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.

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