Development of the egg if the stick insect Didyuris violescens with particular reference to diapause

Abstract

The paper provides a comprehensive account of the development of the egg of the phasmatid D, violescens, a serious defoliator of Euca~ptus forests in the highlands of south-eastern Australia. The occurrence of parthenogenesis and the incidence of the two diapauses are compared in several spatially isolated populations. The observed differences are consistent with the hypothesis that D. violescens has tended to lose its capability for parthenogenesis while developing its competence for diapause as the species extended its range from subtropical to temperate regions. This hypothesis receives strong support from a recent discovery that there are at least seven genetically distinct races of D. violescens in south-eastern Australia. The influence of constant temperatures on diapause development and embryogenesis is described for several geographic populations. Irrespective of the origin of the populations, the optimum temperature for diapause development is about 2"C, and embryogenesis proceeds more rapidly as temperatures increase from a lower threshold of about 7" to 27°C. Both processes can occur simultaneously at temperatures between the two optima. The following section shows the total effect of genotype and environment on the pattern of development and hatching in the field. In the southern outbreak areas the populations are of two kinds: highland populations, in which the occurrence of the two diapauses ensures that the eggs overwinter at least twice before hatching; and foothill populations, where the absence of the first diapause means that the eggs may overwinter either once or twice before hatching, depending on when they were laid during the summer. This partly explains how a particularly cool summer can initiate the 2-year cycling of population which is the most obvious characteristic of the outbreaks of D, violescens in south-eastern Australia.