Quantitative estimation of the stage of embryonic development in the locust, Schistocerca gregaria.

An understanding of the physiological and structural development of an embryo requires a knowledge of the precise state that development has reached. Without this knowledge it is not possible to compare findings from experiments performed on different embryos or to relate them to structural changes during development. In previous work on Acridid embryos two methods have been used in estimating the stage of development. Some authors have defined embryonic stages by their age from the time of oviposition, while others have described development in a series of arbitrarily chosen stages, which are based primarily on changes in the external morphology The eggs of Schistocerca gregaria are laid in pods of 30-90. Pods were collected and kept under constant conditions in order to investigate the development of the embryonic muscle (Tyrer, 1968, 1969). During this work it was found that there was considerable variation in the differentiation of eggs from different pods of the same age, even though most of the eggs in any one pod were at the same stage. This is reflected by the length of the development period of the eggs, which at a temperature of 28 °C, ranged from 13 to 20 days with a mean of 15-25 days (Fig. 1). Similar variations have also been observed by other workers on Acridid Because of these large variations, time alone cannot be used to define the stage of development accurately , especially in the later stages where the effects of variation will be more marked. This difficulty can be avoided by defining development in a series of stages based on the changes in external morphology, but this has the disadvantage that internal changes may take place without changes in external features, and may therefore occur in the undefined period between stages. Shulov & Pener's series for S. gregaria (1963) defines the early stages in detail and it is these which are the main concern of the embryologist, interested in the origin and formation of organs. The physiologist, however, is concerned primarily with the function of