On the other hand, fatty substances, such as beef suet, lard and butter, do not undergo any appreciable change. Moreover, the worms soon dwindle away, incapable of growing. This sort of food does not suit them. Why? Apparently because it cannot be liquefied by the reagent disgorged by the worms. In the same way, ordinary pepsin does not attack fatty substances; it takes pancreatin to reduce them to an emulsion. This curious analogy of properties, positive for albuminous, negative for fatty matter, proclaims the similarity and perhaps the identity of the dissolvent discharged by the grubs and the pepsin of the higher animals. Here is another proof: the usual pepsin does not dissolve the epidermis, which is a material of a horny nature. That of the maggots does not dissolve it either. I can easily rear bluebottle grubs on dead crickets whose bellies I have first opened; but I do not succeed if the morsel be left intact: the worms are unable to perforate the succulent paunch; they are stopped by the cuticle, on which their reagent refuses to act. Or else I give them frogs' hind legs, stripped of their skin. The flesh turns to broth and disappears to the bone. If I do not peel the legs, they remain intact in the midst of the vermin. Their thin skin is sufficient to protect them. This failure to act upon the epidermis explains why the bluebottle at work on the animal declines to lay her eggs on the first part that comes handy. She needs the delicate membrane of the nostrils, eyes or throat, or else some wound in which the flesh is laid bare. No other place suits her, however excellent for flavor and darkness. At most, finding nothing better when my stratagems interfere, she persuades herself to dab a few eggs under the axilla of a plucked bird or in the groin, two points at which the skin is thinner than elsewhere. With her maternal foresight, the bluebottle knows to perfection the choice surfaces, the only ones liable to soften and run under the influence of the reagent dribbled by the newborn grubs. The chemistry of the future is familiar to her, though she does not use it for her own feeding; motherhood, that great inspirer of instinct, teaches her all about it. Scrupulous though she be in choosing exactly where to lay her eggs, the bluebottle does not trouble about the quality of the provisions intended for her family's consumption. Any dead body suits her purpose. Redi, the Italian scientist who first exploded the old, foolish notion of worms begotten of corruption, fed the vermin in his laboratory with meat of very different kinds. In order to make his tests the more conclusive, he exaggerated the largess of the dining hall. The diet was varied with tiger and lion flesh, bear and leopard, fox and wolf, mutton and beef, horseflesh, donkey flesh and many others, supplied by the rich menagerie of Florence. This wastefulness was unnecessary: wolf and mutton are all the same to an unprejudiced stomach.