Home › Volumes › Insects › Chalcis Flies

Chalcis Flies — Part 2

The poor caterpillar, which up to this time has shown no signs of being affected, except by its sickly yellowish color and by its very slow growth, collapses and dies as soon as a single one of the parasitic larvae withdraws, and the same fate overtakes those parasitic maggots which are at the time less advanced in their development. If one of the larvae be removed by hand, Schwarz found, both the victimized worm and the remaining parasites quickly dry up. After the larva turns yellowish white and relaxes its hold on the caterpillar, it works its way around underneath the belly of the host and spins a series of silk threads attaching the caterpillar, which is now a mere empty skin, to the leaf. Then the parasitic larvae take their places side by side across the under side of the caterpillar skin, fasten it for nearly its whole length to the leaf, spin a little more loose silk of yellowish white color, and transform to pupae. This silken web does not form a series of cocoons since it is so loose that the black pupae can plainly be seen between its strands. The caterpillar skin protects these pupae just as a roof would do. After first transforming, the pupa is dark yellow, but soon becomes black, especially on the head and abdomen. In from three to eight days the adult parasite emerges.

Just think what a speedy development this means!--two days for the egg stage, three days for the larval stage, and three days for the pupal stage--an entire generation in eight days. Even the prolific and rapid-breeding house fly cannot beat this. It is altogether the shortest development of any Hymenopterous parasite that has been studied, and it is due to this great rapidity of development of this parasite, together with the abundance of certain other parasites, that the famous cotton caterpillar of the South, an insect which used to damage the cotton crop annually to the amount of fifteen millions of dollars, is periodical in its attacks and while very abundant some years is very scarce in other years. During the summer when Mr. Schwarz studied the species in Central Alabama he found that there was an almost complete destruction of the caterpillars in the early part of October, and that this destruction was principally due to this parasite.