Home › Volumes › Insects › Syrphus Flies

Syrphus Flies — Part 1

(Family Syrphidae.)

The syrphus flies (for they have no other vernacular name), comprise many of the most interesting of the dipterous insects. It is a very large family and more than three hundred species are known to occur in the United States. As a rule they are rather stout-bodied flies, varying greatly in color. Some are metallic greenish as in Microdon and Psilota, while others are banded with yellow in different ways. As a rule the abdomens are rather broad and are rather apt to be flat, but in some, as in Baccha and its allies, the abdomen is slender. The syrphus flies are flower flies par excellence. They fly in the sunlight and are easily taken by sweeping flowering plants. Almost all types of bees and wasps are mimicked by them and so generally does this occur throughout the family that syrphus flies form the most striking instances of protective mimicry. There are syrphus flies like honey bees, bumblebees, social wasps and solitary wasps of several kinds. They are rarely to be seen except in the middle of sunshiny days, some of them resting occasionally upon leaves, but more frequently they are to be found about flowers, while others seem to be almost constantly upon the wing.

The habits of the syrphus flies in their early stages vary greatly. Very many of them in the larval state feed upon plant lice and other small, soft-bodied insects. I have seen currant bushes upon which there was hardly a leaf which did not support a thriving colony of plant lice and which had not become curled and distorted in consequence, and yet within a very few days, while the distortion of the leaves remained, not a plant louse was to be found but under each leaf instead of the flourishing group of lice was a fat, full-grown syrphus larva which had destroyed all of the previous inhabitants and was now ready to transform.

These larvae do not have a distinctly differentiated head. The external mouth-parts are either entirely lacking or there are two or four usually dark-colored hooklets. The body is smooth and usually glistening. When ready to transform, the last skin of the larva contracts and hardens and assumes an oval shape and a darker color and the pupa is formed within it. When the fly is ready to emerge, the front end of the old skin is pushed out and the perfect fly escapes. The hooklets on the mouth of the larva occur with those syrphus fly larvae which feed upon other insects and they serve to grasp and pierce the body of the prey. Those larvae which do not have such hooklets have other habits. They may feed in the decaying wood of old trees or logs; they may live in manure or soft mud impregnated with decaying vegetable matter; they may be found in the sap of trees or in the stems of certain tender plants or in fungi. Still others are common in ants' nests and others again are guests in the nests of bumblebees. With such variable habits there must necessarily be considerable variation in structure and as a result of this mode of life those forms which live in soft mud or manure, which may be almost a liquid, and some of those which live in very damp, decaying wood, have long slender projections at the end of the body bearing spiracles or breathing holes at the tip, so that when the body of the larva is buried in the semi-liquid mass in which it is feeding this long tail still protrudes to the air, enabling it to breathe in comfort. These larvae have been termed "rat-tailed maggots" and are very curious objects which are frequently sent to entomologists for name.

Those which live in ants' nests belong to the genus Microdon and are among the strangest insect larvae known. They do not look like insect larvae, and, in fact, resemble certain land shells. Curiously enough, they have been described and named as species of mollusks. In fact, certain insects have given shell students a good deal of trouble, for, as will be shown when we study the caddis-flies, certain cases constructed by these insects have also been described as shells. The Microdon larva does not appear to be jointed and the upper surface of its body is covered with a network of bristles which usually hold a coating of dirt. There is no trace of any head and the sides of the body project, forming a sort of fringe around the edge. The soft pupa is formed within the last larval skin and does not alter its shape. Just what these larvae do in the ants' nests is not well understood. Perhaps the ants gain some secretion from them. As a matter of fact they are sometimes found elsewhere. The adult flies of this genus are usually dull-colored, are slow fliers and are found on the borders of low-growing woods. The flies have been seen laying their eggs in the ant hills and the ants have been seen to drive them away but they returned again, undiscouraged by the impolite rebuff. This fact would not seem to indicate that the larvae are of any service to the ants.