Pisauridae: Nursery web spiders

This is one of my favourite spider families, which displays a treasure trove of fascinating natural history. The family has just three medium size or large species in the UK, a terrestrial wandering hunter, Pisaura mirabilis, commonly known as the nursery web spider, and two semiaquatic species, the fishing spiders or raft spiders of the genus Dolomedes, able to run on water and dive to escape danger. I am yet to see a Dolomedes, but you can watch a video of the raft spider Dolomedes fimbriatus, a rare, protected species in the UK, with footage of courtship, egg sac building and maternal behaviour. Adult nursery web spiders do not make webs for hunting, but they build nursery webs for their young. The young (and some non UK species) do hunt with webs. These spiders have long, stout legs and they often hold the first and second pairs of legs extended together, like in the photo below.
Nursery web spiders are easy to recognise, they often have a pale line on their cephalothorax and parallel lines along the body. They are quite variable in the tone and colour of the lines, as you can gather from the individuals shown here.

Pisaura mirabilis basking in typical posture

Food for sex and feigning death

The reproductive behaviour of Pisaura mirabilis has attracted the attention of many researchers. When adult, males will catch an insect, wrap it in silk and search for females. It is thought this behaviour is elicited by the male coming across female silk drag lines, possibly impregnated with pheromones. When they find a female they present her the gift in their chelicerae while raising his body and front legs and palps (watch this video of the approach). If the female accepts the gift they will mate. Males without a gift have reduced chances of mating with females, but sometimes males 'deceive' females by wrapping some insect carcass or a piece of debris to offer the female. Presenting a silk wrapped gift increases the chances that mating will be completed before the females grab the parcel and run away. If the female shows aggression, the male can immediately become immobile (death feigning), while still holding his present. When the female renews her interest in the parcel the male comes back to life and resume mating.  

Male Pisaura mirabilis in another typical resting position. Note its swollen palps.

This male has captured a greenbottle fly.

Maternal behaviour
During the summer, in the lower level of meadows and other open, sheltered places, amongst grass and wildflowers, you might be lucky to come across a female Pisaura mirabilis carrying her large, white egg sac. She holds the egg sac on her chelicerae and palps until the spiderlings are ready to hatch, and despite her long legs, she looks like is walking on tiptoes with her cumbersome load. Once the young start to hatch, she loosens the egg sac and attaches it to a blade of grass. She then builds her nursery tent around the egg sac, where the spiderlings will reside for a while.

Female Pisaura mirabilis with fresh egg sac. Compare with the texture of the older egg sac on the top shot, note that the female on the top shot has a piece of egg sac covering between her chelicerae, so she might well have already loosen up the covering of the sac in preparation for building her nursery.

Nursery web with spiderlings. The spiderlings will form balls like in Araneus diadematus, and the balls also 'explode', with the spiderlings running away from each other when disturbed.

A female standing guard by her nursery web. The spiderlings will disperse after their second moult. Later in the summer, she may lay another batch of eggs.

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).

Nursery web spiders and their nurseries

The population of nursery spiders at the wildlife garden is particularly obvious now, as their nursery tents hang from their high positions from grasses and meadow cranesbill leaves, closely guarded by females. There are at least 20 nurseries scattered over the meadow at different stages of development. In some the spiderlings have already dispersed, after spending a few days safe inside their tents. This is what W.S. Bristowe had to say about the female after mating takes place:

During the first half of June the female may be see trundling along with a huge light-coloured egg sac under her sternum which is attached to her spinnerets by threads and is held in her chelicerae. At about the time the eggs are beginning to hatch, she loosens the outer covering and attaches it to a blade of grass. The she weaves a tent over it on the outside of which she stands on guard [...] The young emerge in late June or early July, after moulting once inside the egg sac. Now they cluster together in a ball inside the tent for a few days before moulting for a second time and dispersing.

