The Green Meshweaver gets to Hull

 

Another post on a rapidly expanding species, this time a small spider, the Green Meshweaver, Nigma walckenaeri. All started on Saturday, during the YNU AGM in the University of Hull, where spider expert  Geoff Oxford, showed us a specimen he had just collected from the university grounds, a tiny green spider that is currently expanding its range across the UK and now colonising Yorkshire. We all admired the individual, which had been collected with its web on a leaf. 

The first Nigma walckenaeri record from Hull.

After returning from the meeting, I searched the garden ivy, and since I've been searching ivy and holly, which are favoured leaves to weave it's mesh. Nigma walckenaeri  chooses curled leaves, so it can take advantage of this to hide under its little web. As other regular character in this blog, Amaurobius, Nigma is a cribellate silk weaver. 

Today, I took a local walk and came across a magnificent mature ivy, south east facing. I started searching its leaves, wondering if I had the wrong search image in my mind. It didn't take long to find a Nigma.

Nigma walckenaeri  habitat.

There it was! A light-touch web of blueish threads with the green spider sitting, well camouflaged, underneath.

I moved part of the web aside to have a closer look (also top shot). This is a female, with her whole body green. Once I had seen one, I found another, and another, almost every ivy leaf had it's little Nigma in it. 

A male, with its brown cephalothorax.

This individual was busy weaving its web.

I even spotted a male (with reddish-brown opistosoma) on the edge of a female's web.

Until 1993 it was only found in London and the home counties, and it is associated to parks and gardens. Since then it has been steadily spreading north. I'll keep a look for it in the garden!

Pirate wolf spiders in retreat

This afternoon I walked to local pond in search of dragonflies. Only Azure damselflies were active, half a dozen males bobbing about searching for females on the pond edge, a pair mating. As I looked for exuviae on the plant leaves, I noticed two female Pirate wolf spiders, possibly Pirata piraticus, in silky retreats at the base of branched burr-reed leaves, both carrying their bright white egg sacs. Apparently, unlike other wolf spiders, they build silky tubes, sometimes partially submerged, from where they hunt passing insects.

Pollen gathering spiders

If you go out into the garden and search for orb webs amongst the bushes, you are likely to surprise a garden spider holding onto a fly, an aphid or other small insect tightly wrapped on silk. In the autumn, when they are at their largest, they will be able to subdue large prey including butterflies and droneflies. Indeed, spiders are generally known as predators and the purpose of the almost invisible orb web is to trap flying insects with the sticky droplets that cover the silk threads, right? Orb spiders, including the common garden spiders, Araneus diadematus, have challenged this assumption. When garden spiders hatch in spring, and after spending a few days in a tight ball with their siblings, the tiny, millimetre long spiderlings will disperse and make their first orb web (top shot). Until then, they have been living of the yolk from their egg stage, but now they have to find food. At that time of the year there are not many flying insects, and spiderling mortality due to starvation is high.

Young orb weaver, Zygiella sp, with an aphid prey.

  Now, when you look at a spider web under the microscope you can see not only insects and other small invertebrates trapped on it, but also pollen grains and fungal spores. In spring there is an abundance of pollen of wind pollinated flowers and trees. Given that orb spiders rebuild their web at regular intervals, pulling down and eating the web before spinning a new one, the young spiders could potentially be feeding on the pollen collected, benefiting of this plentiful resource. Risa Smith and Thomas Mommsen, from the University of British Columbia and Dalhousie University carried laboratory experiments to test exactly this. They compared the survival and web building frequency of individually housed spiderling, which were fed exclusivelly on birch pollen or fungal spores with unfed controls and spiderlings fed on aphids. Their results show that spiderlings not only feed on the pollen, but pollen feeding increases their survival compared to starved spiderlings or spiderlings given fungal spores. however, only the spiderlings fed on aphids moulted into the next instar. Smith and Mommsen reckoned that given that pollen from wind pollinated trees is highly deficient in the aminoacid tyrosine, essential for cuticle formation, spiderlings could be unable to thrive only on pollen, but would need to capture the occasional insect to moult into the next stage. The researchers noted that pollen grains are too large to be consumed whole by the spider, therefore, they couldn't have been accidentally swallowed. The spider indeed treated pollen grains like insect prey: grabbing the pollen grain, dissolving a hole in its coating and sucking the contents! More recent experiments estimate, using isotopes, that about 25% of the diet of young orb weaving spiders can be made of pollen. Spiders might be best considered omnivores rather than carnivores!

