I expect you will have guessed what the caterpillars are for. They are food for the wasp grub when it hatches out of the egg. Generally each hole has between twenty and thirty little caterpillars in it, and sometimes, when caterpillars are scarce, the Mother Wasp has to work hard for three or four days. If you dig into a hole yourself and look at the store of little caterpillars, you will see there is something the matter with them. They seem to be alive and yet they don't seem to be able to crawl. Wise men say that the wasp stings them just enough to make them drowsy so that they can't crawl out of the hole, and can't hurt the wasp grub by jostling up against it. It wouldn't do to kill them, because then they would go bad in the hole before the grub had time to eat them. This sounds rather cruel, but I don't think it is really, because it is quite certain that the caterpillars cannot feel as we should perhaps feel, and we may be quite sure that in the wonderful Nature World everything is arranged for the best, so that only the right number of wasp-grubs may be properly fed and grow up to do what it is their duty to do, and only the right number of small green caterpillars may grow up also. [Illustration: The little beetle that the caterpillars turn into. It is sitting on its own open-work cocoon, from which it has just hatched out. The picture makes it about twice its real size] You will wonder, I expect, why the Mother Wasp troubles to make the little tube above the hole. I think I can tell you one reason and you must remember this, because it was just by chance that I found it out. One hot morning in June I watched Mother Spinipes bringing seven caterpillars to her hole. Then a heavy thunderstorm came on, and the rain came down in buckets, and I had to run away for shelter. Late in the evening when it had cleared up a little, I thought I would like to see what had happened to the tube I had been watching, and I went back to the place and found that the rain had knocked it all to pieces. But I saw something much more interesting than this. The tube had been on the face of a sand-cliff, and in a crack close by there was an ants' nest. I found that the ants were running down the wasp's hole and bringing out the caterpillars as fast as they could (I saw them take six away), and taking them along the face of the cliff into their own stronghold. Now the tube that stands out from the sand somehow frightens the ants (I never saw an ant climb out along the tube and down inside it), and so I think that one of the reasons for the tube must be that it keeps away ants and creatures of that kind who crawl about on the face of the sand cliff and like eating caterpillars. [Illustration: BEFORE THE THUNDERSTORM] [Illustration: AFTER THE THUNDERSTORM]