Cellar spider catches a woodlouse (arachnophobes look away now)

 

The cellar spider Pholcus phalangioides is a welcome guest in our house. In summer it's a very effective fly killerand it also catches and eats much larger spiders, of the kind that sometimes turn up in the bath or race across the ceiling and floor when the cold weather brings them indoors, in autumn. A few cobwebs in corners of the ceilings are a small price to pay for these services.

Sometimes these spiders catch woodlice, which is what this one has trapped in its web, alongside a fly that has already been encased in silk. But this woodlouse was a prize catch, because its a female that was carrying baby woodlice, known as mancas, in its brood pouch, under those armoured plates. They are the tiny translucent woodlice, as yet with no hardened armour, that you can see in the photograph below.

Female woodlice carry their young in a brood pouch under their body, and at this stage the young only have six pairs of legs. After their first moult they develop an extra body segment and after the second moult they grow an additional pair of legs, so they are then classified as juveniles with the full adult complement of seven pairs of legs.

For a closer look at woodlice, check out my Beyond the Human Eye blog here.

Bibble-bugs

Thursday's Guardian Country Diary is about these charming little crustaceans - woodlice. They are reputed to have at least 65 colloquial names - more than any other animal in the UK fauna - and are locally known as bibble-bugs, coffin cutters, sow-bugs, tiggy-hogs, sow bugs, sink-lice and slaters.

I disturbed these while I was moving a pile of bricks under our hedge. Well over 100 were packed into the recessed face of one brick and once they were exposed to sunlight there was panic, as they fell over each other in their hurry to escape.

Eventually only this one remained - a fine specimen of Oniscus asellus, the common shiny woodlouse. I particularly like the elegant curves of its articulated armour plating and their saw-tooth outline.

Woodlouse senses are centred around the jointed antennae and these simple eyes that have only about 25 individual ocelli – probably enough to detect light and shade and largish moving objects, but incapable of forming images with a very high degree of resolution.

The tail segment of a woodlouse is called the telson, flanked by two appendages called uropods , and its shape is often an important species identification feature.

All woodlice have only six pairs of legs in their infancy (when they’re known as mancas) and the full complement of seven pairs, visible here, only appears after their first moult, a day after they’re released from the brood pouch of their mother who carries them around. 

From below you can see the mouth at the head end, between the antennae .

Woodlice are omnivores but will eat other small animals if they can catch them, so have two pairs of jaws – crushers at the front and lethal-looking pointed ones behind.

The armour is an obvious defensive adaptation to surviving terrestrial predators like spiders but the woodlouse’s main problem is keeping moist, because it obtains oxygen by diffusion over these plates at the tail end. 

Generations of schoolchildren have conducted simple experiments offering woodlice a choice of moist or dry environments but the outcome is never in doubt – in a dry environment a woodlouse will suffocate, for lack of dissolved and diffused oxygen.

Mother and child

Woodlice face a regular crisis when they need to moult in order to grow. They do this in two stages, shedding the old carapace from the hind segments first and then easing themselves out of the cladding on their front segments. You can see in this photograph that the exposed outer covering on this animal's newly exposed front segments has yet to harden. 

I made my excuses and left ........

The compost bin at the bottom of our garden recycles all our vegetable-based kitchen waste and every time I lift the lid it reveals a new population of animals, most of them small ........

.... like this hairy little owl midge, about three millimetres long. 

This minute juvenile woodlouse, also just a few millimetres long and looking like it's not old enough to be out on its own, was lurking under the bin lid. To give some idea of how small it was, compared with a full-sized woodlouse ....

........... here's a slightly larger juvenile (about three times the size of the one above), under the watchful compound eyes of a full grown woodlouse.

The prize for the most numerous insects in the compost bin must go to these minute flies, smaller than the owl midges, that breed with amazing speed and in incredible numbers on the decaying fruit and vegetable peelings. Every time I lift the lid I have to step back as a small cloud of them takes to the air. I rarely see singleton flies of this species, because they seem to be ......

.... perpetually mating, in this push-me-pull-you conformation.

When I was a callow youth I used to frequent a barber's shop where they always had a pile Reveille and Tit-bits magazines for customers to leaf through while they waited for a trim. For those unfamiliar with these fine publications (long-since defunct),they specialised in 'tasteful' glamour photos of ladies who seem to be having trouble keeping their clothes on, together with sensational and highly improbable stories (of the 'My ordeal amongst cannibals' variety), Daily Express style editorials about the country going to the dogs ('What has happened to our national pride?) and investigative journalism that lifted the lid on the licentious behaviour of the ruling classes. The latter always involved intrepid undercover reporters visiting sordid establishments in Soho, witnessing goings-on that mostly seemed to involve clergymen, Members of Parliament and burlesque dancers ..........

.......... engaged in this sort of activity. 

The pay-off line for such stories was almost always "disgusted, your reporter made his excuses and left". 

