Sunday, October 18, 2015

Courting the Hallowe'en pumpkin

Pumpkins aren't the only orange globes in October.  A female Araneus Diadematus in October is typically plump with eggs.  Her large, unwieldy size and cooler metabolism-slowing temperatures makes her move slowly compared to her youthful self mere months earlier.  At this point, she relies on her sticky silk rather than speed to capture prey.

Chomping on a silverfish that mysteriously fell from the sky

I've talked about the life cycle of the cross spider before.  The one most important event I've never documented is how they mate.

Spiders exemplify sexual dimorphism, meaning that males and females have different physical traits, and in many spiders it is so extreme that one could mistakenly conclude that males and females are two different species.  While the two sexes of the giant house spider have roughly the same legspan when mature, the females of A. Diadematus are many times the size of the males, such that the male can (and does) often become a snack for the female.  (Most notorious for this behaviour (and sexual dimorphism) is the black widow)

Recall what I mentioned in the last post about home advantage - this is clearly the case for A. Diadematus as well.  Nearly blind, the male traverses walls, branches, until he finds the threads or webbing of a mature female.  He identifies species and readiness to mate by tasting the threads.  He coaxes the female out by tapping on a thread of the web, but gently and with a different frequency from an insect in distress.  The female approaches the male slowly, but the male must still use caution - the much larger female could at any instant turn on him.  His escape strategy is to tie his anchor line further away, so in a panic, all he has to do is let go to swing to safety.

See the male's pedipalps at the front of his head

The male stores his sperm in his pedipalps, the two leglike appendages at the front of his head.  He needs to reach the female's seminal receptacle at the underside of her abdomen, which incidentally is also the most vulnerable place to be.

A. Diadematus is the largest and most abundant orb (spiral) web weaver in the Lower Mainland, but nearly 100% of the webs you'd ever see are those of females.  Males will start their search for females soon after molting for the last time, and rarely build webs of their own in this last stage (and if they do, they tend to be quite small).  Meanwhile, females will continue building webs well after maturity, as they need to gain the nutrition to build up an egg mass.

A. Diadematus is the most easily observed spider (and dare I say, urban wildlife) in the Lower Mainland and many other parts of North America (I've even seen them in the UK and China)...a great starter spider, if you want to learn more about spiders in general, and the micro-ecosystems that surround us.  Their entire life cycle is at eye level; their existence targets the only insects in these parts that prey on humans (ie. mosquitos); they don't leave behind messy webs; and they live outdoors which reduces the ick factor.  They are one of the last bastions of wilderness that thrive despite humanity.

1 comment:

Susannah Anderson said...

Great photos and a wonderful video! I've never caught them at it. I kept expecting the female to pounce.