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Insect guide > Flies > Tachina Flies
This is a large and important group of flies, the members of which have no common name except that of "tachina flies", by which they are generally known to everyone who has studied insects, even if his studies have not carried him into the order Diptera, for all or nearly all of these creatures are parasitic upon other insects and a person engaged in rearing caterpillars will often have his ultimate design frustrated through the work of the larva of these flies.
As a rule they are medium sized or rather large flies of a gray tint, rather unattractive in appearance and perhaps resembling the common house-fly as a rule. In fact, one may say that they belong to the house-fly type. The gray body color is frequently striped with dark or lighter stripes and there are some marked exceptions to this general colorational scheme as, for example, in the dark-winged, sometimes reddish-bodied Trichopodas, the slender Xanthomelaenas and Hemydas,the redbodied Echinomyias and those species of the genus Archytas which look like blue-bottle flies.
In general the wings are clear, the bodies are somewhat bristly and the insects fly with a buzzing sound which is not very pronounced but like that of a house-fly. They are active and fly usually in the sunshine, being much less in evidence on cloudy days. In their relations with man the tachina flies are beneficial the most beneficial group of Diptera, with the possible exception of the syrphus flies. With the tachina flies, however, the habits are much more uniform and the larvae feed only upon living insects. By far the favorite hosts of these flies are the leaf-eating caterpillars and the numbers which are destroyed in a single season by these parasites is quite beyond computation. I have seen vast armies of the army-worm, comprising unquestionably millions of individuals, and have been unable to find a single specimen which did not bear the characteristic eggs of a tachina fly. These flies were present in such numbers that their buzzing, as they flew over the army of caterpillars, could be heard at some distance and the farmers were unnecessarily alarmed since they conceived the idea that the flies were the parents of the caterpillars and were flying everywhere and laying their eggs in the grass and wheat. As a matter of fact, one great outbreak of the army-worm in northern Alabama in the early summer was completely frustrated by the tachina flies, aided by a few other parasites and predatory insects.
They also attack grasshoppers, bugs and beetles, saw-flies and saw-fly larva and bumblebees and wasps.
Their eggs are usually white in color, oval in shape and are stuck by some sort of a gummy substance to the surface of the insect on which the future larvae are to feed. The small white eggs are frequently seen sticking to the back of some unfortunate caterpillar. From the under side of each egg there hatches a little maggot which bores its way through the skin of the host insect and penetrates into its body, where it lives, nourishing itself upon the fatty matter and lymph, until it reaches full growth, usually if not always destroying before it emerges some vital organ so as to cause the death of the host insect. It almost invariably issues when full grown from the body of the insect attacked and transforms at or near the surface of the ground within the last larval skin, which hardens into a brown, oval puparium.
Breeding is rapid and there may be several generations each summer. In issuing from the puparium the fly breaks away the entire end of the hardened larval skin. It used to be thought that every caterpillar upon which these eggs were placed was doomed, but it often happens that the mother tachina fly, with a faulty instinct, places her eggs upon the back of a caterpillar which is about to cast its skin and in such instances it frequently moults before the eggs have had time to hatch, so that when they do hatch the young larvae find themselves out in the cold world instead of revelling in the interior of a well-fed caterpillar. So frequently does this occur that a very large proportion of tachina eggs are wasted by the mother flies. The observations of Fernald and his assistants in their work upon the gipsy moth in Massachusetts have given us exact figures in regard to this matter. In one instance 250 caterpillars, each bearing eggs of tachina flies, were fed and carried through their transformation without the appearance of a single adult fly. In another instance 235 caterpillars, each bearing from one to thirtythree eggs, were fed and watched and from these, 226 moths were reared and only nine were killed by the tachinas.
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 two 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.