Thursday 8 October 2015

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.


Unknown said...

Great piece of writing and links a terraced house with no garden in Liverpool, wildlife was scarce! Except for a few spiders in metal biscuit tins, fed on bluebottles and flies, my earliest wildlife experiences!! Cheers, George

RayHolden said...

You are doing a great daily Arachtober blog, (did I miss the 7th).

I've just been introduced to the Bolas spiders - fascinating - have you any knowledge of them? These guys , but we don't appear to have them in Europe.

The most unusual UK spider I know is this guy . What would be your candidate-species for that role?


Africa Gomez said...

Hi Ray, thank you! something weird happened to the dates, I hope now sorted. I haven't seen any bolas spiders myself, so many spiders have unusual behaviours, you could fill a documentary series with them
Thank you for leading me to Cyclosa conica, amazing indeed! I haven't seen any myself either, they don't seem to occur in East Yorkshire.

Anonymous said...

I’m also from Hull UK and have never seen Pisaura mirabilis until today, this is how I’ve found your blog. I wasn’t sure what it was at first, as the body was unlike any spider I’ve seen before.