Insect guide > Flies > Thick Head Flies

Thick Head Flies

(Family Conopidae.)
The flies of this group are rather closely related to the syrphus flies. They may be called, after Comstock, "the thick­head flies", because their heads are large and conspicuous. The flies themselves are rather large, but are generally slender and the abdomen is stalked, like those of some wasps. The wings are usually dark and the insects themselves are dark­colored, but some have yellow bands on the abdomen.

Those which belong to the genus Myopa are stouter and have hairy legs, almost like those of a robber-fly. The big-head flies are found upon flowers with the syrphus flies and their larvae are parasitic, chiefly upon bumblebees and wasps, but they have also been found, according to Williston, in the bodies of grasshoppers.

The larvae of these flies live in the bodies of the full-grown wasps and bees. It has been supposed that the flies enter the bees' nests and place their eggs on the larvae or pupae, but the adult flies always issue from the adult bees or wasps, having occupied the interior of the abdomen.

When full-grown they frequently completely fill the abdomen. Williston has seen a Conops following a bumblebee and repeatedly flying against it and thinks that the eggs are deposited upon the body of the bee and that after hatching the larvae bore into the abdominal cavity. In one instance a big-head fly was reared from the body of a bumblebee several months after the latter had been killed and pinned in a collection.

There is a peculiar genus in this family, Stylogaster, in which the female has an ovipositor which is longer than the entire body. Rather more than thirty species of thick­head flies, distributed in seven genera, are known to occur in the United States.