Saturday, 10 October 2015

Orb-weaving Spiders: nature's great architects

In this blog post I shall be focusing solely on the beautiful and fascinating Garden Spider (Araneus diadematus). This species is a member of the typical orb-weaving spiders (family Araneidae), and is one of the the most noticeable arachnid species encountered from July through to November, occurring across the UK in gardens, hedges, parks and scrub.

Anyone rising early in the last few weeks on a calm, crisp autumnal morning may have been treated to the stunning display of orb-webs that cling delicately to areas of scrub, hedgerows and other favourable hunting grounds. If you are really lucky, then a heavy dew may have descended overnight and vividly highlighted the individual strands of the webs with small water droplets, turning the webs into a beautiful piece of art. This has certainly been a common, and very welcome, sight for me in the last few weeks, and has lead me to try writing a short piece about these superb creatures...

Garden Spiders and their beautiful webs in the early morning light, taken at a nearby nature reserve called the Bissoe Valley

So, who do we have to thank for adding these ethereal scenes to our early mornings? The culprit is the humble Garden Spider, Araneus diadematus. This species, like many others in the Araneidae family, spin large orb webs to snare unwary prey. Typically about 13mm in length in females, and up to 8mm for males, they can grow to a chunky size- the former in particular can spin sizeable webs capable of catching a diverse range of prey items, from butterflies and grasshoppers through to midges, shieldbugs and species of bees and flies.

Female Garden Spider lying low after a bought of heavy rain overnight. I wonder what effect dew and water droplets have on the catch of Orb-weaving Spiders?

The pregnant females are particularly bulging in appearance, due in no small part to the swelling of eggs. These will be packaged into a silk egg sac, which is laid onto branches, in sheds, or in other such sturdy places. The small eggs will hatch out in the spring, into a burst of tens of small yellow Spiderlings. These will then disperse, grow, and eventually set about building their own delicate masterpieces- with no tuition or input from the now-deceased mum.

Another female Araneus diadematus, clinging onto to its central cluster of threads. This web has had a productive day's catch, as can be told by the 'by-catch' items that have just been left. My friend and excellent photographer Will Hawkes is undertaking an interesting study into the relationship between size of spider and number of prey items left over (i.e. not consumed)

So how do they actually construct these architectural masterpieces? It is amazing to think that these spiders are able to spin webs across gaps which they can't actually physically cross, and which can be over 50 times the length of the spider! As a side note before I attempt to summarise the process, it really is worth watching this super cool time-lapse of a Garden Spider creating its web.
To begin the process (which the Orb-weavers typically carry out at night, before the main 'hunting' period during daylight hours), the spider spins a very fine thread of silk, which it allows to drift from one side of a gap to the other. Once this thread has made contact with the far side, it adheres and allows the the spider to pull it tight and strengthen it numerous times with additional threads. Once the main thread is strong enough to support the rest of the web, two additional threads will gradually be added to form a Y-shaped 'web'. This Y will be the primary foundation for the rest of the this point the spider will now go about adding a number of radials to the web, which sets the base ready for the sticky circular threads. An amazing feature of this stage is that the spider will construct a non-sticky spiral of thread from the centre to the periphery, which allows it to move around during construction without getting stuck!

After strengthening the central 'perch' with about five circular strands, the spider now makes the fundamental circles of sticky threads that perform the crucial role of ensnaring victims. The spider will start from the outside moving to the centre, and will use the non-sticky spiral to move between the layers; as it makes these closely-spaced circular threads, it will remove the non-sticky spiral. The reason that the circular sections of the web are so exact in distance between each other is simply because of the way they are made: the spider picks the thread from its spinner with its hind leg, and fastens it onto one of the radials- it then repeats this as it continues round, securing the circular strands to a radial line each time one is encountered. Eventually, the web is finished, and the creator can now sit happily in the centre, and await its prey.

A web in construction, with the spider in the process of spinning the sticky circular sections as it works from the outside to the centre

Perfection: a near flawless masterpiece awaiting a productive day's hunting

The spiders use a variety of mechanisms to prevent themselves from falling victim to their own success...besides the spinning of non-stick threads in the course of construction and operation of the web, the spider also has very fine hairs and a non-stick surface on the tips of its legs which make sticking less likely. Most of the species in the family Araneidae have three tarsal claws, and the third of these is thought to have a particularly non-sticky function, thus forcing the spider to walk solely on these in some circumstances.

The Garden Spider may rebuild these webs every day, as anyone who continually walks through awkwardly-placed webs may testify. The spider will often consume what is left of the old web so that the proteins used to make the web can be recycled and re-used in the next one.  

 Another chunky female Garden Spider

Garden Spiders spinning up an unfortunate Common Green Grasshopper

When prey items become tangled in the sticky threads, the Garden Spider will quickly pick its way over to the victim, and promptly wrap it up in a layer of silk. It will then devour the victim straight away or return sometime later.

 It is great to see the sticky-ness of certain strands at work in all of the dew-covered webs above: can you see that most of the droplets are only sticking to the circular sections of the web? Most of the radial sections and the spider's central perch (non-sticky) are devoid of droplets. 

So there we are...a bit of a focus on the species responsible for the amazing webs which hang unassumingly in parks and scrubland areas all across the UK. Will you look at one of these superb arachnids in a different stead next time you see one?


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