Structure and Function of Aphid Cornicles1

Abstract

The cornicle area of 18 species of aphids was studied histologically. In Cinara spp. and Longistigma caryae (Harris) (Lachnini) a multicellular sac, the wall of which is a layer of flat cells with compressed nuclei, extends into the cornicle. The sac is surrounded by vacuolated fat cells, and a mass of lipid is found in the sac. In Monellia costalis (Fitch) (Callaphidini) apparently an epidermal syncytial glandular sac extends into the very small cornicle. Stegophylla sp., also in this tribe, has a structure similar to that of the Lachnini. In the Aphidini ( Aphis, Urolcucon, Macrosiphum, Cachryphora, Hyalopterus, Amphorophora ) large fat cells collect in the cornicle and subcornicle area, some with a central nucleus, and many fat vacuoles, others with a large central vacuole and a peripheral nucleus in a ring of vesicular cytoplasm. In Neophyllaphis podocarpi Takahashi (Thelaxini) large granular cells are situated near the cornicle valve, with many smaller cells with central nuclei in the subcornicle area. The presence of lipid in the vacuoles and cornicle sacs was demonstrated by staining frozen sections with Sudan Black B. All species possessed a muscle originating on the venter below the cornicle and inserting on the cornicle valve. Its position is such that in addition to opening the valve it may elevate the long cornicle of some species. A dorsoventral muscle immediately anterior to the cornicle but not attached to it also may help to elevate the cornicle, but it appears to be used in association with other dorsoventral muscles in compressing the body contents, which causes emission of the droplet. The droplets of cornicle liquid consist of haemolymph, haemocytes, and lipid from dissociated fat cells. The lipid appears to be triglyceride. Aphids fed various concentrations of sucrose in water produce varying amounts of cornicle lipid, depending upon the sugar concentration in the diet. The primary function of cornicle droplet emission in aphids appears to be that of removal of excess lipid metabolized from a diet high in carbohydrates.