I found the first nursery with already emerged spiderlings on the 20th of June. Bristowe was of the opinion that Pisaura will have two batches of eggs. Although the female below was carrying an egg sac on the 4th of July, I am not sure if this represents variation on the date of reproduction of different individuals as they mature, or evidence of a second batch.

Female carrying her disproportionate egg sac. They appear to walk on tiptoes holding it. I don't think the female is able to hunt while carrying her egg sac.

This spider has a fresh nursery web and appears to be manipulating the egg sac to allow the spiderlings to emerge.

Unlike the neat, smooth new tents holding clustered spiderlings the tent above has a few holes (made by the female?) allowing spiderlings to disperse. The spiderlings are clustered forming a ball just visible at the top right hand corner of the nursery tent.

The spiderlings are often clustered together in a ball like garden spider ones. The ones above look like they are ready to leave their tent.

If you click to enlarge the photo you might be able to see some tiny spiderlings on top of the tent already leaving.

I haven't found any information as to the function of the nursery web. Certainly it keeps the spiderlings dry during rain, and the mother is often found underneath also during poor weather. It could offer protection from parasites or predators. Many predatory insects will readily take spiderlings, including wasps, and it appears that the nursery web is not totally wasp-proof, as these photos on Flickr show.

Wildlife garden bug safari

After my encounter this morning with the basking Nursery web spiders, I popped back to the wildlife garden after a storm had passed and there was some promise of a sunny spell. The spiders weren't out, but I found a few interesting invertebrates about, including two new species for the garden.

A Peacock, which together with a couple of large whites were the only butterflies about.

 The first nice surprise were a couple of Green Thistle Tortoise Beetles (Cassida rubiginosa), which I had never seen before, both of them on knapweed:

Then I found some Rophalus subrufus, including a couple mating.

A Green shieldbug came out on the leaves to bask.

And then I came across some nursery web spiders, one of them a male prowling...

...demonstrating how agile they are moving across leaves, hiding behind them quickly if I moved a bit too fast or too close.

...and a placid female sunning herself on the oregano of the herb garden.

 Just before leaving, another new species for me, a very pale grass spider, Tibellus oblongus, contrasting with the green leaves.

Colourful nursery web spiders

The population of Nursery Web Spiders is thriving in the meadow of my local wildlife garden. I counted no less than 11 today, sunbathing in the long sunny spells between showers. They are striking, easy to spot spiders, that like to stretch to sunbathe like wolf spiders, but keeping their front pair of legs stretched forward together. Many were visible close to each other in various leaves of the lush vegetation, which made apparent how variable in colour they are, from ashy grey to brown and orangey or reddish. They all have longitudinal stripes, with the darker stripes on the cephalothorax. They will probably be mature or mature soon, so males would be capturing flies, wrapping them in silk and offering them to females, while they play dead in order to survive the encounter and make copulation more likely.

Male

Female

More information
British Arachnological Society page.

The sunny side of the fence

The outside of our garden fence is east facing. Many insects and spiders like to bask on it: even normally ground loving invertebrates climb up on it to get the sun a bit earlier. In the last few days I have been watching (mainly) spiders on the fence early in the morning. The male Nursery Web spider (Pisaura mirabilis) on the top shot, warms up on the side of the fence, in almost the same spot every day.

After warming up yesterday he went down to the weeds at the base of the fence to hunt, and caught a greenbottle fly

A crane fly resting, legs outstretched on the fence.

A male Xysticus cristatus, much darker in tone than the female. This is a ground dwelling crab spider and he was basking about 10 cm from the ground, but it is much easier to spot on the fence.

And an eager male Wolf Spider displaying to the female. Alternatively lifting each palp, trembling and getting closer to her very slowly on tiptoes. She kept tapping the ground with their front legs, and didn't miss a movement. When he got very close, almost touching her, she retreated cautiously, tapping regularly.

