once losing hold of the prey the Bembex scratches a little in front of her just where she alighted, pushes with her head, and straightway enters clasping the Dipteron to her body. The sand falls in, the door closes, and the Hymenopteron is at home. I have watched the Bembex return home a hundred times, yet it is always with fresh astonishment that I see the keen-sighted insect at once detect an entrance which nothing indicates, and which indeed is jealously hidden—not indeed when she has entered (for the sand, more or less fallen in, does not become level, and now leaves a slight depression, now a porch incompletely obstructed), but always after she comes out, for when going on an expedition she never neglects to efface the traces of the sliding sand. Let us await her departure, and we shall see that she sweeps before her door and levels everything scrupulously. When she is gone, I defy the keenest eye to rediscover the entrance. To find it when the sandy tract was of some extent I was forced to have recourse to a kind of triangulation, and how often did my triangle and efforts of memory prove vain after a few hours’ absence! I was obliged to have recourse to a stake—in other words, a grass stalk planted before the entrance—a means not always effectual, for it often disappeared during the frequent settings to rights of the outside of the Bembex’s nest. XVIII A PARASITE—THE COCOON I have just described the Bembex hovering, loaded with her prey, above the nest, and descending with a vertical flight—very slow, and accompanied by a plaintive hum. This cautious, hesitating mode of arrival might suggest that the insect was examining from above in order to find her door, and trying to recall the locality before alighting. But I shall show that there is another motive. In ordinary conditions, when nothing alarms her, she comes suddenly, without hovering or plaintive hum or hesitation, and alights at her threshold, or close by. So faithful is her memory that she has no need to search about. Let us find out the cause of the hesitating arrival just described. The insect hovers, descends slowly, mounts again, flies off and returns, because serious danger threatens. That plaintive hum is a sign of anxiety, and is never produced unless there is peril. But who is the enemy? Is it I, sitting by and watching? Not in the least; I am quite unimportant—a block unworthy of notice. The dreaded enemy—the foe who must be avoided at any price—is on the ground, perfectly still upon the sand, near the nest. It is a small Dipteron—nothing at all to look at—of inoffensive aspect. This petty fly is the terror of the Bembex. That bold assassin of Diptera, who so promptly twists the neck of colossal gadflies, full fed on blood from an ox’s back, dares not enter her home because she sees herself watched by another Dipteron—a mere pigmy, which would scarce make one mouthful for her larva.