Walking with woodlice

The day started bright and warm, but I had my allocated wild slot in the early morning, so I decided to look and photograph some common woodlouse species. We tend to take these common invertebrates for granted, but woodlice are a diverse group with over 30 species in the UK with a range of habitat requirements, not all found on wood, and many species have quite restricted distributions. Five species are relatively easy to find and you are likely to have at least three in your garden, so my aim today was to find these common species.
  The Yorkshire Wildlife Trust's Wildlife Garden by Pearson Park was a good place to start, as Pill Woodlouse can reliably be found. And voila, a couple of Pill Woodlouse, Armadillidium vulgare, walking on the path greeted me, although they were in a bit of a rush and didn't stay around long. This species can survive in relatively dry habitats, and is not unusual to see it active in the open during the day. It can be found in sandy beaches, often under stones. Together with other related species it has the feature of being able to roll into a ball when disturbed, with its tough exoskeleton affording some protection to its softer underside.

Pill Woodlouse, Armadillidium vulgare, crossing the path at good speed

I then moved into the woodpile area. This is a shady, moist spot amongst trees. I looked under some logs and, squinting and getting really close I could see some Common Pygmy Woodlice, Trichoniscus pusillus. They are reddish and shiny, not very flattened and very tiny (up to 4 mm).

Common Pygmy Woodlice, Trichoniscus pusillus, on the underside of a log

 These little woodlice were in the company of the Common Shiny woodlouse, Oniscus asellus (above, one found under a brick) which looked like giants in comparison. This species reaches a maximum length of 16 mm, and are amongst the largest native species in the UK. The common shiny woodlouse is shiny and is quite flattened, mainly grey with paler patches and spots (where it stores calcium to form its exoskeleton).

 I went into a drier area, in the orchard, where there are loose bricks and pots over concrete. There I found one of the commonest species, the Common Rough Woodlouse, Porcellio scaber. It is a large species, slaty grey, with clear tubercles on its back. This species often forms large aggregations of various ages under logs, stones or bricks. This behaviour is thought to help reduce humidity loss.

Common Rough Woodlouse, Porcellio scaber, found under a pot

Under the same pot I found a Common Stripy or Fast Woodlouse, Philoscia muscorum. This species tends to have a dark head and a dark stripe along its back. It is quite colourful for a woodlouse and it keeps its body high when moving, which it can do quite quickly (hence its common name).

Fast Woodlouse, Philoscia muscorum, under a pot

That completes the group of common woodlouse species. But I had seen a few Rosy Woodlouse while moving some pots a few days ago. This is not very common, I tend to find single individuals under larger stones or under large pots that haven't been moved in a while, where there is enough damp organic material or mud. I thought I'd give it a try when I got back home. After lifting the pot and looking closely I did find a single one! They are 5 mm long, pink to orange, and have a yellow stripe on their backs and despite their size, quite obvious eyes. They have a rough surface, but my photo really didn't make them justice, see this one for a close look.

A Rosy or Pink Woodlouse, Androniscus dentiger, found under a large pot.

More Information

An identification key for the most common woodlouse species by the Natural History Museum, here.

Woodlice Parade

Woodlice are the easiest crustacean group to spot in the garden. Sure, your garden pond will likely contain several aquatic crustacean groups: Water Fleas, Copepods and Ostracods are likely. Woodlice are amongst the few crustacean groups that are truly terrestrial - although aquatic forms both freshwater and marine are also found.
Despite their terrestrial habits, woodlice species differ in how much humidity they need to survive. Some can live in very dry habitats, while others need a constantly high level of dampness.
 There are over 45 British species of woodlice, of them I have found six locally. Here I have put together a parade which might help you identify your garden woodlice.


1. Common Pill Woodlouse, Armadillidium vulgare. Pill woodlouse are quite resistant to low humidity conditions and live in dry, sandy habitats. When disturbed they roll into a ball, protecting their legs and antennae.

2. Rough Woodlouse, Porcellio scaber. Another species that can live in relatively dry habitats, like under pots. Recognised by its matt, bumpy grey surface. They tend to grip the ground with their legs when disturbed, making it difficult to dislodge them as their flared segments form a continuous surface with the ground, and then they can walk more or less fast. Very common and often found in large aggregations in suitable habitats (top shot).

