Some months ago (vol. vii. p. 443) I sent you an extract from a letter from Mr. Hague, a geologist residing in California, who gave me a very curious account of the terrifying effect on the other ants of the sight of a few which he had killed on one of their paths.² Mr. Traherne Moggridge saw this account in Nature, and wrote to me that he had heard from a gentleman who had lived in Australia that merely drawing a finger across the path deters ants from crossing the line.³

Mr. Moggridge tried this experiment with some ants a Mentone with similar effects. I therefore sent the letter to Mr. Hague, and asked him to observe whether his ants were alarmed by the smell left by the finger, or were really terrified by the sight of their dead and dying comrades. The case appears curious, as I believe no one has ever observed an invertebrate animal realising danger by seeing the corpses of a fellow species. It is indeed very doubtful whether the higher animals can draw any such inferences from the sight; but I believe that everyone who has had experience in trapping animals is convinced that individuals who have never been caught learn that a trap is dangerous by seeing others caught.

Here follows Mr. Hague’s letter, fully confirming his former statement.⁴

Charles Darwin