More information
Smith, R. B. & Mommsen, T. P. Pollen feeding in an orb-weaving spider. Science 226, 1330–1332 (1984).

Eggs, B. & Sanders, D. Herbivory in spiders: the importance of pollen for orb-weavers. PLoS One 8, e82637 (2013).

Thanks to Robert Jaques for sharing the paper that started this blog.

Two Woolly Jumpers

There is a thriving population of the small jumping spider Pseudeuophrys lanigera in my office building at University, which, with its species name meaning 'woolly' I think it deserves the common name of woolly jumper. I spotted one on the windowsill as I was going to close the blinds, and then another one descended from a thread of silk and ended up near the first one. Both were males and, after some swaggering and lifting of the front legs, one of them, the newcomer, retreated and followed on his way. It is the females that truly look 'woolly'. Males are very colourful, with dark and large front legs, red markings around the eyes and white moustaches and palps. These spiders are found on and inside buildings. They must consume minute prey, such as bark lice, springtails or small flies, although jumping spiders are able to tackle prey their own size. The adults can be found year round, although their peak season is June for females and September for males.

The smaller spider number 1, who held his ground.

The newcomer.

A Walnut orb weaver

It's not every day I find a new species of spider in the garden, so I was pleased this afternoon, when opening the garden gate I disturbed a young Walnut orb weaver, Nuctenea umbratica (above). It crawled up and hid in a crack between the planks of wood. This is a large and distinctive species, the females can reach 14 mm.
umbratica means 'of the shadows' in latin, which refers to their liking for dark cracks and crevices, where they hide during the day, aided by their wide and flattened abdomen. When discovered, they are often crouching in a characteristic position, which is the only explanation I can see to another common name: 'toad spider'. They can be very dark, almost all black, but this individual shows the wavy leaf-shape pattern on the abdomen, surrounded by a pale rim and pinkish sides with annulated legs. They like dry microhabitats, including wooden structures, like fence posts, dead wood, window frames (e.g. in birdwatching hides), greenhouses, and cliffs. They are mostly nocturnal, they emerge as it gets dark and make their stout, large orb web, of a similar shape to that of its relative Araneus diadematus, although they can also sit on their web during the day. Their tattered webs can be useful to detect the spider when found in suitable habitats during the day.
 Males peak in July and August, while females are found year round. I wonder if being a very large spider, like the garden spider Araneus diadematus, females may take two years to reach maturity.
Although I might have overlooked it due to its retiring habits, I keep finding it more often in recent years.

Adult N. umbratica in a window frame of a hide. (11/7/16, Tophill Low)

One of several N. umbratica adult female inside bird hide (11/10/15) Alkborough Flats.

A fresh egg sac found inside the same hide with several N. umbratica. It looks very similar in colour, shape and size to an Araneus diadematus egg sac.

An adult female N. umbratica sitting on her web during a dark November day inside a greenhouse (Thwaite Botanical Gardens, 3/11/15).

Sputnik spider and egg sacs

Yesterday evening, while pruning the plum tree, I found the distinctive egg sacs of the theridid Paidiscura pallens: they are white with pointy projections that make them look like the Sputnik satellite. The tiny mother, less than 2 mm in length, sat atop hers oversized egg sac, which was attached to the underside of the leaf and to the spiders loose, tangled mess of silk threads. She nervously moved back and forth from the sac and around it, like sensing that something was amiss. Despite finding the egg sacs regularly, this is the first time I see the female spider guarding it. I found another pair of egg sacs today under a plum leaf, also guarded by the mother.

