A Study of the Embryonic development of the Gooseberry Sawfly, Pteronidea ribesii

Abstract

ABSTRACT The following points in the descriptive embryology of the sawfly appear to be important especially for comparison with the higher Hymenoptera, e.g. the honey bee. The earlier stages of development (formation of the blastoderm, germinal layers, &c.) take relatively more time in the honey bee than in the sawfly, while the later stages of development (organogenesis) take more time in the sawfly. Only two well-defined germ layers are formed—the outer (ectoderm) and the inner (mesoderm). Cells proliferate from the mid-ventral area of the germ band and spread out to form the inner layer in the sawfly, whereas in the honey bee it forms as a middle plate. Localized proliferations at the anterior and the posterior end of the germ band form two cell-clumps; from these differentiate the midgut epithelium, mesoderm of the procephalic region of the head, muscle layer of the proctodaeum and stomodaeum, and also the germ cells. These are here called the mesendoderm rudiments. The coelom sacs are not well defined from each other but form a more or less continuous tube as in the honey bee. Nucleoli appear in the cells and the cytoplasm becomes basiphil immediately after the formation of the inner layer. Changes in the basiphilia of cells during development are described. The proctodaeum first appears as an invagination in the upturned (dorsal) part of the germ band, which is all used up in its formation, and the edges of the ventral germ band rise up to complete the dorsal closure. The Malpighian tubules develop from the blind end of the proctodaeum and are regarded here as ectodermal; in the honey bee they develop before the proctodaeum is formed. In the advanced embryo the midgut cells have big vacuoles in them and throw out pseudopodia into the lumen of the gut, and the cells of the salivary gland acquire large lobed nuclei. Agreement is expressed with Snodgrass’s view (1938) that the head is composed of four segments and a procephalic region. The labrum has a paired origin (in the honey bee it is unpaired in origin); and as in the honey bee the premandibular segment is not developed. The rest of the body is composed of three thoracic and ten abdominal segments. Paired appendages appear on the thoracic and the second to seventh and tenth abdominal segments (in the honey bee abdominal appendages are not formed). The germ cells arise from the posterior mesendodermal rudiment. In the honey bee they appear to be derived from the genital ridges. There are no blastokinetic movements as in the honey bee. The serosa persists till the hatching of the embryo and secretes a cuticle at an early age. The amnion is not well developed and is transitory; in the honey bee only one embryonic envelope forms.