Articles - Information About Butterflies, Caterpillars & Plants

The Mysteries of Life
November 2008
by Regina Cutter Edwards

Black Swallowtail Butterfly Laying EggsButterflies go through four stages in life that are considered to be great mysteries. They are real survivors even with life’s many twists and turns. Butterflies have been able to adapt and make it through unimaginable obstacles. Butterflies are holometabolous, experiencing a complete metamorphosis, or in other words, a complete change in body form. They begin life as a larva and pupate into an immobile state and emerge as a butterfly, looking nothing like their pre-pupated state. Read on to learn more about these complicated yet fascinating stages of a butterfly’s life!

Butterfly Egg
Black Swallowtail EggThe 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 first meal for the newly hatched larva. There is a small opening called a micropyle which allows sperm to enter the egg for fertilization while inside the female.

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. For protection, the eggs are often laid on the underneath side of the leaf so as not to be visible to predators. It is essential for the eggs to be laid on the host plant for that particular butterfly species. Eggs are sometimes laid in groups, although many butterflies such as the Monarch lay their eggs singly. 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.

Butterfly Larva (Caterpillar)
Black Swallowtail larva, also known as a caterpillarThe larva stage of the butterfly’s life cycle is a time for growth. The larva, also known as a caterpillar, spends its time eating and growing. The caterpillars feed on their host plants, which can range from a single species to many different species.

As caterpillars grow, they shed their exoskeleton. The exoskeleton is their support and protective structure outside their body. This process is called molting or ecdysis and is triggered primarily by hormones. The caterpillar forms a new cuticle underneath the old cuticle. Enzymes are secreted between the layers dissolving the muscle attachments to the old cuticle so the new cuticle can form without being exposed and the old cuticle can be shed. Each stage between molts is called an instar. Caterpillars usually have five instars before pupating, in which the same molting process occurs.

Black Swallowtail OsmeteriumCaterpillars have developed some interesting techniques to protect themselves. A common characteristic is camouflaging. Many caterpillars resemble plant parts, bird droppings, or may just look frightening to scare off predators. Many swallowtail caterpillars have huge ‘eye spots’ on their thorax regions and are able to inflate their body so that they appear snake-like. Swallowtail caterpillars also have an osmeterium, an interesting looking Y-shaped gland that protrudes out from above their head producing a smelly scent. A few caterpillars are brightly colored and they usually are able to save plant toxin in their bodies, making them poisonous to predators. Or, they may mimic other poisonous caterpillars while being completely harmless. The same is true for caterpillars that are covered in body hairs - some have stinging body hairs to prevent getting eaten.  

Black Swallowtail CaterpillarAs the caterpillar reaches the final instar, the wing pads have already begun developing. Right before the pupa stage the pads are forced out by the hemolymph, insect ‘blood,’ and by the time the larva has shed the exoskeleton to form the pupa, the wings are strongly attached to the body. This is why you can see the shape of the wings on the chrysalis.

Pupa (Chrysalis)
The pupa stage of the butterfly’s life cycle is a time for complete change. The pupa is also known as a chrysalis and is a vessel in which the slow moving caterpillar changes into a winged butterfly. The process begins with the prothoracicotropic hormone (PTTH) triggering the caterpillar to quit eating and start wandering to a place to pupate, which is usually on twigs or other safe areas around their host plants but not necessarily on the host plants. Some butterfly species that over winter as a chrysalis may pupate in the leaf litter close to the ground.

Black Swallowtail Pupa (chrysalis)The chrysalis is immobile so it is important that it be camouflaged into the environment. For example, the Spicebush Swallowtail’s chrysalis takes on a brown or green color depending on the color of the environment. Many chrysalides are leaf shaped and vary in color from yellow, brown, green to silver and gold. Some chrysalides have the ability to wiggle and make a hissing sound in order to frighten predators.

Butterflies go through a really amazing metamorphosis when being transformed from a caterpillar to an adult. The process inside the chrysalis is very intense. The insect’s body basically is liquefied by digestive fluids and the body is restructured using specialized formative cells. This process is called histogenesis, in which undifferentiated cells are used to build different body tissues. This is similar to the building of tissues that can be done with stem cells in other animals.

Adult Butterfly
Black Swallowtail Butterfly EmergingThe adult stage of the butterfly’s life cycle is a time for reproduction. The adult is also known as an imago, they have fully developed wings and are sexually mature. When the adult first emerges from the chrysalis, their wings are wet and crinkled. They are able to pump fluid through their bodies into the wings to help them expand. Once their wings are fully expanded, they may take 30 minutes up to a couple hours to dry. They are then ready to fly. Many times right after the butterfly emerges from the chrysalis there will be a colored fluid that is released from the chrysalis, this is meconium, excess fluid the butterfly did not need.

Female Black SwallowtailThe adult butterfly’s primary instinctive behavior in life is to search for a mate of the same species and produce viable offspring to carry on their genes. This includes making sure the eggs are laid on the correct host plant. Some butterflies lay their eggs in groups, while others lay them singly because the first instar caterpillars can be cannibalistic and will eat the other eggs.

Predictability of Life
Butterflies have been around for millions of years. They have evolved to withstand many obstacles they have encountered throughout the years, including predators and environmental occurrences. The dramatic differences in their life stages have given them the extra benefit of the larva and adult not having to compete with one another for resources, such as food. These mysteries to us have proven over time to be the predictable life for a butterfly.
 

View an illustration of the life cycle of a Black Swallowtail as it transforms from egg to larva (caterpillar) to pupa (chrysalis) to adult.

Photo Credits: Fred Miller and Diane P. Brooks

References:
Borror, D. J., Triplehorn C.A., Johnson, N.F. 1992. An Introduction to the Study of Insects (6th Ed). New York: Saunders College Publishing.

Powell, J.A. 2003. Lepidoptera (Moths, Butterflies). Encyclopedia of insects. Ed. Vincent H. Resh, Ring T. Cardé. New York: Academic Press. p 631-664.

Riddiford, L.M. 2003. Molting. Encyclopedia of insects. Ed. Vincent H. Resh, Ring T. Cardé. New York: Academic Press. p 733-738.


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