Orb Weaver spider mating - a dangerous business
In Orb Weavers, as in many other spiders, there can be a big difference in size between the male (little guys) and female (BIG!). St. Andrew's Cross Spider (Argiope keyserlingi), commonly seen on the block from late Spring through Summer, illustrates this size disparity nicely.
The female A. keyserlingi is 16-20mm long from the front to the back of the body (excluding legs). This one is sitting in the web in a typical resting posture - on each side of the body, the first and second legs are held together as are the third and fourth.
This photo shows the size difference between the female - here seen from the belly side - and the male, which is only 5mm long. The white X-shaped structure at the centre of the web is called a stabilimentum. Its function is controversial - there is some evidence that it highlights the web to prevent birds from accidentally flying into it.
A close up of another male. Note his drab colouration.
One consequence of this male-female size disparity is that mating is potentially a hazardous process for the male. He must avoid being mistaken for a prey item - at least until he has successfully copulated. To this end, the male Argiope approaches the female cautiously and uses a complex behavioural routine to safely attract her attention and make her receptive to his amorous advances. I was privileged to witness this behaviour taking place in an A. keyserlingi web in our vegetable garden in early Summer and was able to video the whole event. The first video shows the courtship sequence. I have edited this sequence, reducing it from 10 minutes to 2:30 minutes.
The male in this video has apparently made a previous mating attempt, which has led to the loss of five of his legs. He was very lucky - in a related species, only 20% of virgin males survive their first copulation attempt. Undeterred, he now makes repeated advances to the female, who responds by probing with a leg and then moving towards him. However, each of his advances ends in a sudden retreat on his part and finally, she returns to the centre of her web. Frustrated? Notice the enlarged, black appendages at the front of the male's head. These pedipalps hold a packet of his sperm which he will place into the female's genital opening - that is how copulation takes place in all spiders.
This video sequence shows the climax - the final advance of the male, followed by copulation. Soon after the start of this movie, you can see him pulling on a thread that runs to the female's body. He vibrates this to excite the female. She moves towards him with her pedipalps extended. He suddenly rushes towards the female and they embrace. She holds him tightly to her abdomen with three pairs of legs so that his pedipalps are within reach of her genital openings. As the male copulates with her, she dangles from a bundle of threads which she has just woven. After 20 seconds or so, the deed is done. She then pulls the male up to her mouth, jabs him with her fangs and proceeds to dine out on him.
This movie shows the female devouring the male at her leisure - a phenomenon called sexual cannibalism. Having fulfilled his reproductive function, the male becomes a valuable food source for the female.
A close up image of the female with the remains of the male in her fangs. In the end, all of this sex and violence came to nothing. We saw the female in her web for the next couple of weeks, then one day she was gone - probably taken by a bird.
