The ultrastructure and secretory activity of the ventral prothoracic exocrine gland of the walnut caterpillar (Lepidoptera : Notodontidae)

Abstract

The ventral prothoracic exocrine gland (VPEG) of larvae of Datana integerrima is an unpaired sac arising as an ingrowth of the epidermis. The VPEG lies in the ventral body cavity, beneath the prothoracic nerve ganglion; it narrows anteriorly and opens to the outside via a transverse slit in the prosternum, anterior to the forelegs. The size of the VPEG in 5th instar larvae averages 1.0 mm long and 0.6 mm at its widest diameter. The gland is not eversible, and is held in place with large lateral tracheae. Neither associated muscles nor nerves were observed. Histologically, the VPEG is composed of a single layer of ca. 124 large, polygonal Class 1 epithelial cells surrounding a central lumen. Cells measure ca. 50 um in diameter and 35 um deep. The gland is separated from the hemolymph by a basement lamella of medium, variable thickness. The basal plasma membrane is infolded to varying degrees, creating extracytoplasmic channels. Nuclei are large and centrally located; they may be highly branched, with a single nucleolus often seen. The cuticle bordering the gland lumen is composed of a thin outer cuticulin layer, a procuticle and an inner thick, lamellated endocuticle, and bears many small papilla-like projections. The secretory activity of the VPEG is described according to the ultrastructure of cells in 7 sequential stages, from pre-molt 5th instar to prepupa. The cells are very asynchronous. All cells appeared to be of the same physiological type, with the exception of Stage 4D in which two cells types were observed. VPEG cell ultrastructure is similar to that of cells which elaborate lipids or steroids. As the cell matures, the perinuclear cytoplasm changes from a dark band rich in rough endoplasmic reticulum (RER), with many large mitochondria, to a lightened region free of RER and mitochondria and containing mostly smooth endoplasmic reticulum (SER)...