feeders.

One of our many many garden ants (Lasius sp.) tending, maybe 'milking' honeydew from, A Brown Peach Scale insect (Parthenolecanium corni).

The diversity of arachnid forms writ large - a Trombidium mite and a Pardosa wolf-spider

While on the subject of spiders, our garden is full of Garden Orb-weavers (Araneus diadematus) which implies there's plenty of food - unsurprising given the array of tiny things that crawl and fly here. However, it was only recently that I saw the stage in their development just prior to scattering (somewhat literally) to the four winds.

Shhh... the spiderlings are sleeping...

What was that? They've started to stir...

Aaaagh, run for it!

Some bungee from the bottom of the nursery web, others make a run for it - the one top right is winning...

Oh, it wasn't anything dangerous - they start returning to the web to reform the ball, some climbing back up bungee-ropes, others crawling back along various strands. If it had been a predator or other threat, the chances are some would have escaped as they scatter in all directions.

And so, there is space for a wide range of species even in our fairly small garden - it just needs some thoughtful planting, a few other features (feeders, bird-baths, a pond is in the construction stage), diversity of structure, and a willingness to leave things alone as much as possible. This is an important little bit of nature conservation most of us can do to some extent, and if you enjoy recording species, gardens can be important sources of biological records (in the UK there are popular garden-based bird and, to a lesser extent mammal, recording schemes designed for non-specialist volunteers). Anyhow, enough from me today - more soon, and eventually I imagine I will eventually compile a garden species list...

Ants round my home, woodlice in theirs...

My entomological side is getting twitchy - after months of near-invertebrateless cold, I can see a variety of multi-appendaged beasts crawling, flying and generally being busy. So, during a trip to dig the allotment, I couldn't help noticing quite a bit of ant activity. They haven't found the house (i.e. the kitchen) yet but are busily working in the garden, around all kinds of cracks and various bits and pieces. As I left through the back gate, there were even some that had helpfully started work at head height (in and around two bolt-holes in a gatepost) allowing me to look at them closely without crouching down: most thoughtful...

Black ants busy around a hole in a gatepost.

Looking more closely, it seemed as if they were clearing out the remains of previous occupants - there looked to be bits of woodlouse and spider coming out. Closer still, building work was going on with 'cement' plates being formed inside the hole - luckily I managed to capture one ant placing a mouthful (mandible-ful?) of building material...

Note on the left - one ant placing building material

They look like one of the common dark garden species, but it's always worth checking so I brought a couple indoors to see if I could key them out (using Skinner & Allen 1996). The single-segment waist with well-developed petiole (a little plate sticking up from the waist), circular spiracle towards the rear of the propodeum (segment just in front of the waist) and a lack of rows of bristles on the underside of the hind tibia brought me to the genus Lasius (not unexpectedly...). From there, various features including the presence of standing hairs on the antennal scape and hind tibia (but not the double rows seen in other groups) took me to the common species L. niger.

Hind tibia with standing hairs

Head, including antennal scape with standing hairs (see below the eye)

OK, so I've been distracted - time to dig the allotment and off I go... digging (finally) takes place and I'm almost finished when I notice (a) some orange-yellow ants and with them (b) some tiny white woodlice. This looks straightforward enough - the ant is Lasius flavus nesting in the grassy path and the woodlouse is the Ant Woodlouse Platyarthrus hoffmannseggi.

L. flavus and P. hoffmannseggi - both around 3-4mm long

P. hoffmannseggi - the yellow colour is just due to lighting/image processing...

P. hoffmannseggi is an interesting beast - it is white (in the top photo you can just see dark gut contents through its dorsal surface), blind and lives closely associated with ants of almost any species (Hopkin 1991), often found in their nests where it may feed on pellets regurgitated by the hosts (Williams & Franks 1988) and/or may be a more general scavenger in the 'rubbish-dump' parts of nests (Pontin 2005). It has also been noted as following lines of ants when nests have moved (Donisthorpe 1927). This is unusual behaviour for a woodlouse (it is the only myrmecophilous - 'ant-associated' - woodlouse in Britain), but not unique - in Malaysia there is a species of Exalloniscus which 'hitchhikes' (maybe 'stows away' is more accurate) on the pupae of ponerine army ants. Although not often seen without digging, it is fairly common in southern England and Wales, becoming scarcer northwards (will climate change extend its range, and if so, how does it disperse?). Beyond this, little is known about P. hoffmannseggi - an area for a keen researcher to work in maybe?

References

Donisthorpe, H. St. J. K. (1927). The Guests of British Ants.Routledge, London. [interesting if you can find it, but can be expensive]
Hopkin, S. (1991). A key to the woodlice of Britain and Ireland. Field Studies 7(4): 599-650. [this has also be published in separate reprint form]
Pontin, J. (2005). Ants of Surrey. Surrey Wildlife Trust, Woking. [another volume in the excellent series of Surrey atlases]
Skinner, G. J. & Allen, G. W. (1996). Ants. Richmond, Slough. [excellent keys to British species]
Williams, T. & Franks, N. R. (1988). Population size and growth rate, sex ratio and behaviour in the ant isopod Platyarthrus hoffmannseggi. Journal of Zoology 215: 703-717.