3. Smooth Woodlouse, Oniscus asellus. A large, flattened woodlice with a shiny surface. As the rough woodlice, it tends to freeze and sit tight, making it hard for predators to dislodge it from the ground, although it can also move away. It is one of the largest woodlouse species in the UK.

4. Common Striped Woodlouse, Philoscia muscorum. Amongst leaf litter. A fast woodlouse, which rapidly hides when disturbed. Beautifully patterned, shiny with a dark head.

5. Rosy Woodlouse, Androniscus dentiger. A small species which favours damp habitats with rotting wood or organic material. Pinkish with a yellow dorsal stripe and dark contrasting eyes. It is the rarest in my garden.

6. Water Slater, Asellus aquaticus. Found in the leaf litter at the bottom of ponds. Long antennae, although many lost due to fights, then they regenerate (see photo below). Males larger than females.

More information
A key to British woodlice by Jon Rosewell, in the iSpot resources.
Walking with Woodlice, a Natural History Museum project.
Browse BugBlog's woodlouse related posts.

Marsupial water slaters

 Water slaters or water-lice, Asellus aquaticus, scurry about the leaf litter of my indoor pond. They are fascinating pond inhabitants, always active, sometimes three individuals walking in a line across the tank walls. Most of the water slaters had broken antennae when I collected them, but they have now grown back, the new segments were still thin and transparent a few weeks ago (above), but have now fully developed. Unlike insects, which have very limited regeneration abilities, crustaceans can regenerate lost or broken appendages - as even when adults they carry on molting, and in each molt, they regenerate a bit more of the missing leg or antenna.

Water slaters tolerate a wide range of ecological conditions and are distributed across much of temperate Europe, Russia and North America. They are often abundant in polluted water and live in lakes, rivers, springs and even caves - where albino subspecies with smaller eyes have evolved. Being scavengers, they just need some detritus and fallen leaves and organic material to subsist.

In the last few days, a female has laid eggs into her brood pouch or marsupium. While she climbed over the leaves beside the tank wall, I took a photo where her yellow eggs are visible through the transparent brood pouch.

The marsupium is made of overlapping flat blades coming out of the female's anterior four pair of legs, and allows the mother to hold the brood under her body. This structure is a common feature of all Isopods - which include also terrestrial woodlice. The female not only carries the eggs but also the developing larvae. The larvae will emerge as miniature versions of the adults (see the figure below).

Fig. I. Marsupial stages of Asellus aquaticus. Top plate: A, stage A showing two membranes (arrows). B, stage B showing one membrane (arrow) and cleft yolk mass. C, stage B shedding membrane to reveal lateral outgrowth (arrow). D. stage C with its comma shape surrounded by a membrane (arrow). Bottom plate: A, stage C shedding membrane (arrow) to reveal stage D with free appendages. B, stage D with lateral outgrowth (arrow) and appendages covered i n a closely-fitting membrane. C, stage E showing setose body and appendages (arrows). It is at this stage that the juvenile leaves the marsupium to become free-living (from Holdich & Tolba 1981)

Occasionally females expel some larvae from the pouch - possibly due to physical limitations of how many she can carry as they grow - and larvae can also develop normally outside the marsupium from developmental stage B. Probably they need the ventilation and extra oxygen produced by the mother in the initial stage. Carrying the eggs and larvae is also bound to offer them some protection from predators, so it probably evolved as a form of maternal care.

Although water slaters normally stop reproducing during the coldest months of the year, my indoor pond is warmer than outside, and this might have triggered the start of reproduction; a situation that also occurs in wild populations downstream thermal power stations where water is warm all year round. 

More information

Vitagliano, G., Fano, E., Marchetti, E., Colangelo, M., & Vitagliano, E. (1991). Importance of longevity, growth, and diapause in the evolution of Asellus aquaticus Bolletino di zoologia, 58 (2), 113-117 DOI: 10.1080/11250009109355740

Holdich, D., & Tolba, M. (1981). The effect of temperature and water quality on the in vitro development and survival of Asellus aquaticus (Crustacea: Isopoda) eggs Hydrobiologia, 78 (3), 227-236 DOI: 10.1007/BF00008519

Leaf sleepers

This is a good time of year to search for bugs in the leaf litter. On the top layer, ladybirds are set for the winter in the cosy, dry folds of the fresh fallen leaves. They usually cuddle up close to other ladybirds of the same species to overwinter, but it is not unusual to see two species together, like the 22 spot and 7 spot above. If you search a bit deeper on the soil, you can find centipedes, earthworms, and woodlice. With the colder temperatures, they are more torpid than usual, and then easier to photograph.

