Butterfly life history

Butterflies, like other insects, are often referred to as "cold-blooded" animals, but this term is probably better left to malicious criminals than to butterflies. Technically, butterflies are called "poikilotherms," which refers to animals that do not have an internally controlled body temperature, like mammals and birds, but have a body temperature influenced by the air temperature, radiant heat such as sunlight, and muscular activity.

In reality, butterflies require a warm thoracic temperature similar to that of mammals to operate effectively. It is common to see moths and other insects in the morning vibrating their wings, seemingly unable to fly, in order to heat up the thoracic muscles to be able to fly. Butterflies normally do this by basking in the sun and absorbing radiant heat. Most butterflies bask by spreading their wings to the side to expose the greatest amount of wing surface to the sun, but some butterflies (e.g., most sulphurs and satyrs) fold their wings over the back and bask sideways with their wings closed over the back.

In colder regions, where air temperatures are well below the required body temperatures needed for butterfly activity, most butterflies are dark in colour to more effectively absorb radiant heat from the sun, and their bodies are covered with hair to better retain the heat. Arctic butterflies crawl up onto rocks or plants and position their bodies and wings for maximum exposure to the sun; species such as arctics and alpines that bask with wings closed on the side actually tilt to one side so that they can absorb the maximum amount of heat from the sun. On cool or cloudy days butterfly activity is minimal.

Butterflies have four distinct stages in their life cycle, termed complete metamorphosis: egg (or ovum), larva (or caterpillar), pupa (often called a chrysalis in butterflies), and adult.

The eggs of butterflies are termed "upright eggs," which refers to the fact that the tiny pores that allow sperm entry (micropyles) are clustered at the top of the egg rather than to the side. Upright eggs are rare in moths but they occur in Noctuidae and their relatives, in the Hedylidae, and in several small families of moths related to Geometridae. Typically, butterfly eggs have a series of ribs both horizontally and vertically that creates a reticulate pattern of raised ridges and rectangular depressions. The eggs usually are laid (glued) singly or in small groups on the larval host plant. In cases where the host plants are herbaceous and disappear through the summer, such as violets, eggs may be laid on the ground where the host plants will grow the next year. In species where the larvae feed on tree foliage, such as those of many hairstreaks, the eggs are laid at the base of the buds of the next year's leaves so that the eggs do not fall to the ground in autumn with the falling leaves, and the larva emerges from the egg the next spring at the base of the newly expanding leaves.

As butterfly caterpillars grow, they shed their skin (moult) from time to time because the skin cannot stretch indefinitely and so must be shed. The caterpillars that hatch from the egg are called first instar (or first stage) larvae; they usually pass through five instars (some groups of butterflies have four or six or even more instars), each one larger, with the last instar larva moulting to reveal the pupal case rather than a larger caterpillar. There is no simple way to distinguish butterfly caterpillars as a group from those of moths, so butterfly caterpillars are best recognized by the family or subfamily characteristics.

Skipper (Hesperiidae) caterpillars can be recognized by the distinctive constricted 'neck,' and the grasping hooks (crochets) on the ends of the four pairs of fleshy false legs (prolegs) on the middle of the body are arranged in a complete circle rather than a semicircle as in other butterflies. The caterpillars of swallowtails (Papilionidae) are characterized by the presence of an osmeterium, a pair of retractable smelly orange tubes on the back behind the head that are everted when the caterpillar is disturbed or threatened. Swallowtail caterpillars tend to be boldly patterned with stripes and spots, some with brightly coloured fleshy protuberances, or marked with spots that give them a snake-head appearance. The caterpillars of whites and sulphurs (Pieridae) are cylindrical, tapered towards the rear, and covered with an abundance of tiny hairs that gives them a "peach fuzz" appearance. The caterpillars of coppers, hairstreaks, and blues (Lycaenidae) are also fuzzy, but the body is sow-bug-shaped and the head is obscured, being held under the front of the body or retracted into a depression in the thorax. Most nymphalid caterpillars have distinctive branching spines on the head and body (sometimes only on the head or only on the thorax); in the subfamily Satyrinae the spines are absent, the end of the body is extended into a forked process that looks like 'tails,' and there is often a pair of conical 'horns' on top of the head; Monarch caterpillars (subfamily Danainae) also lack spines, and have two long fleshy filaments on the back near the front of the body and two more near the rear.

Butterfly pupae are openly exposed to the elements rather than being encased in a cocoon as in moths, and thus tend to be cryptically coloured to blend with the surroundings. They may be highly ornamented with spines and resemble dead, curled-up leaves, as in many nymphalids, or they may be green and resemble plant leaves, as with many pierids. The pupa is usually attached to a plant by a series of hooks at the rear end (a "cremaster") locked into a silken pad left by the caterpillar, and may be (swallowtails and pierids) supported around the middle by a silken girdle much like the safety strap used by hydro-workers and lumberjacks to climb poles.

The pupal stage is sometimes referred to as a resting stage because so little seems to be happening during the several weeks that it takes before the adult butterfly emerges from the pupa. In reality, this phase is hardly one of resting. Tiny buds of cells that were present but dormant in the caterpillar begin to grow and merge to form a complete and fully grown butterfly, except that the wings are very small and thick.

When the butterfly is ready to emerge, it splits the pupal case open at the head end and crawls out; at this point the body of the butterfly is large and distended with fluid, which it begins to pump into the wing veins. As it does this the wings gradually expand to the size and shape typical for the species. This process, together with a time for the wings to dry and harden, can take an hour to several hours, during which time the butterfly is highly vulnerable to predators.

Butterfly species in Canada pass through the winter in a dormant state technically termed "diapause." Diapause may occur in any of the four life stages, but this is normally constant within a group. Most butterflies, including most nymphalids, sulphurs (Pieridae: Coliadinae), most skippers (Hesperiidae), and many blues (Lycaenidae, Polyommatinae) diapause as larvae; most hairstreaks pass the winter as eggs; but elfins and other species of Callophrys pass the winter as pupae. Parnassians (Papilionidae: Parnassiinae) and most coppers (Lycaenidae: Lycaeninae) also pass the winter as eggs. Swallowtails (Papilionidae: Papilioninae), whites (Pieridae: Pierinae), and some hairstreaks and blues diapause as pupae. Only a few butterflies in Canada spend the winter in the adult stage, and some of these are migrants that survive the winter only in the south. Genera that hibernate as adults in Canada are Nymphalis (Mourning Cloak and tortoiseshells) and Polygonia (anglewings); genera with species that hibernate in the south and migrate into Canada each summer are Vanessa (Painted Ladies), Danaus (Monarch), and Polygonia interrogationis (Question Mark). Other than those that overwinter as adults, most butterflies live from several weeks to several months. In general, small fragile species like blues and hairstreaks have shorter lifespans than larger, stouter species like the larger nymphalids.

© 2003.Reproduced with permission from The Butterflies of Canada by Ross A. Layberry, Peter W. Hall, and J. Donald Lafontaine. University of Toronto Press; 1998. Specimen photos courtesy of John T. Fowler.