Wednesday, 21 October 2015

Atypidae: purse web spiders

This is the only family of mygalomorph spiders in Northern Europe, relatives of the tropical American tarantulas. Although the Western Mediterranean region holds over 40 species of Mygalomorphs, including trap door spiders and funnel-web weavers, there is a single species in the UK, Atypus affinis, the purse-web spider. These spiders have massive forward facing chelicers which open and move vertically and are furnished with saw-like teeth underneath. They are multipurpose tools, and are used for digging, moving soil about, cutting its silky tube and crushing prey.

Silken socks
Purse-web spiders spend most of their lives underground, at the end of a sock-like silk tube sealed at both ends. A part of the tube is underground (up to 50 cm deep), and the rest, likened to the finger of a glove, runs across the surface, partly camouflaged with debris and soil. When the spider feels the vibrations of an insect walking on the exposed tube, she darts up, punctures the tube with its opened fangs, stabbing the insect and dragging it into the tube through a slit cut with her fangs. She will then repair the tube and may eventually throw the remains of the prey out.

Wandering males
When males mature in September-October, they leave their purse-web and wander in search of mature females. This is the only stage of their lives, apart from the initial dispersal of spiderlings from their mother's burrow, that they leave the comfort of the underground. If a male finds a female's tube he freezes and taps it a few times with his palps, pauses and taps again. If the female does not respond with aggressive tugging, the male makes a slit in the tube and enters. After mating the male lives with the female for the winter. He might die and be eaten by the female or it might escape in the spring, when males may again be seen wandering in the open.

Slow development
The female makes her egg sac at the bottom of the tube in the summer and the spiderlings hatch in the following spring, some eighteen months after she mated. The young (about 100 per brood) remain with their mother until they are almost one year old, and then, a sunny day of spring, they will emerge from their natal tube, climb onto the surrounding vegetation, and then disperse by a form of ballooning. They will make their first tube before night falls. They won't reach maturity until they are 3-4 years old. The female may rear another brood in her life, which could reach 7-8 years or longer.

Habitat specialists
Individual longevity and slow development means this species is sensitive to habitat changes, as adult individuals don't move from their tube once settled. They live in undisturbed open habitats, on south-facing slopes of sand or friable soils with sparse vegetation of trees and or heather, either inland or coastal, often under the protection of rocky outcrops, anthills or bushes. Although the purse-web spider can be found at high densities, it is a scarce, very local species. Given that a population has recently been discovered at a Yorkshire site (the first record for Yorkshire), it is possible that, due to their secretive habits, some populations of this species still remain to be discovered.

More information
Enock, F. 1885. IX. The life-history of Atypus piceus, Sulz. Transactions of The Royal Entomological Society of London, 33, 389-420. Available from the Biodiversity Heritage Library here. This, according to Bristowe, is the only account of a spider natural history from the 19th century, it is a pleasure to read.

2 comments:

RayHolden said...

"These spiders have massive forward facing chelicers which open and move vertically and are furnished with saw-like teeth underneath.
They are multipurpose tools, and are used for digging, moving soil about, cutting its silky tube and crushing prey.
" - My garden needs at least one vegan, giant-sized, robot-version of these spiders.

I knew nothing about these spiders - until I read this Arachtober 21st piece, bow I am fascinated by them.

The spiderlings are lovely: https://flic.kr/p/6dpQhS and https://flic.kr/p/6dpQkC .

Africa Gómez said...

It has taken me a bit by surprise how much I am enjoying the writing of these posts. I was a bit apprehensive about writing about spider families that I hadn't seen before, but It has actually been easy given the vivid descriptions of their natural history in Bristowe's book and others. This, the last few lines of Enocks account of Atypus, fits the photos you shared beautifully:
"In bringing my notes to a conclusion, I think I cannot do better than just recall one fact in the history of this interesting spider - that when the first young one emerges from the tube it takes an upward course, leaving behind it a silken cord, which is taken hold of and added to by each one as they emerge and follow on. cannot we follow their example by adding our small amount of knowledge, and so make the pathway stronger and easier for our fellow-students who may come after us?"