Bearing in mind that the most common places for stitching to become loose is around the backside (rear) of the trousers - that might explain why he wears his pants on the outside.
I guess that it is an all in one stretchy material, or very quickly spins it around himself when he turns round and round very fast, like a spider wrapping a web around them, or the silk of a glow worm.
But then we get onto spiderman, who probably really does spin a fine silk web around him with lycra ability.
The second most likely place for the split in costume would be the biceps and triceps area.