Araneidae: typical orb weavers

It's #arachtober, a whole month dedicated to the celebration of spider awesomeness and diversity! For this reason, I will be writing a daily post on the natural history of a British spider family.

For the first post, I have chosen Araneidae, the typical orb weavers. This is one of the largest spider families and include many familiar species, such as the garden spider (Araneus diadematus), the window frame spider, Zygiella X-notata, the wasp spider Argiope bruennichi, and the small cucumber spider Araniella sp. There are 33 species in the UK.  The drawing on the left shows the eye arrangement in the family (based on a photograph by Kiron Basu)

In foggy weather, orb webs become very visible, heavy with dew. The spider is sitting in the central hub.

A. diadematus spinning its web.

The spider's web
The most characteristic feature of araneids is that they spin flat, rounded webs with radial threads connected by a spiral net-like pattern of threads, which they attach to vegetation or other structures  The threads are sticky, and able to trap flying insects. The spider either sits in the middle of the web (often at night) or hides in a retreat on a corner of the web, with one of their legs touching a signal thread connected to the hub of the web, which allows them to detect the vibrations produced by any struggling insects. Orb spiders often eat their web at the end of the day, and build a new one in the morning. As they adopt a sit and wait strategy to catch prey it is easy to find particular individuals in the same spot every day.

Dangerously close. The male (on the left) was dispatched before he had the chance to mate, after a lengthy and careful approach.

Mating and cannibalism
Adult females are readily distinguished by their larger size and swollen abdomen. Females often cannibalise males (often right during mating), and males might die after mating a couple of times so at the end of the season, only mature females might remain.

Female Araneus diadematus with its fresh egg sac.  Once their egg sac is finished, the female sits on it and will die in a few days.

Egg sacs and spiderlings
Females lay their eggs, often over a thousand of them, and spin an egg sac around them. Females might die shortly after egg laying. The spiderlings hatch, moult inside the egg sac and then they emerge. In some species, like the garden spider, they spin a communal web and cluster together in it until they are ready to disperse.

A spiderling ball, newly born A. diadematus

At the end of the summer, full size Araneus diadematus can subdue quite large insects such as droneflies, butterflies and wasps.

A. diadematus are very variable in background colour with a range from sandy to chocolate brown, although the white markings on the abdomen tend to be more uniform.

Velcro spinning spider

I found this female Amaurobius similis in the same spot in the kitchen than in January last year. I wonder if it will be the same individual or this is an ideal spot for this species. At around 11 pm, she was carding her silk using the calamistrum or comb in her rear legs (click here for close up photos of this structure). She changed position every now and then. I have posted on the beautiful blue woolly silk of Amaurobius before. In this photo the silk colour is not visible, but I liked to see the spider in action preparing and adding her silk strands around her retreat.

W.S. Bristowe, a wonderful spider watcher, described how the spider makes her 'velcro' snares in detail:

Ciniflo's [now named Amaurobius] spinning operations take place short after dark and as the spider usually stops spinning soon after the light of an electric torch is shone on her it may take a little time before her exact methods are accurately worked out. The abdomen is tilted slightly upwards and the femurs of the hind legs stand out laterally, almost at right angles. The remaining segments of one hind leg are directed backwards and inclined somewhat inwards, while those of the other leg are bent inwards at the tibia nearly at right angles in such a way as to allow the tarsal claws to clasp the other hind legat the base of the metatarsus. In this position the calamistrum, or comb, on the metatarsus of one leg is situated just behind the spinnerets. The spider remains stationary ... [while the threads] spun by the posterior pair of spinnerets are combed out by a rapid oscillation of the legs into loops and flounces. At the same time she moves in such a way as to stretch out the ribbons she has spun.

A side view of the spinning spider.

And a video showing the behaviour. I was a bit nervous of supporting the camera on the windowsill just in case I disturbed her, hence the shakiness!

More information
Bristowe, William Syer. The world of spiders. Vol. 38. London: Collins, 1958.

The blue lace web spider

I have done a lot of bee watching this week. Several solitary bees species have emerged and males are patrolling the cherry tree, which is now blossoming. A male Red Mason bee landed to bask on the ivy that covers an east facing wall in the garden. Its legs got tangled on the silk threads of a lace web spider, Amaurobius similis. The spider quickly came out of her burrow, deep in the ivy. The bee disentangled itself before the spider got near and flew away, but the spider carried on checking, with short pulls of the web with her forelegs. There is nothing in my view better than sunlight to get a good shot. This is now my favourite Amaurobius portrait. On top of the natural light, the dark ivy background brings out the warm colours of the spider and also its beautiful blue silk.
Amaurobius produce an uncommon type of silk using a silk spinning organ called the cribellum. This special silk emerges from thousands of spigots on one or more plates instead of normal spinnerets. Each fiber is extremely thin, and the spider then cards the fibers with a series of bristles in the shape of a comb on her back legs - the calamistrum - into a woolly silk, which has properties like the woolly side of velcro. Insects provide spines that get easily tangled into this silk with no need of glue. In Amaurobius, the silk is blue when fresh, with the spider working on new threads in the night.