Fig. 572.—Microgaster larvæ; spinning their cocoons: a, enlarged view of 5.—After Marshall.
The first appearance of the parasite is represented in Fig. 572, 1. A warty excrescence appears on the back of the caterpillar, which slowly emerges until it is seen to be a larva enclosed in a delicate transparent membrane, as represented in 2. This it soon succeeds in bursting, and, rising to its full length, balances itself a moment as in 3, then, bending double, it ejects from its mouth a glairy liquid, which instantly changes to silk, and fastens the posterior end to the skin of the caterpillar, as shown in 4, side view. It now begins to spin its cocoon by attaching a silken thread to the silky mass by which it had previously fastened itself to the caterpillar, and forming a series of loops of uniform size, first from right to left, and then back again from left to right, as represented in the front view, 5, and better in the enlarged view, 5a, the arrow heads showing the direction in which the head of the larva moved while forming the loops. The ends of the series, numbered 1, 2, 3, 4, are fastened to the edges of the ventral side of the body, which thus serves as a measure of the width of the cocoon, and also acts as a support for the frail fabric in the first stages of spinning. After the larva has fastened the fabric as far up on its ventral surface as it can, conveniently, it then begins to spin free, as shown in the side view, 6, where it is represented as just completing the first half of its cocoon, which resembles in form a slipper. This accomplished, the larva ceases to spin for the time being, bends its head, as in 7, towards its ventral surface, and pushes the half cocoon free from its body. The form of the silken fabric enables it to stand unsupported, while the larva, sliding its head down to the base, holds on firmly until it swings its posterior end into the toe of the slipper.
Figure 572, 8, shows it in the act of changing end for end, and in 9 the larva is seen erect, beginning at the base to complete the other half of its cocoon; 10 shows the larva contracting its body as it spins upward for about half the length of the cocoon, when it again changes end for end, as shown in 11, where it is beginning at the upper part to unite the two sides, finally enclosing itself as represented in 12.
It may now be seen, under the microscope, through the meshes of its cocoon actively engaged in lining the interior with layers of very fine silk ejected from its mouth in great abundance. One half of the cocoon is first lined by a forward and back movement of its head, and then reversing its position, it lines the other half in a similar manner.
In one case the larva was disengaged from the skin of the caterpillar, after beginning its cocoon. It, however, began again, and spun a portion while lying on the table. This was removed, when it began a third time, and completed its cocoon.
In about 10 days the insect made its appearance through a hole in the upper end, as represented in 13. The top was eaten off in a perfect circle and hung by a few threads, so as to resemble a lid as it was thrown back.
One caterpillar observed had between 300 and 400 cocoons on its back and sides, and another was dissected after more than 30 larvæ had escaped, and 130 were discovered in the soft integuments of the back.
The figures from 1 to 13 are magnified five diameters, but in order to observe the spinning of the cocoon a power of 50 is required. (Amer. Naturalist, xii, pp. 559, 560.)
Certain differences observed by W. A. Buckhout in a Microgaster parasitic on the different species of Macrosila, are referred to in the same volume, p. 752.