New resident male nursery web spider

We have spent much of the day out, sorting out the garden and pottering about the house. In one of the comings and goings I noticed a male nursery web spider (Pisaura mirabilis) sunbathing by our garden gate. By its size I think it is a subadult. I have blogged on this spider before because of its peculiar mating behaviour. The male offers the female a wrapped up prey and plays dead. The females carry their large egg sac in their chelicerae until the spiderlings hatch, but they build a 'nursery web', in the shape of a tent, which they guard and where her spiderlings develop before dispersing. This is an obvious spider that likes to sit in the open with its legs stretched, relying on its camouflage to remain undetected. The photo above, taken by my 9 yr old daughter with her Canon Powershot A495, shows the eye arrangement and striking colour pattern nicely. A beautiful spider, I hope to come across it more often.

A close up of the spider, showing the enlarged palps

Typical posture of Pisaura mirabilis with outstretched pair of front legs

Nursery web spiders

The nursery web spider, Pisaura mirabilis, is a striking, handsome spider with a surprising range of behaviours. A large spider, with long, but solid, legs and a long, tapering abdomen, it is quite variable in colour, but it always has a pale band in the middle of the carapace, flanked by darker bands. It is found in a range of habitats including grassland, meadows, woodland rides and gardens. Pisaura does not build a web, rather it is an active, highly visual hunter on low vegetation or the ground.

  In the words of Michael Roberts:

When at rest or sensing prey, they often extend the first and second legs and hold them together, straight out at an angle. Pisaura does this on vegetation, sitting quite still with the first pairs of legs held up, and seems almost like a dog sniffing the air.

In the summer, the young spiders mature and males wrap a captured prey into a silk parcel as a nuptial gift for the female, a unique behaviour amongst spiders. Although this might represent some form of paternal care, it is likely that it originated as a way for the male to protect himself from sexual cannibalism, which is very common in spiders, and also occurs in Pisaura, often before mating takes place. The male will carry the gift around until he finds a mature female. He will offer her the gift in a ritualised way and if she accepts it and starts feeding on it, copulation will ensue. Females not being offered a gift will not mate. Occasionally the female will interrupt mating and the males display another unusual reproductive behaviour: if the female behaves aggressively or tries to snatch the gift, the male will feign death, a common antipredator behaviour in spiders called thanatosis. He will remain motionless, with legs stretched while being dragged by the female, but still hanging onto his nuptial gift with his jaws. If the female starts opening the present and feeds, the male "comes alive" again and mates. Feigning death is a behaviour that increases male reproductive success, as was shown in experiments by Line Spinner Hansen and coworkers. They found that males feigning death have more copulatory success than males who don't. They explain these results by the fact that males feigning death achieve longer copulations, and are able to continue in contact with the female and resume mating later, for example when females try and run away with the gift.

(from Spinner Hansen et al 2008)

Females make quite large, pale egg sacs which they hold under their bodies with their jaws and palps (top and above), and walk awkwardly on tiptoes white they do this. When the female senses that eggs are about to hatch she constructs a nursery. 

She opens the egg sac and folds some blades of grass and builds a loose tent of silk strands around them. The she mounts guard just underneath her nursery. When the spiderlings hatch around midsummer, they form a cluster in a similar way to the Garden Spider, in the safety of their tent, where remain until after their second moult they disperse. I found the one above this afternoon in my local wildlife garden. The spiderlings are visible at the edge of the tent. I am so pleased to have a thriving population of this amazing spider living at my doorstep.

References
Roberts, M.J. (1995) Spiders of Britain and Northern Europe. Harper Collins, London. 
Spinner Hansen, L., Gonzales, S., Toft, S., & Bilde, T. (2008). Thanatosis as an adaptive male mating strategy in the nuptial gift-giving spider Pisaura mirabilis. Behavioral Ecology, 19 (3), 546-551 DOI: 10.1093/beheco/arm165
Bilde T, Tuni C, Elsayed R, Pekár S, & Toft S (2006). Death feigning in the face of sexual cannibalism. Biology letters, 2 (1), 23-5 PMID: 17148316