Summary 1. Life‐table data for 14 species of Lepidoptera are analysed by the k‐factor technique of Varley & Gradwell (1960). Two factors are shown to be of particular importance in determining fluctuations in abundance from one generation to the next. These key factors are predators and the failure of females to lay their full complement of eggs. 2. Data from 24 studies are reviewed to identify any density‐dependent factors that would be capable of regulating the populations about an equilibrium density. In eight studies no density‐dependent relationships could be identified, and in a further 13 the only density dependence demonstrated was due to intraspecific competition for resources. It is argued that competition is incapable of regulating populations at low density. In the other three studies, natural enemies are thought to be acting in a density‐dependent manner, but their ability to regulate the populations is questioned. 3. The frequency of over‐population and of extinction is reviewed and both appear to occur frequently in Lepidoptera. This, coupled with the failure of most studies to demonstrate the existence of density‐dependent processes capable of regulating populations, leads the author to reject the model of regulation about an equilibrium density in favour of a model of population limitation by a ceiling set by resources. 4. Fluctuations in resource availability may be important in determining variations in the abundance of many Lepidoptera, but at present too few ecologists have quantified the carrying capacity of habitats occupied by the species they study to generalize about this.

Summary 1. The eggs of Acrididae are well adapted to survive adverse weather and mortalities in field populations from this cause are low in most years. The eggs of tropical species are generally only laid where conditions are suitable for growth, while an egg diapause is present in many temperate species, during which time eggs are resistant to extremes of both temperature and moisture. Only prolonged flooding or drought can cause high mortalities and then only at certain stages in the egg's development. All eggs must absorb water to complete their development. 2. Nymphal and adult populations are far more dependent on the right weather conditions. Prolonged wet, cloudy weather greatly increases the mortality of both. Newly hatched nymphs are particularly susceptible. The way in which wet weather affects survival is little understood, but since the rate of feeding of Acrididae is so dependent on warm sunny conditions, it is possible that inclement weather may lead to starvation. 3. Fecundity is also greatly reduced by cool wet weather and the level indicated by laboratory studies is rarely reached in the field. 4. While both survival and fecundity are enhanced by hot, dry conditions, both are dependent on the presence of green food, which can be destroyed by excessive drought. 5. These opposing requirements of Acrididae make their numbers extremely sensitive to variations in weather and cause large fluctuations in their populations. 6. The eggs of most acridids are laid in bare ground, while vegetation is required by nymphs and adults for food and shelter. These requirements are again opposing. High‐density populations are often found in natural and man‐made ecotones, in which vegetation and bare ground occur in a mosaic pattern, thus providing both in their maximum availability. Ecotones are often unstable and the extent of the two vegetation components (bare ground and plant cover) is greatly influenced by weather. Variations in weather may then further affect acridid populations through effects on the carrying capacity of the habitat. 7. In locusts, gregarious behaviour and swarming are brought about by increases in population density, and variations in the relative extent of the components of the vegetation mosaic may cause crowding of one or other of the stages in the life cycle. 8. The influence of weather on acridid populations is so marked that correlations have been found for a large number of species between population size and particular weather conditions. The most important factor determining numbers varies between different species occupying different habitats and geographical regions. 9. Acridid populations are attacked by a large array of natural enemies–diseases, parasites and predators. Little quantitative work has been done on their effects, but biological considerations show that it is extremely unlikely that any are sufficiently density‐dependent to act in the way suggested by Nicholson (1933, 1954, 1958). It is likely that they do little more than damp the peaks in population fluctuations. ...

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Numbers of the orange-tip butterfly were recorded on a permanent transect in Monks Wood National Nature Reserve between 1982 and 1993, together with numbers of its only larval food plant in the wood, Cardamine pratensis. Females of the butterfly lay their eggs on the newly opened flower heads of Cardamine and the larvae feed on the developing seed pods. They are extremely selective in their choice of plant for egg laying, choosing mainly large flower heads growing in open sunny locations. Larval survival is greatly reduced if the flower head is more than 8 days old at the time of egg laying, because the seed pods become too tough for feeding. Earlier studies showed that only one larva can survive on a flower head, because larvae are cannibalistic, but females tend to avoid plants carrying more than one egg, in response to an oviposition-deterring pheromone laid down at the time of egg laying. The numbers of flower heads of Cardamine fluctuate enormously between years, and the numbers of eggs follow these closely. There is a strong correlation between number of eggs laid and the availability of suitable flower heads, and this correlation was shown to be real and causal by the experimental provision of extra flower heads, which resulted in more eggs, laid over an extended period of oviposition. The main causes of mortality of the butterfly's young stages were egg infertility, cannibalism, predation, starvation, the grazing of flower heads by deer, and possibly the failure of the newly hatched larvae to feed adequately. Adult numbers were only weakly correlated with the numbers of young stages in the wood, and there appears to be a considerable amount of movement by the butterfly through the wood and surrounding farmland.