Monday, 16 February 2015

Garden Centre Spider (II)

Another visit to the garden centre. I spend much of it looking up most, in search for the Garden Centre Spider, Uloborus plumipes. I must have been a funny sight. My efforts are rewarded quickly. First, and most obvious I spot the webs, stretched across the plumbing by the glasshouse roof. They are laid almost horizontally and is an orb web. This one had a stabilimentum, a decoration in the centre of the web, in this case a linear one.
The spiders were a bit harder to find. After a while I saw some piece of debris hanging from some web, see the spot in the middle of the photo?
Zooming in, it revealed itself a a superbly camouflaged spider. It is hard to think it is a spider from a distance. Once I had spotted one, more followed, and I counted five in a relatively small area. 
The nicer thing, though, was to find its spiky egg sac too.
Sorry for the poor photo quality, but the spiders were 3 m above me and the light already fading.

Sunday, 8 February 2015

Sunbathing Nursery web spider

A sunny day, we visit the wildlife garden. We find little in the way of invertebrates, but, as we are about to leave, on the south facing Ivy, I spy this young Nursery Web spider, Pisaura mirabilis, which makes my day. First she is sitting like some wolf spiders or crab spiders, sometimes sunbathe, with their front legs curled up, looking very relaxed. Then she seems to detect my presence, and it stretches out her front legs in the more typical position of the species (above).


Wednesday, 28 January 2015

Supercool snails


In the last couple of months, I've regularly come across active Girdled Snails on my way to work. Damp, but often very cold even during frosty mornings I see these small snails on they way back to their day retreats on the pavement by a front garden, presumably after having been active, feeding? during the night (above, on the 23rd of January at 8:48 am). This nonchalant cold hardiness is in stark contrast to common or garden snails (Cornu aspersum), which have been dormant for a good while, and won't become active until March or April. Why is that? how cold resistant are snails? do they differ in their cold hardiness?
  Land snails are a very useful indicator species for ancient environments, as their shells fossilise very well and can be often identified to species level. Despite this, surprisingly little is known on their cold tolerance. Amazingly, 35 snail species live north of the arctic circle and 44 species over 2,000 m of altitude. How do they survive ice-cold temperatures? As other invertebrates unable to migrate snails have two strategies to survive sub-zero temperatures: freezing avoidance and freezing tolerance. Species that engage in freezing avoidance can actually be active in sub-zero conditions by supercooling. A supercooled snail will be at a temperature under 0 oC, but it won't be frozen, that is cool indeed! They can do this by producing large amounts of small sugar molecules that bind water and make their tissues more dehydrated, and also large antifreeze proteins, which inhibit ice formation even further, allowing them to remain active at sub-zero temperatures. Smaller snails (of shells up to 15 mm) appear to be more freeze avoidant than tolerant, and therefore, they are better at supercooling.
A favourite overwintering spot, with dozens of garden snails of various sizes under a tile lined against a wall in my garden.
We know a bit more about the cold tolerance of garden snails, thanks to the research of Armelle Ansart, from Rennes University and her colleagues. The garden snail has limited supercooling abilities, it is a partial frost tolerant species (they can only survive to a minimum of -5 oC). The are partially freeze-tolerant, avoiding freezing by emptying their guts - gut contents can start the formation of deadly ice crystals, reducing the water content of their body (which makes soluble chemicals more concentrated and decreases the temperature at which ice crystals form) and producing an epiphragm, a hard, thick calcareous layer of mucus that seals their shells shut, keeping the deadly moisture out. As an aside, the epiphragm is also produced in very dry weather, during aestivation, another dormant state in snails, but then the epiphragm keeps the moisture in.
An early waking young garden snail (28 Feb 2011), still carrying its epiphragm attached to its shell.

The preparation for overwintering seems to be kickstarted by the decreasing photoperiod of autumn, rather than temperatures dropping. Garden snails also seek high and dry microhabitats to overwinter and congregate, sometimes in very large numbers in favourable spots, such as the underside of logs, stones or holes in tree trunks. Large garden snails are more resistant to the cold than small ones, as they are better at avoiding the formation of ice crystals, so adults are more likely to survive a hard winter than immature snails.
 As for the Girdled snail, sadly I found nothing, although a comparative analysis of cold hardiness by Ansart and colleagues found out that its congeneric species Hygromia limbata freezes at -7 oC, not too impressive when compared to the tiny Columella edentula, also a British species, which doesn't freeze until the temperature descends to -17 oC, but probably enough to allow it to survive in the mild frosts of Hull.

References
Armelle Ansart, Annie Guiller, Olivier Moine, Marie-Claire Martin, Luc Madec. 2014. Is cold hardiness size-constrained? A comparative approach in land snails. Evolutionary Ecology, 28: 471-493. Here.

Ansart, Armelle, and Philippe Vernon (2003) Cold hardiness in molluscs. Acta Oecologica 24.2: 95-102.

Ansart, A. & Vernon, P. (2004). Cold hardiness abilities vary with the size of the land snail Cornu aspersum. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 139: 205-211.

Tuesday, 20 January 2015

Garden Centre Spider


Bugblog has curled under a dry leaf and enjoyed an extended period of dormancy. The new year has brought some interesting invertebrates so I foresee some blog activity in the next few days. In a trip to the garden centre, a little spider dangled from the ceiling. Something called my attention: its front legs. They were long and robust, and adorned with dark tufts of hairs. The spider held them forward, using them as feelers as it moved. It reminded me of an orb web spider, but not one I had seen before, so I posted a photo on Twitter asking for help. Within minutes, Chris @BHWWildlifeGdn answered:

A close up of the spider in my hand, to give you a sense of scale
And so it was! This species has been expanding in the UK since the early 90s, when it was found in garden Centres in Reading, Liverpool and Southampton. In fact, it is not even mentioned in my spider field guides. Now it is widespread through most of England and large cities in Scotland, where it is almost exclusively found in or around heated greenhouses of garden centres, and is thought to have come with plants from Holland, where it was also found. The original distributions appears to be Africa and the Mediterranean, although it is expanding worldwide thanks to its ability to thrive in garden centres.
 Uloborids are cribellate spiders, they brush their silk with a comb-like set of bristles in their rear legs making the silk sticky. This silk is so efficient immobilising prey that uloborids have lost their venom glands, not that this makes much of a difference to us, as this species is so small its fangs will be unlikely to break the skin.
 I shall keep an eye for this unusual spider every time I visit a garden centre. It builds horizontal orb webs (although it is not a member of Araneae, but of Uloboridae) and sits underneath, resembling a fragment of dead leaf and their egg sacs are white and of an unusual shape too. Chris recommended looking around the lights.

More information
Page for Uloborus plumipes at the Spider and Harvestman recording scheme. Here.
Wikipedia page.

Saturday, 25 October 2014

Overwintering lacewings

On a visit to a local nature reserve a couple of weeks ago I looked to the ceiling of one of the hides, hoping to find overwintering butterflies. I didn't find any, but realised the hide contained several hundred lacewing, loosely clustered together in groups all around the ceiling and wall edges. It is something I had never come across. I believe these are one of a group of similar lacewing species, Chysoperla carnea group. Some of the individuals have started to change colour.