Thursday, 15 October 2015

Theridiidae: Cobweb weavers

A large spider family, including some very famous world spiders such as the North American black widows (Latrodectus sp.), the Happyface spider (Theridion grallator) and the social spider (Anelosimus sp.). The 56 British species are small to medium spiders. They make irregular webs of criss-crossed threads with some of them stretched and loaded on one end with sticky dropplets, which break when an insect struggles to break free and lift the prey in the air like snares. The genus Episinus, however, produce the most remarkably simplified web, a 'H' shaped with four sticky threads connecting the spider to the ground and an over head support. Theridids have a relatively large, well marked abdomen with a diversity of patterns and colours.  This family includes one of the most fascinating examples of maternal care in spiders.

Powerful hunters
These spiders are able to throw sticky threads over prey to immobilise them. This ability, and their rapidly acting venom, makes them formidable hunters able to subdue much larger prey.
Steatoda bipunctata with immobilised Ectemnius wasp
Even the powerful and agressive male Anthidium manicatum couldn't escape Enoplognatha sp.
A wasp? No problem for Enoplognatha
Not even a honeybee. Note the typical web of Enoplognatha, tangled around a flower, ready to catch visiting insects, with the spider neatly hidden under the flower itself.
Enoplognatha doesn't even care about the chemical secretions of shieldbugs.

Colour polymorphism
Enoplognata  comes in several colour morphs. There is a white morph and one with a pink stripe. This is the pretty and rarer pink morph.


Stridulating males
Steatoda bipunctata males have a serration at the end of the cephalothorax which rubs against teeth at the front of the abdomen making an just audible stridulation which is used during courtship. In his field guide to the Spiders of Britain and Norther Europe, Michael Roberts writes:
I first heard this in use as a medical student. Quietly studying the anatomy of the brachial plexus, I became aware of a quiet sound from a guitar standing in the corner. My initial thought was that one of the well worn strings was very slightly creaking as if about to snap and I  clearly remember tensing myself for this not uncommon event. It turned out to be a pair of Steatoda bipunctata mating inside!
This is the male of Steatoda bipunctata. It waves it's large palps as it runs on the ground looking very ant-like. The female (top shot) has a look reminiscent of a fat tick, its cephalothorax almost hidden by her large, shiny abdomen.

Maternal care
Theridids make egg sacs that are attached to their web, or under a curled leaf retreat, and are often guarded by the females. Some species even carry the egg sacs attached to the spinners. The presence of females protects the eggs from predation, for example from ants. The mother will react to the presence of ants by relocating the egg sac more centrally on the web. Enopognatha guards her egg sac under a curled leaf and stays with the spiderlings until her death.
The small Theridion sisymphium female produces a bluish egg sac, which she guards. When the spiderlings emerge she will feed them  mouth to mouth with regurgitated food. Later, she will share prey with them until they are ready to disperse.
The wonderfully weird egg sac of Theridion pallens
The blue egg sac of Enoplognata sp.
The tiny Theridium pictum with her egg sac (note the screw head for scale). This species lives near other spiders webs and steals food from them. 
Enoplognatha with her brood inside a curled leaf.

3 comments:

Wildeye said...

Super series on spiders! Thanks so much.

Karen said...

This is the best Therididae info I've come across on the internet yet, thank you!

Africa Gómez said...

Thank you very much Karen for leaving your comment, much appreciated