Yesterday I was walking down the canal towpath next to Victoria Park in that London, and there's a railing running all the way down this one section, between the canal and the park.

All along the railing there were scores and scores of ladybirds, all different markings and sizes and stuff, walking up and down it. None of them were flying, even when I nudged them. They seemed to be on some kind of mission to get from one end to the other, except some were going one way and some the other.

At one end of the railing they petered out and there some much smaller ones, that appeared to be dead.

Explanations please.