Spider explosion

Mangroves covered in spiderweb
Mangroves covered in spiderweb

I stopped off at one of my inner-city parks three weeks ago and saw the biggest spider colony I’ve seen for quite some time. I was more than slightly boggled by it. Skeins and swathes of web stretched from one mangrove tree to the next along at least 20 metres of creek bank, overhanging the water and within about 1.5 m of water level. (It’s the bank on the far side of Ross Creek from these cormorants.)

Looking into the web ...
Looking into the web …

When I looked more closely I saw long skinny orange-brown spiders (1), lots of smaller chubbier charcoal-white patterned spiders (2), and a few very small ones (3); also lots of egg sacs woven onto twigs.

I reckoned that (3) were very young juveniles, that most or all of (2) were sub-adults and that (1) were adults, all of one species. “Which species?” is the obvious question. Not so obviously, why did all the adults I photographed happen to be males? Do sub-adult males change from type (2) colours to type (1) with their last moult, I wondered, while adult females remain type (2)?

Identifying the adults to genus level is easy, since the enormous jaws are very distinctive. They are in the genus Tetragnatha in the family Tetragnathidae. Both genus and family have the common names ‘Four-jawed Spiders’ (an exact translation of Tetragnatha) or ‘Long-jawed Spiders’. Beyond that, identification is less certain. They may be Tetragnatha nitens, T. rubiventris or another close relation (follow this link for photos of each species).

On a second vist two weeks later I saw many adult females with the reddish coloration, so any gender imbalance was only temporary. The webs were just as extensive and the egg-sacs even more numerous, although spider numbers seemed to be a little reduced.

I normally see adult spiders in this family as individuals (e.g. this collection), often well away from water and often with hardly any web, let alone a tent city like this. The expert I call on for things arachnological suggested that this could be a local ‘population explosion’, for whatever reason. He hadn’t heard of it with this genus but says it does happen with others, mentioning one of the St Andrews Cross species as an example.

What will happen here remains to be seen. I will call in again and report back … if I emerge alive. I’m just joking, of course – they are completely harmless, and the biggest risk to me is that I will fall in the creek while trying to take a better photo.

Part of the web on my second visit


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