By working in alternate directions it manages so as never to over-fatigue one leg, for on its return the leg previously in use is at rest, while the one on the opposite side is now called into duty. The insect thus works on until its trap is completely excavated, the task occupying a variable time; sometimes being finished in half an hour, sometimes even in less, but occasionally occupying several hours, the little labourer being obliged to rest a certain time. Réaumur, who has given a fascinating account of these insects, writes, "I have had at times hundreds of ant-lions in a large box, and I have often been amused with filling up their traps. Some of them would immediately begin to form another; but the greater number in the warm long days of summer deferred executing the work until the sun began to go down. They seldom worked in the heat of the day; but in cold or cloudy weather they would excavate at any hour." We well know what perplexity a chain of rocky hills causes to a railroad engineer, and what vast outlays of money, labour, and time are necessary in order to overcome the obstacle thus presented to the path of the engine and its train. But it may be safely said that we can furnish a parallel instance of difficulty and of patient, all-surmounting exertion from the history of the insect before us. M. Bonnet was curious to know what it would do if a stone or some such obstacle were met with in the process of its excavations, and one day had the gratification of observing the behaviour of the insect under these trying circumstances. Not being able to cast it out with its head, the insect determined to carry it out, if possible, on its back. With this view it contrived by various manoeuvres to place the stone upon its back and to balance it there. This was the least difficult part of the undertaking. The insect had to climb up an inclined plane upon soil, chosen with other views purposely by itself, as shifting and unsteady as possible, and not only so, but to preserve the balance of the stone with which it was encumbered. Undaunted by these difficulties it made the attempt, but the first step brought down a shower of sand, and tumbled the little rock to the bottom. Again and again did the heroic insect attempt the same feat, and with the same ill success, and we might have thought we beheld a realization of the fable of Sisyphus and the rolling-stone, in the vain endeavours of the insect to get rid of its encumbrance. Five or six times did the insect repeat its endeavours, and at last, after one or two narrow escapes, the stone was fairly lodged on the outside of the trap, and the insect returned to its subterraneous recess at the bottom of the cell in triumph.