Why do they do that?

Why is it that as soon as I start to paint a wall insects come from far and wide to check it out?
We are in any case plagued by some black bugs which crawl up the house walls whenever the sun shines. I tried to kill them last year with all things environmentally friendly - I think next year I will have to get the whole garden sprayed with something very nasty. As it is I can't have any windows open because these things can crawl up to the second floor! And when squished they are full of very red bloood. Yuk!