The function and ecological significance of the pharyngeal ridges occurring in the larvae of some cyclorrhaphous Diptera

Abstract

The anatomy and function of the pharyngeal ridges, which occur in the larvae of saprophagous Cyclorrhapha but are generally absent in biontophagous forms, were investigated.The ridges extend throughout the length of the ventral aspect of the pharynx and lie in an evagination of its floor. They enclose channels which narrow and disappear posteriorly but are open anteriorly. Each ridge consists of a longitudinal plate projecting dorsally from the floor of the pharynx, the bifurcated upper edge of the plate bears, one on each side, a row of lateral lamellae, each about 0·3 μm thick. They are set vertically at regular intervals of just less than 1 μm. The lateral lamellae of adjacent ridges are closely apposed and completely roof over the channels between the ridges. Anteriorly the upper arms of the ridges are fused together.The lamellae form a sieve which retains particles of 0·6 μm and above in size as the relaxation of the pharyngeal dilator muscles allows the roof to descend so forcing liquid downward between the lamellae. InCalliphora vicina(Syn.C. erythrocephala) the sieve concentrates a suspension of 2700 particles of 4–7 μm per mm³between five and six times. The natural food consists largely of bacteria.Fungus spores and pollen grains of selected sizes were used to demonstrate the movements of particulate material in the pharynx and to define the upper size limit of the particles ingested.Swallowing is effected by simultaneous relaxation of the anterior dilator and transverse muscles of the pharynx, with contraction of the posterior dilator muscles; the transverse muscles are otherwise maintained in a state of contraction closing the posterior exit from the pharynx.Starved third-instar larvae ofC. vicinacan ingest 50% of their body weight in 6 min.The larvae of the Cyclorrhapha which possess pharyngeal ridges are essentially particle feeders. The ridges form a sieve by means of which food is concentrated and the amount of non-nutritious liquid ingested is reduced. The lack of pharyngeal ridges in larvae feeding on living material is correlated with the constant and high nutritive value of both the solid and the liquid fractions of the food.I should like to thank Dr P. Tate for his help and encouragement during the course of this work which was carried out during the tenure of a Postgraduate Studentship awarded by the Agricultural Research Council. Thanks are also due to the staff of the Plant Pathology subdepartment of the Botany School, Cambridge, for their help in obtaining fungus spores of suitable sizes.