Two Paidiscura pallens egg sacs also from today. 

The female and her two egg sacs from today.

My conservatory spiders

The sun has been shining for a couple of days and it was warm enough that I started taking plants out of the conservatory into the garden and having a little tidying up. While doing this, I found a few spiders. Many were very tiny, very young individuals that I was unable to photograph properly and identify. Others were quite impressive. They have survived the winter in the dry, not too cold environment of the conservatory.
 First up, a Steatoda bipunctata, a false widow spider, which was on the side of a pot, a fully grown female. It has a very tick-like appearance, especially when it drops and tucks its legs in.

In between the bags of compost, I noticed the sheet webs of a Tegenaria. Here she is a bit camera shy, but shows her pretty abdominal patterns.

A few cob-webs came out of a corner, and then a Pholcus phalangioides ambled around trying to settle again.

This slim, fast and jumpy male Clubiona which I call black-face (possibly C. terrestris, but pedipalp examination is needed to confirm species) also featured on the top shot, fell from the foliage of a pot plant.

Finally, a Pardosa wolf spider on the window frame. There are dozens of young pardosa that have overwintered outside and sunbathe in loose groups on tiles and stones just outside the conservatory. This one got in through the crack of the window.

Wandering male lace weavers

In the last week I have found two different males Amaurobius similis wandering, one in the porch and the other in the house. Note the different abdominal pattern in the photos below, especially the dark blotches surrounding the cardiac mark (the midline elongated area over the abdomen). In this species, mature males are most likely to be found between September and November, and they abandon their webs in search of the female retreats. I released them both after taking their photo paying especial attention to the palp (above), which is diagnostic and separates this species from the similar one A. fenestralis. Males A. similis have an inward pointing, curved sharp projection on its palp, which in A. fenestralis is thicker and blunt.

Male 10/11/15.

Male 15/11/15.

Spiders in glasshouses

Today at work, I had the chance of inspecting some glasshouses in the Thwaite Botanical Gardens of the University of Hull. They are heated glasshouses holding cacti, succulents, ferns and other plants from around the world. I was pleasingly surprised by the diversity of spiders in them. I found a single Woodlouse Spider, Dysdera crocata, under a bin (above). Quite fitting as we were collecting woodlouse for a practical on woodlouse diversity. The students were quite impressed!
 The first surprise was to find several Walnut Orb Weavers, Nuctenea umbratica. We dislodged an individual from a tree growing in a pot and it proceeded to play dead, legs drawn in, as they do when disturbed. They are usually nocturnal, spending the day in a crack in bark, but this large female I found later was sitting in the middle of her web, I wonder if the reason is it was a very dark, overcast day. There were many more smaller sized individuals, also sitting out in their webs, and it is quite likely this spider matures on her second year of age.

Mature female Nuctenea umbratica.

A student pointed this female Tegenaria to me, she found under a tarpaulin.

A mature, gravid Araneus diadematus sitting on her web.

This was the second nice surprise. One of the glasshouses held a healthy population of Garden Centre spiders, Uloborus plumipes.

By the toilets, a number of Pholcus phalangioides.

Where there are windows, there are window-frame spiders, Zygiella x-notata. 

I found a single Ero sp. egg sac. These spiders abandon their characteristic egg sacs, and they are not the easiest to find.

On a window ledged, a female Araneus diadematus looking like it has seen better days. Its abdomen shrivelled. Her days are counted after she lays her eggs and weaves her egg sac.

Liocranidae: spiny-legged sac spiders

Credit Gerhard Elsner CC BY 3.0 via Wikimedia Commons.

A small family with 13 British species that used to be regarded as part of Clubionidae. They are mainly nocturnal wandering hunters, except the two species of the genus Phrurolithus, which are diurnal ant mimics, and have been also regarded as a different family.

