ABSTRACT
Although species of some nematode genera, such as Panagrellus, Trichinella and Turbatrix, are oviviparous, where the larvae hatch within the female uterus and subsequently emerge, these are exceptions and the majority of larvae hatch from eggs which are laid by the adult female. Each egg contains a single larva and, despite the vast difference in size between adults of various species of nematodes, the majority of eggs are of similar size and morphology. In many species, it is the first-stage larva which hatches but in most plant-parasitic nematodes the larva moults within the egg and the resulting second-stage larva hatches. ('Juvenile' is the term of choice by Plant Nematologists; however, to conform with usage in this book, rather than personal preference, the term 'larva' will be used here.) In some animal-parasitic species, there is a further moult in the egg and it is the third-stage larva which hatches. Hatched larvae are very vulnerable to environmental stresses and, in plant endoparasitic species, they are viable in the soil for only a short period, usually days or weeks. For example, larvae of Globodera rostochiensis and G. pallida can survive for less than two weeks without feeding (Robinson, Atkinson and Perry, 1987a). In many species the egg is the main survival stage of the life cycle and, in species such as G. rostochiensis and Ascaris suum, the unhatched larva can remain viable for many years. The eggshell affords protection to the enclosed larva and hatch only occurs when environmental conditions are favourable. Physiological adaptations which enhance survival, such as quiescence and diapause, are frequently associated with the unhatched larva (Perry, 1989; Wharton, this volume).
