The male European crab spider... approaches tentatively but, when close to the female, grabs one of her legs. Initially she struggles but later calms down as he moves over her body trailing silk threads, which bind her to the ground. He then lifts her abdomen, crawls under and inserts his palps [the organs that carry his sperm]. However, this bondage appears to be purely ritualistic because it is not difficult for the female to break free: it is likely that it helps to pacify her.
Just one example of spiders' unusual mating practices described in Paul Hillyard's book The Private Life of Spiders.
You can find more factoids from the book here.