A shiny woodlouse, Oniscus asellus

Centipede, Lithobius forficatus

 A group of 22 spot ladybirds inside a vine leaf

Arion sp. slug

7 spot ladybird and birch shieldbug

Water slater embrace

Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
 Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males.  Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
 Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.

(from Verrell, 1985)


References
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127

The fast woodlouse

I have featured two common woodlice species before. There is a third common garden species, the fast woodlouse, Philoscia muscorum. As it name implies, it is quite fast, with long legs that keep its body well above the ground. Its dark, rounded head, and a longitudinal dark stripe along its shiny, marbled body, make it easy to identify (for a simple key to ID British woodlice click here). The fast woodlouse is a native European species which is also found in the U.S. It is of medium size (around 1 cm.) and inhabits a range of habitats from sand dunes to woodland. A P. muscorum population living in the dunes of Spurn Head investigated by Grundy and Sutton showed the phenomenon of year-class splitting. This means that there are two reproductive strategies amongst individuals born each particular season. The 'fast growers' reproduce when they are one year old and die, the 'slow growers' breed for the first and only time when they are in their second year.

Figure from Grundy and Sutton 1989. Note that the dark (slow) growers are joined in their second year by a new cohort (in grey).

Grundy and Sutton's experiments showed that females only reproduced with they reach large size and they have one or two broods (rarely three) with an interbrood period of 5.4 weeks. This and the positive relationship between temperature and growth rate means that most individuals born in the first brood are able to reach a large size before winter sets in and start growing faster when the breeding season arrives. Most individuals born in the second brood (a bit more than a month later) miss out on the benign growing conditions and are too small after the winter and when the next breeding season arrives they are still too small and waiting until the following breeding period makes sense to maximize their chances of reproducing successfully.
More information
Grundy, A. & Sutton, S. (1989). Year class splitting in the woodlouse Philoscia muscorum explained through studies of growth and survivorship Ecography, 12 (2), 112-119 DOI: 10.1111/j.1600-0587.1989.tb00829.x

Night prowl

I had to go out late at night in the garden a few days ago. I took the torch as I dislike squishing slugs or snails walking on the path. Indeed, there were a few yellow slugs (Limax flavus), garden slugs (Arion hortensis) and Common rough woodlice, but, surprisingly, prowling in the middle of the path, I found a shockingly bright-coloured female woodlouse hunting spider (Dysdera crocata) - of which I wrote a post recently.

The spider seemed uninterested in woodlice, by the way, as it walked around one of them. A pleasure to observe this nocturnal spider in its environment.

The common woodlouse

With this photo of a Common Woodlouse (also known as shiny or smooth woodlouse) Oniscus asellus, let me round up the topic of woodlice. Woodlice are crustaceans, a mostly marine group. They have 7 pairs of legs when adults. They are quite gregarious, and usually many individuals are found together. The common woodlouse is not as drought resistant as the rough woodlouse, as their bodies are more permeable to water, and therefore is present in more humid microhabitats. It has a very shiny cuticle with dark and light patches, although colour varies, their antenna are longer than in the rough woodlouse. We found these large individuals (this is the largest UK species and can reach 16 mm in length) today under large logs kept covered with some tiles.

A side view of a different individual.

A front view of the first individual.

The common rough woodlouse

Although woodlice have been mentioned in BugBlog a few times, they haven't been the main character in any post yet, and this has to be addressed. This little group I found yesterday under a pot gives me the perfect excuse. There are over 3,000 described species of terrestrial isopods (woodlice) in the world, with only 35 native to the UK. The Common Rough Woodlouse (Porcellio scaber) - above- is one of the most common. Unlike the Pill Woodlouse (Armadilium), it is unable to roll into a ball, and instead, its defence mechanism consists in grasping the substrate and pressing its dorsal armour against it, so that its more vulnerable underside is protected. Its nocturnal habits and love of humid places reflect the fact that it loses water very easily from its body due to transpiration as their surfaces are not waterproof. Woodlice absorb water from their surroundings when they rest under stones, in compost heaps or similar places and they lose it when they forage away from their refuge. They are detritivores and feed on dead vegetable matter and therefore are involved in the decomposition of dead leaves and the production of compost.
 Woodlice usually have separate sexes, that is, there are males and females. Females brood the eggs and their young in their first instar, suplying them with nutrients and water, in a pouch under their body. In the common rough woodlouse, females can produce three broods (of around 20 young) a year in good conditions. Breeding is very seasonal, and starts when days become longer (16 h light). Woodlice can live several years.

