The Egg or Ovum

Structure

The shape of butterfly eggs is remarkably variable. The examples below show some recurring forms but there are many others, for example swallowtail eggs are smooth and spherical. There is some consistency of shape between closely related species. The egg consists of an outer casing, or chorion, inside which is the females fertilised ovum. There is always a minute opening, the micropyle, which is visible as a small pit at the top of some eggs. This structure allows the male sperm to fertilise the egg and probably allows the developing embryo to breathe.

A Nymphalid egg, Comma, Polygonia c-album

A Pierid egg, Brimstone, Gonepteryx rhamni Rather a poor attempt at a Lycaenid egg. This is from the Long-tailed blue, Lampides boeticus. A Satyrid egg, Wall, Lasiommata megera.

Through her legs the female butterfly can 'taste' plants by a chemical process and so recognise the species, or groups of species which her young will need to feed on. Some butterflies (termed monophagous) only use a single species of plant for their larvae, while others (oligophagous) will use hostplants of similar species and there are some (polyphagous) which will use hostplants from different genera. Once a suitable site is found egg-laying, oviposition, can take place.

Sometimes eggs are laid singly, at other times they may be in bunches, the Map, Araschnia levana,  lays its eggs in vertical columns. All these tactics have their benefits in terms of survival, a parasite may miss one or two eggs in a large group and similarly may miss one or two widely scattered eggs. Usually the eggs are laid on the foodplant, but some species lay nearby. The Silver-washed fritillary, Argynnis paphia, lays its eggs on the trunk of a tree near to a growth of its foodplant, Viola. Butterfly eggs are attacked by various parasitic wasps so as much as possible must be done to safeguard them.

The eggs take a variable amount of time to hatch, indeed some butterflies remain as eggs through the winter, only hatching when the warmth of spring arrives. I guess they are less likely to be eaten when very small and easy to miss. Usually it takes about 10 days for an egg to hatch. There is an easy exit for the tiny first instar caterpillar to escape from the confines of its egg.

Brimstone 1st instar larva with egg

A number of young larvae actually eat their egg shell. For some it is the fuel for their journey to find the foodplant and for others it is the only meal they have before the winter, without it they don't survive.

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All pictures in these pages copyright to Simon Coombes. Permission must be sought and obtained for any use.