The morphological basis of embryonic movements in the light brown apple moth, Epiphyas postvittana (Walk) (Lepidoptera : Tortricidae)

Abstract

A description is given of the morphological basis of the embryonic movements revealed in E. postvittana by time-lapse cinematography. The blastoderm gives rise to a broad germ band and serosal rudiment. The serosa spreads over the germ band, followed by the amnion. The germ band becomes cup-shaped, elongates, and turns on its side in the flattened egg before gastrulation begins. Only a small amount of the yolk of the egg is enclosed in the germ band. The remainder fills the amnioserosal space. As elongation continues, mainly through growth in the length of the abdomen, and segmentation takes place, the germ band becomes spirally coiled and convoluted within the flattened egg space. At the completion of elongation, the nervous system is well developed, segmental myoblasts are present, and the tubular stomodaeum and proctodaeum are linked by paired midgut strands. Shortening and dorsal closure eliminate the spiralling and convolution of the germ band and result in a tubular embryo with a large ganglionated nerve cord and large stomodaeum and proctodaeum, but with musculature still at the myoblast stage and midgut strands unchanged. Paired sheets of cardioblasts extending from the body wall to the midgut strands divide the ventral haemocoele from the dorsal haemocoele in the middle region of the body. A mesodermal sac covers the inner end of the stomodaeum and opens in the dorsal haemocoele. The tubular embryo now elongates, doubling its volume, reverses its position in the egg, and tucks the tail in beside the head. During elongation, the segmental myoblasts differentiate as muscle fibres. Towards the end of elongation and reversal, the midgut strands give rise to the midgut tube and the cardioblast sheets to the middorsal heart. When elongation and reversal are complete, the stomodaeal mesodermal sac is transformed into proventricular mesoderm. After further differentiation of striated muscle and secretion of the cuticle, the embryo ingests the yolk in the surrounding amnioserosal space and digests it before hatching takes place. Comparison of morphological structure with the movements displayed in the time-lapse record show that all elongation, rotation, spiralling, and convolution of the embryo before the onset of shortening is due to growth by cell proliferation and to accommodation of this growth within a fixed egg space of specific shape. In contrast, controlled muscular activity plays a major role in shortening and dorsal closure, in elongation and reversal of position, and in ingestion of excluded yolk. During shortening and dorsal closure, slow, generalized, possibly myogenic contractions are produced by the somatic myoblasts and appear to be responsible for the shape changes observed. During elongation and reversal, contractions are confined to the stomodaeal myoblast layer and result in rhythmic filling and emptying of the terminal stomodaeal sac. By this activity, fluid is transferred from the excluded yolk to the embryo and pumped along the dorsal haemocoelic channel, bringing about controlled elongation and reversal. In view of the coordination exhibited, and of the precocious development of the nervous system, it seems likely that this activity is neurogenic, even though the contractile units are still myoblasts. The final movement of reversal, tucking in of the tail, is due to somatic musculature, now differentiated as muscle fibres and showing twitches of probably neurogenic causation. Ingestion of excluded yolk is a precocious feeding activity, brought about by coordinated action of the larval feeding apparatus, and occurs only when the appropriate musculature has become differentiated as striated muscle. Although the morphogenetic movements of E. postvittana show modifications associated with the flattened form of the egg, they and their structural basis are, on comparative evidence, common to all lepidopteran embryos.