Immature Stages of Syrphidae
The eggs of the entomophagous representatives of the Syrphidae are quite similar in form and appearance, being sub cylindrical, about 1 mm. in length, 1/4th as wide as long, and slightly arched, with a small conical micropyle. The surface bears a waxy covering which ranges in color from a glistening white to grayish‑white, with fine reticulations or longitudinal ridges, most frequently the latter.
Heiss (1938) has given an extended account of the classification of the larvae and pupae of the family, exclusive of the aquatic forms. Characters and keys are given by means of which the various genera can be distinguished. An outstanding contribution to the biology, morphology, and anatomy of the larvae of a series of aphidophagous species was presented by M. L. Bhatia (l939). The reader is referred to these two papers for a detailed discussion of the characters of the larvae and pupae, which will be described here only in very general terms.
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The three larval instars are slug‑like in general appearance, and those of the entomophagous species are very similar in form. The larvae are distinguished from those of other cyclorrhaphous families by two characters: (1) The posterior respiratory tubes of the second and third instars are contiguous and partly or completely fused. (2) Each body segment bears 12 spines, in definite positions. Prolegs are lacking whereas they occur frequently among the saprophytic species. The body usually tapers anteriorly, and the body contents arc distinctly visible through the thin integument. The anterior spiracles are relatively small, whereas the posterior spiracles of the mature larvae (Fig. 181) are large, contiguous or nearly so, and often situated at the tip of a sing1e heavy stalk. They bear three slits, which, among the different species, vary in position and direction with respect to the spiracular button. Many if not all representatives of the Syrphidae possess so‑called rectal gills, which are rarely found extruded in the entomophagous forms. Each gill consists of a pair of simple finger‑like processes joined at the base.
The body color and markings are typical for each species, the former usually being due not to pigmentation of the integument but to coloring matter contained in the fat bodies, the arrangement of which also differs among species. There is likewise a variation in color of the body fluids, which is often due to the food consumed by the larvae.
The puparium (Fig. 182) consists of the indurated exuviae of the 3rd‑instar larva and is of tear‑drop form with the posterior segments markedly narrowed and the anterior portion broadly rounded. The integumentary spines and processes of the larvae of the species that bear such ornamentation are shrunken and distorted. The prothoracic cornicle of the pupa does not protrude through the puparial wall, and the minute anterior spiracles of the larva are almost invisible. The operculum is circular and divided transversely into a dorsal and a ventral piece, which separate upon emergence of the adult.
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