THE ORIGIN, DISTRIBUTION AND FATE OF THE MOLTING FLUID PROTEINS OF THE CECROPIA SILKWORM

Abstract

1. Molting in insects is always accompanied by the production of a molting fluid which fills the exuvial space between the new and the old cuticle and digests the inner layers of the old cuticle. In Hyalophora cecropia, molting fluid is secreted at the outset of adult development and persists until two days before eclosion, whereupon it is absorbed. 2. The present report examines the protein composition of the molting fluid of Cecropia, the origin of the molting fluid proteins, the relation of these proteins to blood proteins and the exchange of macromolecules between the molting fluid and the blood. It also examines the sites of absorption of molting fluid. 3. Disc electrophoresis on acrylamide gels reveals that the molting fluid of Cecropia contains about fifteen protein bands which can be resolved at pH 8.6. Some of these protein bands are detected in the molting fluid at all stages, whereas others appear only at specific times. About ten of the bands are peculiar to molting fluid and are not detected in the blood. About five bands are detectable in both blood and molting fluid, but none of these common bands appears to be a major component of the molting fluid, and only one is a major blood protein. In contrast, the epidermis contains most of the major protein bands found in molting fluid but lacks all but one of the major protein bands present in the blood. 4. Immunological analysis reveals that blood and molting fluid share five antigens. At least four of these common antigens also occur in the epidermis which appears to secrete these antigens into both the molting fluid and the blood. 5. Native and foreign proteins do not penetrate from the exuvial space into the blood or vice versa. Apparently the epidermis and cuticle act as a barrier to the exchange of most macromolecules between the blood and molting fluid. The exuvial space is clearly a separate fluid compartment. 6. In addition the exuvial space itself is compartmentalized and the fluids in the compartments do not admix several days before eclosion. 7. Absorption of molting fluid during the final two days of adult development occurs most readily through particular regions of the integument. In the abdomen the principal sites of absorption are pits which represent the points through which tonofibrils make attachment to the old cuticle. Two days before ecdysis, the attachments between the tonofibrils and the pupal cuticle rupture, exposing the points of attachment on the new cuticle. It is through these exposed surfaces that much of the molting fluid is absorbed. Molting fluid is also absorbed in the head and thorax through various flexible membranes at the bases of the appendages.