Peppered moth ecology

In Britain, the peppered moth is univoltine (i.e. it has one generation per year), whilst in south-eastern North America it is bivoltine (two generations per year). The Lepidopteran life cycle consists of four stages; ova (eggs), several larval instars (caterpillars), pupae and imagines (adults). During the day, the moths cryptically rest on trees, where they are predated by birds.

Contents

1 Pupae 2 Imagines 3 Imaginal diurnal resting behaviour 4 Cryptic resting behaviour 5 Mating behaviour 6 Larvae

Pupae

The pupae overwinter in the soil.

Imagines

The imagines emerge from the pupae between late May and August, the males slightly before the females (this is common and expected from sexual selection). They emerge late in the day and dry their wings before flying that night. The males fly every night of their lives in search of females, whereas the females who release pheromones to attract males only fly on the first night. Since the pheromone is carried by the wind, males tend to travel up the concentration gradient, i.e. upwind. During flight they are subject to predation by bats.

Imaginal diurnal resting behaviour

A mating pair or a lone individual will spend the day hiding from predators, particularly birds. In the case of the former, the male stays with the female to ensure paternity. The best evidence for resting positions is given by data collected by the peppered moth researcher Dr. Mike Majerus, and it is given below. These data were originally published in Howlett and Majerus (1987), and an updated version published in Majerus (1998), who concluded that the moths rest in the upper part of the trees.

Majerus continues;

Creationist critics of the peppered moth have often pointed to a statement made by Clarke et al. (1985): "... In 25 years we have only found two betularia on the tree trunks or walls adjacent to our traps, and none elsewhere". The reason now seems obvious. Few people spend their time looking for moths up in the trees. That is where peppered moths rest by day.

From their original data, Howlett and Majerus (1987) concluded that peppered moths generally rest in unexposed positions, using three main types of site. Firstly, a few inches below the a branch-trunk joint on a tree trunk where the moth is in shadow; secondly, on the underside of branches and thirdly on foliate twigs. The above data would appear to support this.

Further support for these resting positions are given from experiments watching captive moths taking up resting positions in both males (Mikkola, 1979; 1984) and females (Liebert and Brakefield, 1987).

Cryptic resting behaviour

Majerus et al (2000) have shown that peppered moths are cryptically camouflaged against their backgrounds when they rest in the boughs of trees. It is clear that in human visible wavelengths, typica are camouflaged against lichens and carbonaria against plain bark. However, birds are capable of seeing ultraviolet light that humans are can not. Using a ultraviolet sensitive video camera Majerus et al showed that typica reflect ultraviolet light in a speckled fashion and are camouflaged against crustose lichens common on branches, both in ultraviolet and human visible wavelengths. However, typica are not as well camouflaged against foliose lichens common on tree trunks; though they are camouflaged in human wavelengths, in ultraviolet wavelengths, foliose lichens do not reflect ultraviolet light.

Mating behaviour

The female releases pheromones which attract the males. The males guard the female from other males until she lays the eggs. The female lays about 2000 pale-green ovoid eggs about 1mm in length into crevices in bark with her ovipositor.

Larvae

The caterpillar (see fig 1c) is a twig mimic, varying in colour between green and brown. Late in the season it goes into the soil, where it pupates in order to spend the winter, and the life cycle continues.

Peppered moth
Ecology | Genetics | Evolution | Taxonomy | Predation experiments
Researchers: Bernard Kettlewell (The Evolution of Melanism) | Mike Majerus (Melanism: Evolution in Action) | Laurence Cook | Cyril Clarke | Bruce Grant | E.B. Ford | Philip Sheppard J.W. Tutt
Alternative theories: Craig Millar | Ted Sargent
Creationism: Jonathan Wells (Icons of Evolution) | Judith Hooper (Of Moths and Men)
References