Distinctive egg sacs
Some Agroeca have distinctive stalked egg sacs, like inverted wine glasses (top shot), sometimes covered with soil, which are seen more often than the spider, as they are not guarded. Other species guard their egg sacs.

Parasitoids
Spiders have many species of external and internal parasitoids, most of them ichneumonid wasps. Some external parasitoids attach themselves to adult spiders and feed on them, other species search for cocoons and inject their eggs on egg sacs, with the parasitoid larvae developing on the eggs. The characteristic egg sacs of Agroeca makes them easy to identify and collect, so in a study of spider adult and egg sac parasitism in Germany, up to 66% of Agroeca egg sacs were parasitised. Four species of parasitic wasps emerged from egg sacs: two small Gelis wasp species, Bathythrix formosa, and Thaumatogelis audax. Few or no spiderlings emerged from the parasitised egg sacs, indicating how important parasitoids can be as a source of spider mortality.

More information
Finch, O. D. (2005). The parasitoid complex and parasitoid‐induced mortality of spiders (Araneae) in a Central European woodland. Journal of Natural History, 39: 2339-2354.

Anyphaenidae: buzzing spiders

Credit gbohne, CC BY-SA 2.0, via Flickr.

A mainly American family with a single UK species, Anyphaena accentuata, that can be recognised by arrow head markings on the abdomen. They are good climbers that live on the foliage of trees in woodland and scrub, where they hunt and mate. They are active hunters that pounce on insects that sit on leaves such as leafhoppers, aphids and flies.

Buzzing spider

Males of this species (top shot) use acoustic cues in courtship, vibrating the abdomen and tapping with the palps while sitting on a leaf. This produces an audible noise (which may be undetectable for older people), so Bristowe suggested the name buzzing spider for them. This is a short clip of this behaviour. Females may respond to these signals by approaching the calling male. Females attach their egg sac to the underside of leaves and guards it in a thin silk cell.

Cold adapted
Immature buzzing spiders are active year round and have physiological adaptations to activity during cold temperatures, even below the freezing point. In the winter they prey on small insects that live on bark.

Dictynidae: mesh weavers

Credit: Drriss & Marrionn CC BY-NC-SA 2.0 via Flickr.

These are small cribellate spiders, less than 5 mm long, some with characteristic colour patterns. They build an irregular sheet web usually with a central retreat. Many of the 18 British species are uncommon. Some of the species live on plant foliage, others are ground dwellers.
 The top shot shows Nigma walckenaeri a green and white spider that builds sheet webs on the top surface of leaves. This species is mainly found in the Thames and Severn valleys, where it can be common in gardens.

'Unusual friendship' between males and females
 Dyctina arundinacea, the most common species, makes its web atop a dead shoot or flower head. The spider adds more threads as the summer progresses, for, unlike orb weavers, the web is a permanent structure. Bristowe remarked that in this species, the male spends an unusually long time in company of the female in her web (a month or more). Once a male enters a receptive female's web, he signals to her and she signals back with vibrations and tactile signals. The male then goes on to modify the female's web and adding threads to build a canopy, where mating happens. They signal to each other every time they meet. Males and females can even share capture and eating of prey. The female sets her egg sacs inside the web, and she can produce up to six in a season. While a male have to gain from staying around a female, for example, preventing rival males from mating with 'his' female, it is unclear what the female has to gain from accepting the male, unless his cooperation in prey capture helps she build up resources for egg production. More research is needed on these spiders natural history. There are tantalising observations of  males possibly participate in egg guarding. Some non European Dictynidae are social, so it is possible that the roots of social behaviour lay on these unusual male-female associations.

Zodariidae: ant spiders

Credit: Christophe Quintin, CC BY-NC 2.0 via Flickr.

This relatively large family is distributed mainly in tropical and warm temperate regions. There are four species in the UK, all from the genus Zodarion, and all of them appear to be recent arrivals with first records from 1979 onwards, although there have been suggestions that at least one of them is native. They are very local and tend to be found in the south of England where they live in dry, warm and open habitats in association with ants.

