A monarch butterfly in flight.
Most people are almost completely unaware of the tremendous diversity of insects that can be found even in a typical city backyard, and indeed, even if an insect is noticed it will likely take an expert to identify it to the species level. But butterflies are an exception. Most North American butterflies can be identified relatively easily by anyone with a good butterfly book, and some of the butterflies are exceedingly conspicuous insects recognized readily by the novice.
As butterflies go, there are few as well known as the monarch. Monarchs are well known not only because of their conspicuous appearance, but also because of their dramatic yearly migrations. In the eastern part of North America, the entire monarch population migrates to a small mountainous region in Mexico where they spend the winter (in the western part of North America the monarchs migrate to coastal regions in the general vicinity of Pacific Grove, California, near Monterrey Bay).
A monarch butterfly feeding on the nectar of cup plant flowers. Though butterfly caterpillars have chewing mouthparts for feeding on leaves or other plant parts, adults have mouthparts specialized for feeding on nectar.
Many people grow milkweed plants in their gardens specifically to attract monarch butterflies, which lay their eggs only on the various species of milkweeds. Milkweeds are toxic to most insects and other animals, but the monarchs are able to sequester the toxins in their wings without harm to themselves. In doing this they become toxic to predators, and their bold coloration serves as a warning to birds that the monarchs are best avoided.
The butterflies in these photos were photographed shortly before the Fall migration in September 2007 near Lake Hiawatha in Minneapolis. Though monarchs lay their eggs only on milkweeds, the leaves of which are the only food the larvae will eat, the adults feed on the nectar of a wide variety of flowers. By Lake Hiawatha that day they were actively feeding at the flowers of wild sunflowers (Helianthus spp.) and cup plants (Silphium perfoliatum, which are the flowers in these photos).
Why such large wings?
Butterflies in general have among the largest wings of any insects, and monarchs are large even by butterfly standards. Though butterflies look delicate, they actually are very strong flyers, and the size of their wings surely plays a role.
Aerial plankton vs. strong directed flight:
The flying monarch above has a black spot on the hind wing, which indicates this is a male. The black spot is a scent gland used in courtship
Many insects disperse long distances by flight, but most small insects are at the mercy of the wind currents once they are high in the air. Most small insects control their long-distance movements by choosing to fly up into, or drop out of, the air currents at specific times and places, but once they are high in the air they have little control over where they go (they do have control closer to the ground and in the vicinity of vegetation of course). These small insects high up in the air are often referred to as aerial plankton because they are at the mercy of the wind patterns much the way planktonic creatures in the ocean are at the mercy of the ocean currents. A great many insects disperse this way, and the air is filled with insects in the summer, up to fairly high altitudes. The variety and abundance of larger animals that specialize in harvesting this airborne bounty (including swallows, martins, swifts, nighthawks, and bats, all of which specialize in feeding on flying insects captured on the wing) is testament to the great abundance of flying insects in the air.
But large butterflies like monarchs (and also some other large insects such as dragonflies, large moths, and some others) are different. Their flight is strong enough that they can more directly control where they go, which of course is essential if they are to congregate in specific overwintering sites each year. Interestingly, in addition to the well-known migrating populations of monarch butterflies, there are also other non-migratory populations of monarchs in tropical regions and on some tropical islands. Recent research has found that monarch wings in the migratory populations are significantly larger than in the non-migratory populations, which lends support to the idea that larger wings are important for strong, long-distance flight.
See this article for more about this very interesting study:
‘Supersized’ monarch butterflies evolved to fly far
And a few more interesting articles on monarchs:
This article sheds some light on how migrating monarchs navigate:
Butterfly ‘GPS’ found in antennae
And this article discusses threats to monarchs due to illegal logging in their overwintering area in Mexico:
Mexico logging threat to butterflies