The Structure and Development of the Larval Cuticle of Diataraxia oleracea (Lepidoptera)

Abstract

The main layers comprising the soft cuticle of the Diataraxia larva have been denned and a study made of their formation during development of the 5th instar larval cuticle. In soft cuticle the epicuticle is thrown into minute tubercles and consists of three layers--the cuticulin layer, the wax layer, and the cement layer. The cement layer lies outside or partly embedded in the wax layer, but it is either absent or extremely thin over the tips of the tubercles. A polyphenol layer is absent and it is suggested that this layer is present only in hard cuticle. Beneath the epicuticle is a thin, lightly tanned exocuticle approximately 0.5µ thick. The 7-10µ thick outer endocuticle is perforated by well-defined pore canals and consistsof lamellae containing chitin fibres varying from 100 to 2,000 Å in diameter. Pore canals are absent in the inner endocuticle which when fully developed may be more than 50µ in thickness. At 25°C, moulting of the 4th-5th instar takes less than 48 hours. The cuticulin layeris laid down about 12-24 hours before ecdysis. At about 6-12 hours before ecdysis the exocuticle is formed beneath the cuticulin layer in the outer part of the developing outerendocuticle. Formation of the wax layer begins about 1 hour before ecdysis, while the cement layer is secreted only a few minutes prior to ecdysis. The inner endocuticle is laid down during the 5 days after ecdysis. Prior to ecdysis the pore canals connect the hypodermis to the epicuticle and open into theexuvial cavity at the tips of the epicuticular tubercles. Probably the pore canal lumen is responsible for conduction of the moulting fluid. Each pore canal contains a well-defined strand of material which occupies but a small part of the pore canal lumen. Thfc pore canal strands, conduct protein and dihydroxyphenol from thehypodermis to form the exocuticle. Exocuticle formation is complete by about 1½ days after ecdysis. At this stage the pore canal strands become chitinized and sclerotized and the pore canals are cut off from the hypodermis by the developing inner endocuticle. The pore canalsare not responsible for transport of regenerative waxes from the hypodermis after abrasion of the epicuticle. In areas of hard cuticle there is a relatively thick and heavily tanned exocuticle which continues to develop during the life of the instar. The pore canals maintain contact with the hypodermis throughout the instar and probably remain functional as a conducting system for transport of exocuticular materials. A study of Verson's glands suggested that the large gland cell secretes a lipoprotein whichis discharged on the surface of the epicuticle to form the cement layer. The intercalary cell secretes a phenol which is responsible for the tanning of a plug that blocks the opening of the gland duct after discharge.