Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Saturday, January 19, 2008

A few local Spiders.

Here is a link to what is named as a "Crab Spider" (Diaea evanida). It is also described as a Flower Spider - a name I have been known to use, if somewhat imprecisely. As an observer of flowers, and therefore an occasional finder of spiders - in flowers - it is natural to refer to many different spiders as "Flower Spiders". This little spider found her way into a car, where she posed for this photo (before being lifted to safety). This little orange-red spider is known as a Triangular Spider (Arkys clavatus) Although in body shape it resembles a "jumping spider", it does not have the large forward facing eyes which those creatures need in order to pounce upon their prey. This spider apparently is a "wait and catch" strategist. Quite what it was doing walking around on John Ross's car, I do not know.
This spider not only has a triangular abdomen*** (in outline), its back is noticeably raised (in profile), as you can see by examining it closely. Click on the photo to enlarge it. The first (in this case, lowest) line of yellow dots is clearly higher in the middle section. That part of the abdomen is much higher than the "carapace" (shell on the back of the front section of spiders, which is known as the "cephalothorax" - which simple means head-chest).

*** In spiders, the word Abdomen refers to the second body section, not just the "tummy" as in mammals. It includes what we think of as the "back" of the spider, as well as its belly. Abdomen is the entire second section of the body - behind the "cephalothorax". Spiders, of course, are "invertebrates", so do not have a "back" (or spine).

This next spider is a classic "St Andrew's Cross" Spider (Argiope keyserlingi). These spiders are not angry spiders. They are named for the way they hold their legs held out in the 4-way diagonal paired arrangement, resembling the diagonal cross of St Andrew, patron saint of Scotland. I have just learnt that the "X" shaped cross (crux decussata) derives from the traditional story that St Andrew was martyred, but refused to allowed himself to be crucified in the same manner as Christ (on a vertical cross), so was strapped to a diagonal cross - which is the pattern in the flag of Scotland. Certainly, this spider is living up to her name, and holding her legs in pairs, in the 4-way diagonal cross, as in the illustration linked above, for St Andrew's crucifixion.
Interestingly, there is a large egg sac visible in the background (top right). It is in another web, adjacent to this main insect-catching web. But from photos, it is a classic example of the egg sac of the St Andrew's Cross Spider.

Notice the bold striped patterns of this spider. She is much brighter than one at Jim's place, which was, uncharacteristically, living in an aircraft hangar.

Notice also the zig-zag reinforcing patterns in the web. These are typical of this species of spider. They are called "stabilimentum". Usually these also are positioned in the 4-way diagonal lines in which the spider holds its legs. This spider has only created two such lines.
When in danger (or at least when being photographed), these spiders hold themselves back off the web, and can set up a vibration, pulsing back and forth. I am indebted to David for pointing out this behavioural characteristic.

This spider is a bit of a puzzle to me. From its web, and the central line of "debris" (dead insects) in the middle of its web, it ought be in the Cyclosa genus. However, the photos of that genus of spiders which I have been able to find all show the back of the "head" (cephalothorax) as smooth (as in this image), whereas my spider is distinctly hairy (click on the photo above to enlarge it). In that regard, this spider resembles this "Tent Spider" (Cyrtophora moluccensis) more closely, but its web is not the classical tent spider web. The mystery of its true identity is not yet resolved. As this spider was in the garden at the back of the CTC, I shall return when it stops raining, and try and get better photos of it.


Gaye from the Hunter said...

hi Denis,

beaut spider post !!

Your mystery spider could be a young female Golden Orb Spider (Nephila edulis).

The last picture in the following blog entry is a young Golden Orb and you will notice the pattern on the abdomen. I have noticed that the pattern can vary in brightness and form.....Just a suggestion.


Denis Wilson said...

Thanks Gaye,

Good suggestion about a young Golden Orb Weaver. I had a look at yours, and the head looks right, and, as you say, the abdomen on the juvenile is quite different from the adult.

Thanks for the tip.