第22章 CHAPTER IV LARVAL DIMORPHISM(7)

There are a few Anthrax grubs beside the remains of pupae not far removed from the final metamorphosis; there are others, but very rarely, on Mason bees already in the perfect state. These grubs are sickly and appear to be ailing; the provisions are too solid and do not lend themselves to the delicate suckling of the worms.

Who can these laggards be but animalcules that have roamed too long in the walls of the nest? Failing to make their entrance at the proper time, they no longer find viands to suit them. The primary larva of the Sitaris continues from the autumn to the following spring. Even so the initial form of the Anthrax might well continue, not in inactivity, but in stubborn attempts to overcome the thick bulwark.

My young worms, when transferred with their provisions into tubes, remained stationary, on the average, for a couple of weeks. At last, I saw them shrink and then rid themselves of their epidermis and become the grub which I was so anxiously expecting as the final reply to all my doubts. It was indeed, from the first, the grub of the Anthrax, the cream-colored cylinder with the little button of a head, followed by a hump. Applying its cupping glass to the mason bee, the worm, without delay, began its meal, which lasts another fortnight. The reader knows the rest.

Before taking leave of the animalcule, let us devote a few lines to its instinct. It has just awakened to life under the fierce kisses of the sun. The bare stone is its cradle, the rough clay its welcomer, as it makes its entrance into the world, a poor thread of scarce cohering albumen. But safety lies within; and behold the atom of animated glair embarking on its struggle with the flint.

Obstinately, it sounds each pore; it slips in, crawls on, retreats, begins again. The radical of the germinating seed is no more persevering in its efforts to descend into the cool earth than is the Anthrax grub in creeping into the lump of mortar. What inspiration urges it towards its food at the bottom of the clod, what compass guides it? What does it know of those depths, of what lies therein or where? Nothing. What does the root know of the earth's fruitfulness? Again nothing. Yet both make for the nourishing spot. Theories are put forward, most learned theories, introducing capillary action, osmosis and cellular imbibition, to explain why the caulicle ascends and the radical descends. Shall physical or chemical forces explain why the animalcule digs into the hard clay? I bow profoundly, without understanding or even trying to understand. The question is far above, our inane means.

The biography of the Anthrax is now complete, save for the details relating to the egg, as yet unknown. In the vast majority of insects subject to metamorphoses, the hatching yields the larval form which will remain unchanged until the nymphosis. By virtue of a remarkable variation, revealing a new vein of observation to the entomologist, the Anthrax flies, in the larval state, assume two successive shapes, differing greatly one from the other, both in structure and in the part which they are called upon to play. Iwill describe this double stage of the organism by the phrase 'larval dimorphism.' The initial form, that issuing from the egg, Iwill call 'the primary larva;' the second form shall be 'the secondary larva.' Among the Anthrax flies, the function of the primary larva is to reach the provisions, on which the mother is unable to lay her egg. It is capable of moving and endowed with ambulatory bristles, which allow the slim creature to glide through the smallest interstices in the wall of a Bee's nest, to slip through the woof of the cocoon and to make its way to the larva intended for its successor's food. When this object is attained, its part is played. Then appears the secondary larva, deprived of any means of progression. Relegated to the inside of the invaded cell, as incapable of leaving it by its own efforts as it was of entering, this one has no mission in life but that of eating. It is a stomach that loads itself, digests and goes on adding to its reserves. Next comes the pupa, armed for the exit even as the primary larva was equipped for entering. When the deliverance is accomplished, the perfect insect appears, busy with its laying.

The Anthrax cycle is thus divided into four periods, each of which corresponds with special forms and functions. The primary larva enters the casket containing provisions; the secondary larva consumes these provisions; the pupa brings the insect to light by boring through the enclosing wall; the perfect insect strews its eggs; and the cycle starts afresh.