True Flies — Part 1

(Order Diptera.)

All the true flies, that is, those insects which are called flies and have but two wings, belong to the order Diptera. They are the only insects which possess but two wings, with the exception of the males of the scale insects, and a very few May flies (genera Cloeon and Caenis). Some insects in other orders have one pair of wings so greatly aborted that they appear two-winged as in the genus Psectra, one of the Lacewing flies. The wings are membranous and usually transparent and bear no scales, except in the mosquito family. The hind wings are represented only by two knobbed projections called halteres, or poisers. The metamorphosis is very complete, the larvae being always footless and usually apparently headless maggots and the pupae either somewhat resembling those of butterflies and moths, with comparatively free legs and wings, or they are enclosed in the larval skin. Their mouth-parts are formed for sucking. The true flies comprise an enormous number of species. The most numerous of all of the orders of insects are the Coleoptera, or beetles, the Hymenoptera, which we have just discussed, and the Diptera, and for superiority in point of numbers the precedence must probably be given to the Diptera. About forty thousand species are known and it is estimated that the number yet to be described will bring this number fully up to three hundred and fifty thousand, against three hundred thousand which we have estimated for the Hymenoptera. Not only have the true flies a superiority in point of numbers, but entomologists are concluding that they probably stand at the head of the insect system in point of evolution, that is to say, they are the most highly specialized of insects. While they do not possess the apparent specialization in the way of intelligence and in other respects seen with the bees, wasps and ants, the very completeness of their transformations and the highly specialized or ganization of the adults of several families support this view.

The order is not a popular one among entomologists and collectors. Aside from the fact that observations upon their life history are by no means as interesting as some of those which we mentioned in the preceding order, they have none of the beauty which attracts students and collectors to butterflies and moths and they have not the definiteness of structure characteristic of the beetles and they are much more difficult to preserve in collections in perfect condition. The hard-bodied, easily collected, and readily pinned beetles seem much more attractive. But the Diptera in many respects possess a peculiar interest and their study is of enormous importance from many points of view. Even in point of beauty, many of the families possess species of striking color and graceful shape; and, everywhere abundant as they are, they are easy objects to collect. It is true that with some of the delicate species, especially the mosquitoes and crane flies, it is almost impossible to preserve specimens in good condition. Still, with many of the groups they keep well when simply killed and pinned and preserve their colors much better than do the dragon flies, for example.

Very many species, and in fact entire groups, are harmful to man through damage to growing crops and to livestock. One of the most famous crop enemies in the world, the so-called Hessian fly, is a dipterous insect, and most of the insect parasites of livestock belong to this order. As late as 1884, Dr. S. W. Williston, then of Yale University, an authority upon this order, wrote: "As a whole, the order is a beneficial one to the human economy. While we may resent the troublesome mosquito's and the impertinent house-fly's molestations, and while the black fly and the horse-fly may cause the death of many horses and cattle, yet the larger number are purely parasitic in their habits, either in the larval or adult states, upon other and usually injurious insects. Many others, too, act as beneficial scavengers of unwholesome matters, which would otherwise often bring disease and death." Since Williston wrote these lines, a whole class of baneful work accomplished by flies has been discovered. That is their agency in the spread of disease.