I have something better than a possibility to offer in reply to another and no less important question. Do the Orthopteron-eaters ever vary their diet? Should the favourite type of game fall short, can they not accept a different one? Does the Languedocian Sphex consider that there is nothing in the world worth having but fat Ephippigers? Does the White-edged Sphex allow none but Locusts to figure on her table; and the Yellow-winged Sphex none but Crickets? Or, according to time, place and circumstances, does each make up for the lack of her favourite victuals by others more or less equivalent? To ascertain such facts, if they exist, would be of the greatest importance, for they would tell us if the inspirations of instinct are absolute and unchangeable, or if they vary and within what limits. It is true that the cells of one and the same Cerceris contain the most varied species of either the Buprestis or the Weevil group, which shows that the huntress has a great latitude of choice; but this extension of the hunting-fields cannot be presumed in the case of the Sphex-wasps, whom I have seen so faithful to an exclusive victim, always the same for each of them, and who moreover find, among the Orthoptera, groups that differ very widely in shape. Nevertheless, I have had the good fortune to come upon one case, one only, of complete change in the larva’s nourishment; and I record it the more willingly in the Sphegian archives inasmuch as such facts, scrupulously observed, will one day form foundation-stones for any one who cares to build up the psychology of instinct on a solid basis. Here are the facts. The scene is enacted on a towing-path along the Rhône. On one side is the mighty stream, with its roaring waters; on the other is a thick hedge of osiers, willows, and reeds; between the two runs a narrow walk, with a carpet of fine sand. A Yellow-winged Sphex appears, hopping along, dragging her prey. What do I see! The prey is not a Cricket, but a common Acridian, a Locust! And yet the Wasp is really the Sphex with whom I am so familiar, the Yellow-winged Sphex, the keen Cricket-huntress. I can hardly believe the evidence of my own eyes.