Rincewind half-dragged the ape from his nest and out through the door. He didn't head for the main gates but for an otherwise undistinguished area of wall where a few loose stones had, for two thousand years, offered students an unobtrusive way in after lights-out. Then he stopped so suddenly that the Librarian cannoned into him and the Luggage ran into both of them.

'Oook!'
'Oh, gods,' he said. 'Look at that!'
'Oook?'

There was a shiny black tide flowing out of a grating near the kitchens. Early evening starlight glinted off millions of little black backs.

But it wasn't the sight of the cockroaches that was so upsetting. It was the fact that they were marching in step, a hundred abreast. Of course, like all the informal inhabitants of the University the roaches were a little unusual, but there was something particularly unpleasant about the sound of billions of very small feet hitting the stones in perfect time.

Rincewind stepped gingerly over the marching column. The Librarian jumped it.
The Luggage, of course, followed them with a noise like someone tapdancing over a bag of crisps.

And so, forcing the Luggage to go all the way around to the gates anyway, because otherwise it'd only batter a hole in the wall, Rincewind quit the University with all the other insects and small frightened rodents and decided that if a few quiet beers wouldn't allow him to see things in a different light, then a few more probably would. It was certainly worth a try.

That was why he wasn't present in the Great Hall for dinner. It would turn out to be the most important missed meal of his life.

© Terry Pratchett