The log pile revisited

We made a couple of log piles in the garden last year. Today I dismantled one of them to check what was in it. This was a very 'untidy', and relatively dry log pile, with the logs having been covered on branches from pruning bushes around the garden and the location being under a large Philadelphus bush. The first finding was that a lovely crumbly pile of compost had accumulated around the logs. There was a lot of bug activity in the pile. The most abundant form of -visible- life were woodlice, especially the common rough woodlouse, Porcellio scaber, which made the compost move as they were so many, also some smooth woodlouse, Oniscus asellus and common striped woodlice Philoscia muscorum. I found three species of snails: many common garden snails, Helix aspersa (=Cornu aspersum) most of them hibernating; a yellow banded snail Cepaea (one of 5 specimens we released in the garden a couple of years ago), and several Glass Snails Oxychilus draparnaldi (photo above). Several yellow slugs Limax flavus crawled around. There were quite a few spiders, amongst then a very large Tegenaria and possibly an Amaurobius, which rapidly disappeared amongst the logs. I also found some Blunt-tailed Snake Millipede (Cylindroiulus punctatus), which coiled into a spiral when disturbed. I saw some new springtails, but I couldn't get a decent photo. As for insects, a Harlequin seemed to be hibernating on a log. By far the most shocking thing was a couple of enormous earthworms, Lumbricus terrestris, which I found while they abandoned the log pile - possibly feeling the disturbance - by creeping on the ground. One of them was so large that at first I thought it was a snake! Big fun examining this log pile, I am sure I forgot to mention some bugs. I didn't even go through the whole of it but it yielded quite a number of interesting finds.

A banded snail, some woodlice and an earthworm

Garden Snails

Three species of woodlouse can be seen in the image

A specimen of the millipede Cilindroiulus punctatus

The largest earthworm I've seen in my garden, creeping on the ground

The same earthworm being held, just for scale

A yellow slug, Limax flavus

Woodlice hunting spiders

I occasionally come across Woodlice hunting spiders (Dysdera) in their silky day retreats when moving pots. These relatively large and brightly coloured spiders are strongly photophobic and run away from the light as fast as they can looking like they are dragging their bodies on the ground as they go. I haven't managed to take a decent photo of these spiders outside my plastic bug container (above) because of they dislike of lights. Dysdera spiders have traditionally been considered specialists on hunting woodlice. They have long, thin and highly mobile forward facing chelicerae (fangs) which it uses as pincers, catching the woodlice and then piercing the underside not as armoured side of woodlice. They are known to be able to deliver a painful bite if handled by brave humans. As they hunt during the night, when they come out of their retreats, their diet is difficult to study in the wild and there is some debate as to how specialised on woodlice these spiders are. In captivity, they are not fussy eaters, and feed on a variety of invertebrates, they seem to say no to nothing! What is certain is that they do eat woodlice, and the few times that they have been found in the wild with a captured woodlice suggests they might be an important part of their diet. Dysdera spiderlings raised on woodlice alone or combined with flies grew faster than those raised only on flies, sugesting some degree of physiological specialisation. In addition, not many spiders like woodlice - one of the few is the daddy long legs spider Pholcus- and here is a photo of it.

A Pholcus with its woodlouse prey in the kitchen

More information

S.D. Pollard, R.R. Jackson, A. van Olphen and M.W. Robertson (1995) Does Dysdera crocata (Araneae: Dysderidae) prefer woodlice as prey? Ethology, Ecolgy & Evolution 7:121-275. here.

J. A. L. Cooke (1965) A Contribution to the Biology of the British Spiders belonging to the Genus Dysdera. Oikos, Vol. 16:20-25. here.

Milan Rezac and Stano Pekar (2007) Evidence for woodlice-specialization in Dysdera spiders: behavioural versus developmental approaches. Physiological Entomology 32: 367-371. here.