The value of chemist's pepsin is estimated by the quantity of hard-boiled white of egg which a gram of that agent can liquefy. The mixture has to be exposed in an oven to a temperature of 1400 F. and also to be frequently shaken. My preparation, in which the bluebottle's eggs are hatched, is neither shaken nor subjected to the heat of an oven; everything happens in quietness and under the thermometric conditions of the surrounding air; nevertheless, in a few days, the coagulated albumen, treated by the vermin, runs like water.
The reagent that causes this liquefaction escapes my endeavors to detect it. The worms must disgorge it in infinitesimal doses, while the spikes in their throats, which are in continual movement, emerge a little way from the mouth, reenter and reappear. Those piston thrusts, those quasi-kisses, are accompanied by the emission of the solvent: at least, that is how I picture it. The maggot spits on its food, places on it the wherewithal to make it into broth. To appraise the quantity of the matter expectorated is beyond my powers: I observe the result, but do not perceive the leavening agent.
Well, this result is really astounding, when we consider the scantiness of the means. No pig's or sheep's pepsin can rival that of the worm. I have a bottle of pepsin that comes from the School of Chemistry at Montpellier. I lavishly powder some pieces of hard-boiled white of egg with the potent drug, just as I did with the eggs of the Bluebottle. The oven is not brought into play, neither is distilled water added, nor hydrochloric acid: two auxiliaries which are recommended. The experiment is conducted in exactly the same way as that of the tubes with the vermin. The result is entirely different from what I expected. The white of egg does not liquefy. It simply becomes moist on the surface; and even this moisture may come from the pepsin, which is highly absorbent. Yes, I was right: if the thing were feasible, it would be an advantage for the chemists to collect their digestive drug from the stomach of the maggot. The worm, in this case, beats the pig and the sheep.
The same method is followed for the remaining experiments. I put the bluebottle's eggs to hatch on a piece of meat and leave the worms to do their work as they please. The lean tissues, whether of mutton, beef or pork, no matter which, are not turned into liquid; they become a pea soup of a clarety brown. The liver, the lung, the spleen are attacked to better purpose, without, however, getting beyond the state of a semi-fluid jam, which easily mixes with water and even appears to dissolve in it. The brains do not liquefy either: they simply melt into a thin gruel.
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.
A distant disciple of the maggot's biographer, I look at the problem in a light which Redi never dreamt of. Any flesh of one of the higher animals suits the fly's family. Will it be the same if the food supplied be of a lower organism and consist of fish, for instance, of frog, mollusk, insect, centipede? Will the worms accept these viands and, above all, can they manage to liquefy them, which is the first and foremost condition?