All About Butterfly Eggs

Excerpted from: Facts About Butterfly Eggs

Butterfly Life Cycle They may be small-winged insects, but butterflies are among the world’s most interesting members of the animal kingdom. They are found everywhere in the world and contain thousands of different species, with 750 species found in the United States. Their sizes vary from less than half an inch long, to some species that can reach up to 10 inches long between wing tips. We may know much about their size, color and habitats but lesser is known about butterfly eggs.

Life Cycle Basics

Butterflies grow through a four-stage process known as complete metamorphosis, changing from egg to larva to pupa and finally to adult. The eggs hatch into a larva, which we know commonly as a caterpillar. The caterpillar then grows by molting its outer exoskeleton. Larvae can do this a few or several times before they enter into the next phase of development — the pupa. At the pupal stage, known as a chrysalis in butterflies, the insect is typically non-mobile and appears to be resting. During this stage the pupa is changing drastically, forming the often colorful, scaly wings that butterflies exhibit. Once the mature adult emerges from the pupa, it is ready to find a mate for producing offspring.

Egg Laying

The larvae 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. Even 'oligophagous' species — those that are able to feed on more than one type of plant - have a hierarchal order of foodplant preference, only accepting less nutritional species if they are unable to locate their preferred foodplant.

Monarch Butterfly, Danus plexippus, egg laying Butterflies therefore spend a great deal of time checking various leaves to ascertain whether they are of the correct species for egg-laying. Studies have shown that butterflies initially determine leaf choice by shape and size, but use taste and smell to confirm that the leaf is chemically 'correct'. It is common to see butterflies flitting from plant to plant, alighting momentarily on leaves, tasting the foliage using olfactory sensors on their feet. All female butterflies have spines on the underside of their forelegs. When they land on a leaf these spines puncture the surface, releasing aromas that are detected by the olfactory sensors.

It isn't 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. Female butterflies often spend long periods probing about with the tips of their abdomens, being extremely careful about the positioning of each individual egg.

It isn't 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.

To protect the egg and embryo that is forming, the eggs are attached to a leaf by the butterfly using an adhesive-like substance that glues the egg to the plant leaf. The nature of the 'glue' is still fairly unknown in the scientific community. This glue holds the eggs to the leaf in such a way that they cannot be separated without destroying the eggs. Small funnel-shaped openings called 'micropiles' can be found on top of each egg. This is where water and air enter while the egg is developing. Each egg is surrounded by a chorion, a hard outer shell that protects the larva. Some shells have raised ribs.

Egg Characteristics

Butterfly Egg Detail graphic Butterflies are 'oviparous', meaning they lay eggs. They breed as many animals do — eggs from the female insect are fertilized by sperm from the male. The female butterfly stores the male’s sperm in a bursa, or sac, until she is ready to lay eggs. Depending on the species, females lay eggs one at a time, in clusters, or in batches of hundreds. Butterflies lay an average of between 100 to 300 eggs, although some species may only lay a few dozen, others can lay as many as a 1,000 or more.

The butterfly egg is spherical or ovate shaped, varying in color and size. It is covered by 'chorion', a hard shell that protects the egg, and is lined with a waxy layer that prevents dehydration. It is full of nutrients and is commonly the he newly hatched larva. There is a small opening called a 'micropyle' which allows sperm to enter the egg for fertilization while inside the female.

Butterfly eggs vary in size—from about 1 to 3 mm in diameter. The eggs can be smooth or textured, their shapes can be oval or round, and their colors can be yellow, white, green or other shades, depending on the species. The Zebra Longwing butterfly, for instance, produces eggs that look like tiny cobs of corn while the eastern Black Swallowtail butterfly produces smooth, pale-green, globe-shaped eggs.

The egg shell also is peppered with thousands of microscopic pores called 'aeropyles'. Microscopic examination of the eggs reveals them to be adorned with hundreds of minute hexagonal pits. Tiny hollow spines emerge at the intersections of each hexagon. These are also aeropyles, and act as breathing tubes for the developing larva.

Egg Survival

A female butterfly lays a great number of eggs. They also take special care of their eggs. The eggs need to be kept warm and must have the appropriate moisture or they will either rot or dry out. A big portion of these eggs will not hatch to become butterflies as they are vulnerable to many predators such as birds, spiders, other insects and small mammals. Of the few hundred butterfly eggs laid, very few will reach adulthood.

Egg Development

Question Mark, Polygonia interrogationis, eggs Inside each egg, a yolk can be found that serves as nourishment for the developing larva. A butterfly egg hatches after three to eight days depending on temperature and season of the year. A change in the egg’s color before hatching is normally visible. After hatching, some caterpillars eat their own eggshells as their first meal but most of them eat parts of the plant that the eggs were laid on.

The average time for the embryo to develop is 7-14 days but in instances where the egg is the overwintering stage, the embryo may take months to develop.

Egg Parasites

The eggs of butterflies and moths are valuable sources of protein. In addition to the threats from birds, snails, reptiles, amphibians and grazing mammals already mentioned, they are prone to parasitisation by microscopic wasps and flies. It may seem surprising that something as small as a butterfly egg has its own parasites, but these cause high losses. The main parasites are wasps — as many as 60 of these can emerge from a single butterfly egg !

Further Information

Butterflies and Moths of North America
WisconsinButterflies.org
Southern Wisconsin Butterfly Assn (NABA)
The Butterfly Site