Which is what I did after I lowered the compost bin lid on these three and plunged them back into darkness.

The Snail, the Woodlouse and the Millipede with disgusting but valuable habits...

If this post title sounds like a modern-day Aesop's fable well, in a way it is, because it does have a moral. Not one, but two morals...

Have you ever wondered what millipedes eat? Probably not, but if you have the books will tell you that they are herbivores that sometimes eat gardeners' seedlings and are therefore a pest.

When I found this millipede clinging to the back of a garden snail's shell I wondered what it was up to. Could it be that millipedes attack snails? If so, they'd go up in my estimation.

All was revealed when the snail glided away, almost running over a woodlouse in the process but leaving some tell-tail evidence: snail poo.

Yes, remember you read it here first: millipedes eat snail poo. 

Maybe not the most pleasant addition to the sum total of human knowledge, but there is a moral to the story. 

Millipedes do indeed eat some of the gardener's seedlings but they are also part of a vast community of soil invertebrates that play a role in cycling of minerals. This snail poo might well be all that's left of some lettuce seedlings I planted out a few days ago, but at least I'm secure in the knowledge that, once they've passed through a millipede's digestive system, some of the nutrients will find their way back to the soil and feed the next batch of my seedlings that a snail snacks on....

The second moral is that the only sane way to approach vegetable gardening is to recognise that you are part of the great web of food interactions and nutrient recycling, and to become reconciled to the fact that some of what you plant - sometimes most of what you plant - is going to pass through the digestive system of a snail and maybe a millipede too, rather than your own.

So, I think of gardening as a source of endless photo-opportunities and don't expect to harvest too much. It's the way to reach gardening karma. 

So, on to the next question: if you are a snail, what does it sound like when a millipede walks past? Probably, like this...

Britain's Largest Woodlouse and a dozy seal

Today's Guardian Country Diary describes encounters with two very interesting animals during a stroll on Roker Pier at Sunderland. Here's the pier, below, on a calm, sunny day.

This sea slater Ligia oceanica had crawled out of a crack between the stone blocks on the pier. They usually do most of their feeding at dusk, grazing on algae, and you don't often see them in the open in broad daylight. This is Britain's largest woodlouse species and it's confined to areas just above the high water mark on the seashore. The coin is a 10 pence piece, 25mm. in diameter, so you can get an idea of the animal's size - about four times larger than a garden woodlouse.

These little animals are very well camouflaged, especially when they hunker down so that the skirts of their armour plates are pressed close to the ground and they cast no shadow. It matched the stone so well that it was very lucky that I didn't tread on it.

The most remarkable thing about sea slaters is the ease and speed with which they change colour, due to being able to expand and contract pigment spots in their cuticle, so that they match their background. This is the same animal as the one on the grey stone, after it had been kept on a white background in bright sunlight for a few minutes. The pigment spots have contracted and it's almost translucent.

Sea slaters have large eyes for a woodlouse, perhaps because they do most of their feeding at dusk.

The sea slaters that live between the granite blocks here are often pummelled by waves that break over the pier (see picture below) but can maintain a tenacious grip on the stone with those needle-sharp bristles on the tip of their legs. Their flattened shape is perfect for squeezing into the gaps between the stone blocks.

You can see the jaws quite nicely here, and also the pigment spots in the body armour, fully contracted so they are just small brown dots.

We might have missed this character too if it hadn't raised a flipper to have a scratch. It was sunbathing on Roker beach below the pier and its smooth outline and mottled colouring made it surprisingly inconspicuous against the sand and rocks at low tide. I wonder if seals dream when they are asleep? If so, then it's probably having a very satisfying fantasy about pursuing fat, succulent salmon.

I'd guess that this grey seal must have been born on the Farne Islands last autumn. It had been left high up on Roker beach by an extremely high equinoctial spring tide. It didn't seem to be at all concerned and was content to bask in the sun and wait for the tide to return.  

Lovely animal, isn't it?

This is Roker pier a week later. Somewhere between the granite blocks, under the waves, there'll be sea slaters hunkered down. As for the seal, that'll be somewhere out to sea.

New Suit of Armour

The trouble with being an invertebrate with a hard external skeleton is that you have to change your suit of armour every time you grow a little bigger - which is what this woodlouse is doing. It's a risky business, leaving you exposed to predators until your new exoskeleton hardens, so this cautious crustacean is doing it in two stages. It's already slipped out of the rear half and its new shell plates have become hard and shiny. Now it's extricating itself from the head end, and here you can see that the newly exposed shell plates are still soft, with a dull surface. The animal hasn't quite removed its delicate, all-important antennae from inside their armour yet - if you double-click the image to enlarge you can just make them out inside their old translucent covering. 

This woodlouse is Oniscus asellus and you can find more about its biology here

The whole process is known as ecdysis and all invertebrates that shed their old armour in this way are grouped together as ecdysozoans. You can read all about it here.