Ant-looking ant-eaters
Spiders of the genus Zodarion are ant mimics: they resemble ants in size, shape and also behaviour. The resemblance goes as far as the microscopic pattern of spines, bristles on legs, cephalothorax and abdomen as ants. Even the effect of a shiny ant body is achieved with ridges in their cuticle. They also imitate ant gait and tactile signals: they deceive ants by tapping with their first legs, as ants do with their antennae. Mimicry is thought to offer them protection from spider predators that avoid ants. But they are also aggressive mimics: they use the resemblance to ants to approach unnoticed, as they feed exclusively on ants, using ambushing tactics. Once they capture an ant, they hold its body ahead of them and present it to approaching ants, a behaviour known as 'shielding', at the same time tapping the antennae of the ant. This way they imitate the behaviour of ants carrying dead corpses away from the nest.

Igloo makers
When resting the spiders take shelter in curious igloo-shaped refuges made with small pieces of debris and tiny stones (see photo below). They also lay their eggs in their retreats.

Credit: Korenko S, Schmidt S, Schwarz M, Gibson G, Pekar S , CC BY 3.0, via Wikimedia Commons.

Cybaeidae: water spiders

Credit: Norbert Schuller CC BY-SA 3.0 via Wikimedia Commons

The only British species in this family, the Water Spider Argyroneta aquatica, is the only known spider that lives almost permanently under the water surface. It inhabits clear ponds, ditches or slow moving streams. Despite their ability to dive and swim freely, their adaptations for aquatic life are subtle: the fine hairs of their abdomen and front of cephalothorax trap a silvery lining of air when the spider is under water, and the two pairs of rear legs are covered with rows of long fine hairs, which help with swimming and collecting air bubbles. They are grey and brownish spiders that can reach large sizes (sometimes over 2 cm).

Inside diving bells
The water spider builds an underwater refuge made of a dome of silk attached to submerged plants which she fills with air. This takes several trips to the surface, trapping large air bubbles with the aid of her rear legs, and then releasing the air out when she is under the dome. The dome then acquires a bell shape and effectively acts as an external gill. The spider spends much of its time in the bell, as oxygen diffuses from the water into the air bubble as the spider uses it up. Moulting either takes place inside the bell or on the water surface.

Reverse size dimorphism
Unlike most spiders, water spider males are on average larger than females. This could be due to the different hunting strategies of males and females: males are more active hunters and spend more time diving outside their bells, and large males are better divers than small ones in moving inside water as they are better counteracting air buoyancy. Females build larger bells and, in contrast to males, tend to practice ambush hunting from their bells, sitting head down with her front legs by the entrance, where they can detect vibrations of passing prey. Both males and females take their prey to their bells to eat.

Reproduction

Mature males (top shot) leave their bells in search for females. Mating takes place in the female's bell, although it involves both male and female swimming outside the bell. When mating, females prefer larger males, but they are also more wary of them, as - also uniquely to them - male water spiders are known to occasionally cannibalise females (although the reverse also happens). Females sometimes lose their bells after rejecting a male.
The female lays her eggs in her bell, in a thick white egg sac at the top of the bell. She partitions this area out with silk and remains in the bottom part of the bell herself. She stays there on guard until the spiderlings hatch. After a few moults they disperse and they quickly build their own little bells under water. Females can lay several egg clutches in a season.

Empty snail shells
At the end of autumn, the spider reinforces the bell with more silk, closes it and stays in it through the winter months. An alternative winter home are empty pond or ramshorn snails, lined with silk and filled with an air bubble.

More information
Schütz, D., & Taborsky, M. (2003). Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica. Evolutionary ecology research, 5, 105-117.

Schütz, D., & Taborsky, M. (2005). Mate choice and sexual conflict in the size dimorphic water spider Argyroneta aquatica (Araneae, Argyronetidae). Journal of Arachnology, 33, 767-775.