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Chalcis Flies


(Super-family Chalcidoidea.)
This group of parasitic Hymenoptera is probably the largest in number of species of any of the corresponding Hymenopterous groups. It is a well defined and well limited group structurally speaking and comprises undoubtedly many thousands of species. Only a small proportion of the species have as yet been described. As a rule the insects are so small that they attract no attention from the average collector and the paucity of our specific knowledge of the group possibly arises from this fact.

Almost all of the forms are truly parasitic, the exceptions being certain gall­making species belonging to the genus Isosoma and its allies and possibly the seed inhabiting species of the genus Megastigmus. Nowhere in nature is there a more marked example of the co­relation between structure and habits than occurs in this family. This co-relation descends to the relation between the parasites and their hosts so that it is possible for an experienced person on seeing a new species of Chalcis fly to tell precisely what kind of an insect it will be found to be parasitic upon.

For example, the species of the genus Copidosoma are always parasitic within naked caterpillars. Those of the genus Bothriothorax are always parasitic in small dipterous larvae. The economic importance of the group is great. They are the most effective parasites of many of our most injurious insects. For example, in a certain year in the cotton fields of Northern Florida 95 percent of the eggs from which would have hatched the voracious cotton caterpillar were killed by the minute Chalcidid parasite, Trichogramma pretiosa.

Life History of a Chalcis Fly
(Euplectrus comstockii, How.)
It goes without saying that the full life of the internal-feeding parasites of this group is very difficult and practically impossible to follow so long as they are within the body of the host insect. In our earlier consideration of the super-family Proctotrypoidea we have given some general remarks upon the development of all internal-feeding parasitic Hymenopterous larvae, and those remarks will apply in general to the Chalcis flies.

There are some of them, however, whose larvae do not feed internally. These are especially those which are parasitic upon the larvae of gall-making insects. If a gall be cut open, it is quite likely that there will be found within it the larva of a Torymus (one of the Chalcis flies) feeding externally upon the gall making larva; and there is a quite large group known as the Elachistinae which are parasitic upon caterpillars, the larvae of which feed also outside the skin of the host insect. It is one of these which has been selected for our typical life history. It was studied in the summer in the cotton fields of Alabama by Mr. E. A. Schwarz, but it has a northward spread and is parasitic upon caterpillars of certain Geometrid moths-measuring worms.

The adult parasite, which is a little black, shiny, four-winged fly and which, when seen under a strong lens has a number of strong, dark colored bristles upon its back, lays its eggs to the number of from three to fifteen in a group upon the middle of the back of the wriggling caterpillar.

Each egg is brown in color, almost black before hatching, is elongate oval, strongly convex above, and somewhat flattened beneath. The individual eggs although laid in a group are sufficiently separated from each other to allow for the development of the larvae. They hatch about two days after being laid. The delicate egg shell splits longitudinally in the middle of the back and discloses the white, grublike parasite larva, which gradually works the egg shell more and more down the sides of its body where it remains visible as a black line for some hours. As soon as the grub has freed its head from the egg shell it pierces the skin of the victim, and thereafter remains stationary with its head buried. As soon as it has fairly begun to feed, the white color changes to a bright bluish green, and the spiracles and the constrictions between the segments are readily seen.

The growth of this larva is extremly rapid. In fact, we know of no insect larva which has a more rapid development. In midsummer it reached full growth in three days from the time of hatching. In September this larval growth took four days. When full grown the parasitic grubs crowd each other, and if there are five or more of them on a caterpillar they form a semi-globular lump of very striking appearance. Usually their growth is uniform. A retardation in the development of individuals in the group results in death. When full grown they turn white and relax their hold.

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 for 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.