Some Aspects of the Metamorphosis of the Alimentary System in the Wasp, Vespa vulgaris (Hymenoptera)

This paper describes the morphological changes of the gut of Vespa during metamorphosis, and also the histological changes which occur in the mid‐gut epithelium. The first histological changes are observed in the late larval stage (pre‐pupa), when phenomena of disputed significance occur—the formation of globules and their apparent liberation into the gut‐lumen This is followed by rapid multiplication of the replacement‐cells. The bases of the histolyzing epithelial cells contain large vacuoles, and it is probable that these exert a mechanical force, pushing the epithelium away from the basement‐membrane. This process of casting is probably aided also by chemical disintegration of the cell‐bases and basement‐membrane, by the force exerted by the rapidly growing replacement‐cells, and by severe muscular peristalsis of the intestinal muscle‐layers. The rapid proliferation of the replacement‐cells leads to the formation of the new epithelium. This is at first irregularly disposed and thrown into folds which disappear as the diameter of the gut increases. The cast epithelium lies in the gut‐lumen, and is removed by an undetermined method, probably a combination of “auto‐digestion” and by enzymes liberated from the new cell‐layer. Phagocytosis plays no part in the process. The early epithelium shows certain characteristic features, particularly a refringent edge, which splits away by delamination and gives rise in this manner to the peritrophic membrane which thus arises during the pupal period. There is only one pupal epithelium. The adult morphology is attained before the imago emerges, but the cells of the epithelium are small and “cuboid.” The final histological characters (such as greater size, perinuclear space, etc.), are not at first present, and probably do not appear until after the first meal. A discussion of the time‐relations of these various processes and consideration of the results of other investigations leads to the theory that metamorphosis is started by a single original stimulus, and though some amount of progress may follow from this, its completion appears to depend upon the serial development of other succeeding stimuli each responsible for some particular phase.