Wednesday, 20 March 2013

Flat-backed millipedes

My local - now disused - Victorian cemetery is like an old woodland. There are mature trees with woodpecker holes, tree stumps, dead wood, thick leaf litter. Dead wood is a habitat full of life even in winter. Slugs and snails, springtails, millipedes and centipedes, woodlice, mites, little beetles and their larvae, tiny spiders, and fly larvae: they might be a bit torpid, but still active. In the cemetery, I often come across flat-backed millipedes, Polydesmus on dead wood, often in pairs. They are relatively large and - true to their name -, have a flat, sculptured back. There are five similar species of this genus in the UK that can be only told apart by examining the male gonopods, an 8th pair of modified legs to transfer sperm, or the females' genitalia (which opens behind the second pair of legs), features that can only seen properly when dissected. Males have sturdier legs and appear to have a gap behind the 7th leg (where their gonopods are), where in females the gap appears behind the second pair. The most common species is P. angustus.
 A couple of weeks ago, I surprised this pair of embracing flat-backed millipedes under a piece of rotting stump, maybe mating or just checking each other out? In another photo I can see that the head of the millipede on top is holding onto the rear end of the other one.
Pair of Polydesmus 3rd March 2013
And a few days ago I found a pair of smaller ones, one of them is walking at the top of the post.

Saturday, 16 March 2013

Velcro spinning spider

I found this female Amaurobius similis in the same spot in the kitchen than in January last year. I wonder if it will be the same individual or this is an ideal spot for this species. At around 11 pm, she was carding her silk using the calamistrum or comb in her rear legs (click here for close up photos of this structure). She changed position every now and then. I have posted on the beautiful blue woolly silk of Amaurobius before. In this photo the silk colour is not visible, but I liked to see the spider in action preparing and adding her silk strands around her retreat.

W.S. Bristowe, a wonderful spider watcher, described how the spider makes her 'velcro' snares in detail:
Ciniflo's [now named Amaurobius] spinning operations take place short after dark and as the spider usually stops spinning soon after the light of an electric torch is shone on her it may take a little time before her exact methods are accurately worked out. The abdomen is tilted slightly upwards and the femurs of the hind legs stand out laterally, almost at right angles. The remaining segments of one hind leg are directed backwards and inclined somewhat inwards, while those of the other leg are bent inwards at the tibia nearly at right angles in such a way as to allow the tarsal claws to clasp the other hind legat the base of the metatarsus. In this position the calamistrum, or comb, on the metatarsus of one leg is situated just behind the spinnerets. The spider remains stationary ... [while the threads] spun by the posterior pair of spinnerets are combed out by a rapid oscillation of the legs into loops and flounces. At the same time she moves in such a way as to stretch out the ribbons she has spun.

A side view of the spinning spider.

And a video showing the behaviour. I was a bit nervous of supporting the camera on the windowsill just in case I disturbed her, hence the shakiness!

More information
Bristowe, William Syer. The world of spiders. Vol. 38. London: Collins, 1958.

Sunday, 3 March 2013

Sunbathing wolf spiders

 The sun shone all day yesterday and we had some lovely active garden invertebrates, including the first hoverfly (Eristalis tenax) and the first bee of the year (a male Anthophora plumipes). The first wolf spiders, likely Pardosa amentata, emerged from their winter retreats to bask in the sun and explore. These would be the young produced last summer and will have to moult again to become adults. I spotted the one above climbing the house wall, something they do not normally do.
Sunbathing on leaves...
...and on the conservatory wooden frame.

Rotund disk snails

In a walk to my local cemetery I found this group of disk snails, Discus rotundatus, under a decaying branch. They are small snails up to 7 mm in diameter, with a lovely ribbed shell with reddish banding. It feeds on algae, fungi and decaying vegetation in damp and shady places and shelters in groups under bricks, rocks, logs and at the base of tree trunks in dry weather.

Friday, 1 March 2013

Two freshwater flatworms

The garden barrel is one place where some invertebrate activity is guaranteed any time of the year. Snails graze algae in the winter and Water Slaters, Asellus aquaticus, munch the decayed leaves. There are predators too: flatworms. These innocent looking, slug-like animals, covered in sticky mucus slither on the surfaces of leaves looking for prey. Today I found two species, a small, dark one, probably Dugesia lugubris (top) and a large white one, Dendrocoelum lacteum (bottom). Both species shown here have a pair of simple eye spots, other species have more eyespots.
The LED lights of the hand held microscope reflect on the water surface and bring out the colour of the branching gastrovascular system of Dendrocoelum lacteum. The white band in the middle of the body is the evertible pharynx, on one end of which is the mouth. Small organisms, including Water Slaters, are often prey of flatworms, and are subdued with the mucus and wrappen on the flexible bodies of the flatworm.

More information
Richard Fitter & Richard Manuel 1986 Collins field guide to Freshwater life. 

Helen Mellanby 1975 Animal life in freshwater life. Chapman & Hall.