My old notes give the following description of the caterpillars found in the nest of E. Amadei: a pale-green or, less often, yellowish body covered with short white hairs; head wider than the front segment, dead-black and also bristling with hairs. Length: 16 to 18 millimetres;[6] width: about 3 millimetres.[7] It is more than a quarter of a century since I jotted down this descriptive sketch; and today, at Sérignan, I [[15]]find in the Eumenes’ larder the same sort of game that I saw long ago at Carpentras. Time and distance have not altered the nature of the provisions.
I know one exception and one alone in this fidelity to the ancestral diet. My observations mention a single dish that differs greatly from those which accompany it. This is a caterpillar of the Looper group[8] with only three pairs of prolegs, placed under the eighth, ninth and twelfth segments. The body tapers slightly at either end, is contracted at the junction of the different rings and is pale green with faint black veinings, visible under the magnifying-glass, and a few sparse black cilia. Length: 15 millimetres;[9] width: 2½ millimetres.[10]
E. pomiformis also has her preferences. Her game consists of small caterpillars about 7 millimetres long by 1⅓ wide.[11] The body is pale green, pretty sharply contracted at the junction of the segments. The head is narrower than the rest of the body and is spotted with brown. Pale ocellated [[16]]circles are distributed in two transversal rows over the middle segments and have a black dot in the centre, surmounted by a black cilium. On the third and fourth and also on the penultimate segment, each circle has two black dots and two cilia. This is the rule.
The exception is supplied by two head of game in the whole course of my observations. These two had a pale yellow body, with five longitudinal brick-red stripes and a few very rare cilia. Head and prothorax brown and shiny; length and diameter as above.
The number of pieces served for the meal of each larva interests us more than the quality. In the cells of E. Amadei I find sometimes five caterpillars and sometimes ten, which means a difference of a hundred per cent in the quantity of the food, for the pieces are of exactly the same size in both cases. Why this unequal supply, which gives a double portion to one larva and a single portion to another? The consumers have the same appetite: what one nurseling demands a second must demand, unless there be here a menu differing according to the sexes. In the perfect stage, the males are smaller than the females, are hardly [[17]]half as much in weight or volume. The amount of victuals, therefore, required to bring them to their final development may be reduced by one-half. In that case, the well-stocked cells belong to females; the others, more meagrely supplied, belong to males.
But the egg is laid when the provisions are stored; and this egg has a determined sex, although the most minute examination is not able to discover the differences which will decide the hatching of a female or a male. We are therefore needs driven to this strange conclusion: the mother knows beforehand the sex of the egg which she is about to lay;[12] and this knowledge enables her to fill the larder according to the appetite of the future grub. What a strange world, so wholly different from ours! We fall back upon a special sense to explain the Ammophila’s hunting; what can we fall back upon to account for this intuition of the future? Can the theory of chances play a part in the hazy problem? If nothing is logically arranged with a foreseen object, how is this clear vision of the invisible acquired? [[18]]
The capsules of E. pomiformis are literally crammed with game. It is true that the morsels are very small. My notes speak of fourteen green caterpillars in one cell and sixteen in a second. I have no other information about the integral diet of this Wasp, whom I have neglected somewhat, preferring to study her cousin, the builder of rockwork domes. As the two sexes differ in size, though not so greatly as in E. Amadei, I am inclined to think that those two well-filled cells belonged to females and that the males’ cells must have a less sumptuous table. Not having seen for myself, I am content to set down this mere suspicion.
What I have seen and often seen is the pebbly nest, with the larva inside and the provisions partly consumed. To continue the rearing at home and follow my charges’ progress from day to day was a business which I could not resist; besides, so far as I was able to see, it was easily managed. I had had some practice in this foster-father’s trade; my association with the Bembex, the Ammophila, the Sphex[13] and many others had turned me into a passable insect-breeder. [[19]]I was no novice in the art of dividing an old pen-box into compartments in which I laid a bed of sand and on this bed the larva, with her provisions, delicately removed from the maternal cell. Success was almost certain at each attempt: I used to watch the larvæ at their meals, I saw my nurselings grow up and spin their cocoons. Relying upon the experience thus gained, I reckoned on success in raising my Eumenes.
The results, however, in no way answered to my expectations. All my endeavours failed; and the larva allowed itself to die a piteous death without touching its provisions.
I ascribed my reverse to this, that and the other cause: perhaps I had injured the frail grub when demolishing the fortress; perhaps a splinter of masonry bruised it when I forced open the hard dome with my knife; perhaps a too-sudden exposure to the sun surprised it when I withdrew it from the darkness of its cell; the open air again might have dried up its moisture. I did the best I could to remedy all these probable reasons of failure. I went to work with every possible caution in breaking open the home; I cast the shadow of my body [[20]]over the nest, to save the grub from sunstroke; I at once transferred larva and provisions into a glass tube and placed this tube in a box which I carried in my hand, to minimize the jolting on the journey. Nothing was of avail: the larva, when taken from its dwelling, always pined away and died.