Wednesday, 15 June 2016

A shady individual with WOW-factor: the Walnut Orb-weaver (Nuctenea umbratica)

I cannot tell why Nuctenea umbratica, the Walnut Orb-weaver, is my favourite spider; it just is; it always has been. One could be forgiven for thinking this spider a sinister-looking dame (it's usually a female which is seen): it's abdomen is shiny, stubble-haired, flattened and pitted (these pits, or sigilla, indicate the muscle bases which pull the abdomen flat), it has an alarming zig-zagging oak-leaf pattern or folium around its abdomen and it slinks into the shade during the daytime, emerging only at dusk to weave its silky orb upon which, once complete, it rests in the centre of, darkly, waiting for supper. The reality is that it is no more sinister than many invertebrates and the female, like many spider species, is a devoted mother.

The scientific name, Nuctenea umbratica, is interesting and instructive (I apologise to the reader who understands Latin as a language!): I was tempted to think of the first name as being derived from noctem (night), but this is wrong because nuce or nux translates literally as 'nut', and tenea (less obviously) as 'to hold', 'grasp' or 'possess' - so a possible reference to being nut-like. The second half of the binome, umbratica, is more familiar and means 'sheltered' (umbraticus meaning 'in the shade'). This translation describes both the spider's appearance and habits well, as does the acronym of its vernacular name: WOW!

This is a common and widespread spider, a member of the Araneidae family, which has 3100 members worldwide, including another with wow-factor - the stunning Wasp Spider (Argiope bruennichi). In its natural environment umbratica tucks its flattened self away during the daytime into small apertures behind tree bark, although I have seen it more often on wooden gates, fenceposts and signposts, where nooks and crannies provide an acceptable alternative. They emerge at dusk to weave a fresh web and are often found in little communities of a few or several spiders. Most of the webs I find are about 30cm in size, but they are reported to stretch to more than double this size to 70cm. Typical prey are night-flying insects such as moths. Females are often seen in springtime guarding an egg sac, as seen in the photograph below, which was taken close to home at Mount Pleasant in 2011.


Female umbratica guarding an egg-sac on a gate-post in 2011
The sexes are dimorphic, the female being much larger than the male at about 14mm compared with the smaller male, which is 8 to 9mm. This is a common feature of spider species. Females are apparently active year-round, with males appearing in early summer.

This spider is capable of biting humans, although reports of bites are rare. Venom is, after all, a precious resource and most spiders will bite in defence only when they feel threatened. The wound can burn and itch and form a fiery lesion, but it is not a serious injury. The spider is much more likely to feign death when disturbed and abseil on a silken thread away from danger to the ground, its legs tucked in tight to its body. Most of the time the spider will sit motionless and pose for a photograph. They were once a common sight in my garden but they seem to have declined during the past two or three years, their places having been taken by an ever-expanding colony of Steatoda nobilis (False Widow). I wonder if my favourite spider is being out-competed where the two species coexist?

Should you go dusking for glow-worms, Ghost Moths or Nightjar in June, it is well worth shining your light onto wooden field-furniture for a chance of glimpsing this secretive and beautiful little orb spider with the wow-factor.

Only one individual has been seen at home since 2013

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