The shape, size, colour and texture of butterfly eggs varies enormously from one species to another. Some, such as those of Satyrines and Heliconiines are typically domed or barrel-shaped, adorned with between 8-30 vertical ribs, between which can be seen dozens of lateral ridges. Other butterflies, including most Hesperiidae, Papilionidae and Riodinidae, produce smooth globular eggs. Those of the Polyommatinae ( Blues ) have a finely reticulated surface, and are shaped like flattened do-nuts. Pierines ( Whites and Sulphurs ) produce eggs which are tall and skittle-shaped, with fine vertical ribbing.

All butterfly and moth eggs have a depression at the top, in the centre of which is a hole known as a micropyle, through which sperm enters during fertilisation.

The egg shell is peppered with thousands of microscopic pores, through which air enters to sustain the developing larva within.

Butterfly eggs grow in size as they mature within the female's abdomen. Egg-laying is triggered when they reach a certain size, at which time they pass from the ovariole to the egg chamber. They are fertilised just prior to egg-laying, the male's sperm having been stored until this time within a receptacle in the female abdomen.

Some species, e.g. the Marbled White Melanargia galathea, drop their eggs randomly as they fly amongst tall grasses, but most species have very precise requirements. The Pearl-bordered Fritillary Clossiana euphrosyne for example lays it's eggs singly on dead bracken or dry grass stems that are within a metre of their caterpillar's foodplant, dog violet. The White-letter Hairstreak Satyrium w-album is even more fussy, always laying it's eggs on elm twigs, at the precise point where the new year's growth and old growth meet.

Silver-washed Fritillaries Argynnis paphia lay their eggs in chinks on the bark of oak trees, but the caterpillars don't eat oak - they begin by eating their own egg-shells, and then go into hibernation until the following spring, when they descend the tree trunks to feed on the leaves of nearby violets.

Orange tip caterpillars Anthocharis cardamines normally feed on cuckoo flower or garlic mustard leaves, but if they encounter another caterpillar they become cannibalistic. It would therefore be wasteful if the butterflies laid more than one egg on each plant, so nature has endowed the females with the ability to detect eggs that have already been laid, and thus avoid laying more than one egg per plant.

In the tropics, eggs are often glued underneath the leaves of trees and bushes, where they are protected from rain, and from the desiccating effects of hot sunshine.

Foodplant selection

The caterpillars of most species will only eat the leaves of one or two species of plant, and will die if they find themselves on the wrong type of tree, bush or herb.

From this it can be seen that it is not just enough to locate the correct species of plant. The eggs usually have to be laid on tender young leaves or buds, as the older leaves often contain toxins that can kill them. They also have to be laid on plants that are growing in very precise conditions - just the right degree of shade, just the right conditions of temperature and humidity, and at a height on the plants where they will not get eaten by browsing herbivores. Eggs are often laid on the tips of buds, usually quite high up on the tree or bush. This way they are less likely to be found by ants.

Laying in batches

The caterpillars of most butterflies live solitarily, but some, particularly amongst the Melitaeini, Morphini and Pierini, live gregariously for the majority of their lives. The females of these species lay their eggs in batches, sometimes as many as 500 at a time. The Marsh Fritillary Euphydryas aurinia is a typical example - it lays about 300 eggs in a single batch, on the underside of a leaf of devil's bit scabious, always within a few metres of the spot where the female emerged from it's pupa. Having laid most of it's eggs the female then disperses and sometimes lays another much smaller batch of eggs slightly further afield.

The incubation period varies greatly from species to species. Eggs of tropical butterflies usually hatch within a week, but in temperate areas 10-14 days is more typical. There are however many species, such as Purple Hairstreak Quercusia quercus, Chalkhill Blue Lysandra coridon and High Brown Fritillary Argynnis adippe, in which the eggs hibernate over winter, and in these cases the incubation period can last for several months.

NEXT >> ( caterpillars )