The way in which the wings come out of their sheaths is the most remarkable feature in this casting of the skin. In their undeveloped state they are folded lengthways and much contracted. A little while before they acquire their normal appearance one can easily draw them out of their sheaths; but then they do not expand, remaining always crumpled, while, when the large piece of which the sheaths are a part is pushed back by the movements of the abdomen, they may be seen issuing gradually from the sheaths, and immediately they gain freedom, assuming dimensions out of all proportion to the narrow prison from which they emerge. They are then the seat of an abundant influx of vital juices which swell and spread them out, and the turgescence thus induced must be the chief cause of their coming out of their sheaths. When freshly expanded the wings are heavy, full of moisture, and of a very light straw colour. If the influx should take place in an irregular manner, the point of the wing is seen to be weighed down by a yellow droplet contained between its under and upper surface. After denuding itself of the abdominal sheath, which draws away with it the wing-cases, the Sphex again is motionless for about three days. During this interval the wings assume their normal colouring, the tarsi take colour also, and the mouth-parts, at first spread out, assume their normal position. After twenty-four days as a nymph the insect attains its perfect state, tears its imprisoning cocoon, opens a way through the sand, and appears one fine morning in the light as yet unknown to it. Bathed in sunshine, it brushes wings and antennæ, passes its feet again and again over its abdomen, washes its eyes with its forefeet moistened with saliva, like a cat, and, its toilette made, flies joyfully away. Two months of life are before it. Beauteous Sphegidæ, hatched under my eyes and brought up by my hand, ration by ration, on a bed of sand, at the bottom of an old feather box,—you whose transformations I have followed step by step, waking up with a start at night for fear of missing the moment when the nymph breaks through her swaddling bands and the wings issue from their cases. You have taught me so many things, learning nothing yourselves, knowing without teachers all that you need to know. Oh, my beautiful Sphegidæ! fly away without fear of my tubes, my phials, and all my boxes and cages, and all my prisons for you; fly through the warm sunshine, beloved by the cicadas! Go, and beware of the Praying Mantis, who meditates your destruction on the purple thistles; beware of the lizard watching for you on the sunny slopes. Depart in peace, hollow out your burrows, stab your crickets scientifically, and continue your race, so as to afford to others what you have afforded to me—some of the few moments of happiness in my life. IX ADVANCED THEORIES