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.

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.

Saturday, 11 October 2014

October spiders

I found quite a number of spiders today in a trip to my local wildlife garden. It was mild and sunny. Some spiders basked on the wall, like a couple of young Nursery Web spiders, Pisaura mirabilis, and a wolf spider (Pardosa sp.). Nearby, a pair of Linyphia triangularis. As I went for a walk today, I was surprised by a mature Araneus diadematus female hanging from a tree (above). She might have been disturbed in her web possibly by a bird and dropped to safety on her line of silk. When it is foggy, I have seen these spiders webs quite high up in trees.
Several Pisaura mirabilis, sat on the painted leaves on the sunny wall.
This one seems to have regenerated a few legs, notice that some legs are shorter and paler than the rest. These nursery web spiders will overwinter soon.
As will young wolf spiders, Pardosa sp. which were also on the wall.
A Linyphia triangularis, males guard the female web in this species, fighting any contenders with their long cheliceae. The female is on the left, the male - out of focus - on the right.
Metellina male with present for female? Male Metellina sp. will capture prey before attempting to court a female, and then mate with her as she is entertained with the present ('nuptial gift' as it is called). I found this mature male today and wondered if that is why it was carrying this present. Unfortunately, I didn't see the female.
And on the playground, under a window frame painted with some street art, this pink and fully grown Araneus diadematus.
I found this male Amaurobius similis on the kitchen wall, on the prowl tonight. It measured 8 mm long. I got a good view of palps allowing for species ID.
For more October spiders, check out the Flickr group #Arachtober or on twitter.

Tuesday, 7 October 2014

Hitchhiker crickets

For about a couple of weeks, my six year old daughter has been telling me about the crickets in her school playground. Crickets? surely they would be grasshoppers, I said dismissively. I shouldn't have doubted her identification abilities, as she confidently pointed at oak bush crickets when presented with photos in a field guide, and my husband and son also confirmed it. I was most intrigued, as bush crickets are rare north of the Humber. Last week I spent an embarrassing amount of time searching for the mystery crickets while the kids played after school, while other parents I know looked at me as if I was going nuts. 'I put in on the tree this morning', my daughter would said, 'I've seen four, no... five' I wondered if the kids throwing sticks to get at the conkers was also dislodging the crickets from the chestnut tree at play time. I searched and searched, and, although I did find some Field Grasshoppers, Chorthippus brunneus, nearby there was no sign of the crickets, so frustrating!
  Today, at school pick up time, she told me she had rescued one from a puddle under the chestnut tree. I searched and initially found none, but finally, I founf a live female and a very squished male on the ground, hoorray! Both were collected and taken home, and to my surprise they turned to be the Southern Oak Bush Cricket, Meconema medidionale, distinguished from the related Oak Bush Cricket by its stumpy wings and larger male cerci. Oak Bush Crickets are nocturnal and live in trees canopies, so they are thought to be under recorded, although they are attracted to light, so they turn up inside houses in the summer. Instead of singing by stridulating with their wings like other crickets do, males attract females by drumming with their rear legs on the substrate, and this sound can be audible up to 1 m away. They are predatory crickets, and feed on small insects like aphids and leaf-miners (including those of Cameraria ohridella, the Horse Chestnut leaf miner). Despite their name, they occur in many tree and bush species and are a late species, with adults found from mid August up to the first frosts.
 Since the 1960s, the Southern Oak Bush cricket expanded its distribution range from its original homeland in Italy throughout large areas of Northern Europe, and is now also found in North America. It was recorded in the UK for the first time in the autumn of 2001, and since then, it has spread north up to Nottinghamshire. Given its flightlessness, it is surprising how fast they are expanding. A study systematically searching for this species in the recently colonised Slovak and Czech Republics found that they are found mainly in urban habitats like parks or campsites, often with localised populations near car parks and main roads, suggesting that they might be dispersed passively by vehicles, especially trucks and caravans. They are, unexpectedly, often found on vehicles.
 The fact that several individuals are present suggests that the crickets have been around for a while in the school grounds. Would a teacher returning from a visit down south might be responsible from the introduction of this cricket species in Hull?
The squished male
Side view of the female
UPDATE 8/10/2014
We released the female on the chestnut tree. Although she had lost a leg, she was quite capable of jumping, and hid under a shrivelled leaf. I found a freshly dead male in the same spot, quite intact. Here he is. Look how much longer his antennae are compared to the female.

More information
British Orthoptera & Allied insects page. Here.

Grabenweger, G., Kehrli, P., Schlick‐Steiner, B., Steiner, F., Stolz, M., & Bacher, S. (2005). Predator complex of the horse chestnut leafminer Cameraria ohridella: identification and impact assessment. Journal of Applied Entomology, 129: 353-362.

Vlk, R., Balvín, O., Krištín, A., Marhoul, P., & Hrúz, V. (2012). Distribution of the Southern Oak Bush-cricket Meconema meridionale (Orthoptera, Tettigoniidae) in the Czech Republic and Slovakia. Folia Oecologica 39(2) 155-165.

Liana, A., & Michalcewicz, J. (2014). Meconema Meridionale Costa, 1860 (Orthoptera: Tettigonioidea: Meconematidae)–The First Record In Poland. Polish Journal of Entomology, 83(3), 181-188.