Embryogenesis of the Sand Fly Phlebotomus papatasi (Diptera: Psychodidae): Cell Cleavage, Blastoderm Formation, and Gastrulation

Abstract

Serial sections of dechorionated eggs were used to study the early embryology of the sand fly Phlebotomus papatasi (Scopoli) up to 96 h. Cleavage began between oviposition and the first 12 h and continued until the cellular blastoderm was formed (60 h). Cleavage was synchronous, with most cleavage processes occurring during the first 36 h. Anterior-to-posterior cellular growth became apparent until division temporarily ceased. Cleavage nuclei were observed 1st in the anterior 1/3 of the egg and, as they developed, became more evenly distributed into the periplasm. Primary vitellophages concentrated linearly in the center of the egg started migrating posteriorly, and a few secondary vitellophages appeared at the posterior pole. Approximately 6-10 pole cells originated from the blastoderm and were formed at the posterior extremity. At 48 h, delimiting cell furrows first became evident, jutting inward from the outer periplasmic membrane between the nuclei, and continued until 60 h. Invagination of the germ band occurred 72 h after oviposition and spread bilaterally as paired ventrolateral mesodermal bands. The stomodeum and the anterior midgut invaginated simultaneously at 72-84 h. Formation and invagination of stomodeal cells preceded that of the proctodeum. The proctodeal invagination began at 84 h, enclosing the pole cells, which migrated from the posterior pole to the dorsal side of the egg. Invagination of the pole cells appeared in association with the invaginating posterior midgut proctodeal complex. The amnion and serosa became a loosely attached layer of cells clumped to form a primary dorsal organ, which underwent lysis at 84 h and was totally resorbed into the yolk by 96 h.