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Tachina Flies — Part 2

An interesting point connected with the life of these flies is brought out when we compare them with the parasitic Hymenoptera, the ichneumon flies and the chalcis flies. In the latter case we are struck by the extremely definite relation between the kind of parasite and the kind of host. The parasites of a particular genus will attack perhaps only insects of a certain family and it is a very definite rule that parasites of a given subfamily will attack only insects of a certain order. With the tachina flies, however, it is quite different. The same species of fly will lay her eggs not only upon insects of several different families but upon insects of tow or even three different orders. This would seem to me to indicate that the parasitic mode of life in the tachina flies is one of comparatively recent acquirement and that sufficient time has not elapsed since they began to take on this habit for so great a differentiation, so great a co-relation between the host relation and the structure of the insects, to grow up. The ancestors of the tachina flies were probably flesh-flies and the parasitic mode of life has come from a gradual change from feeding on dead insects to feeding on live ones.

Coquillett has pointed out that in their instincts these flies appear to be much stupider than the ichneumon flies. The latter, for example, seem to know by a touch of their antennae whether or not an insect has already been stung by some other parasite and they only in very rare instances insert an egg in the body of an insect that already contains an egg or larva of another parasite. Moreover, the ichneumon flies seem to grade the number of eggs which they lay in a certain insect to the number of larvae which can successfully grow within it. But the tachina fly will attach to a caterpillar three or four times as many eggs as the number of larvae the caterpillar can maintain. Thus many tachinid larvae perish for want of food while some, which are barely able to exist, produce flies which are dwarfed in size, so that some adult flies are only one-third as large as others.

As above stated, the group is a very large one and fortunately it has been admirably monographed by Mr. D. W. Coquillett, who has also brought together from the records of the U. S. Department of Agriculture an interesting table of these flies in relation to the insects upon which they have been parasitic.