Butterfly of the Week

Eastern Tiger Swallowtail

Papilio glaucus (Linnaeus 1758)

Status: Breeding resident.

Flight: Generally double brooded, with flights from April through June and early July through August. A partial third brood may be present in some years from mid September to first frost. 

Distinguishing features: This familiar butterfly is yellow with black margins and black tigerlike stripes on the front edge of the fore wing. Females are dimorphic. One form resembles the male, with yellow wings and black markings. The other form is melanic: the yellow may be completely obscured by black scaling, but under close examination the tiger stripes may be seen. Some individuals appear intermediate, with the yellow scaling only partially blacked out. Wingspan: 9 - 16 cm.

Distribution and habitat: Map 297. Common throughout the state in a wide variety of habitats. It is particularly frequent along forest edges, where adequate nectaring sites occur. 

Natural history: The larvae feed on a number of common trees, such as ash and wild cherry. They derive protection from predators in at least two ways. First, the dorsal part of the thorax has two large eyespots. When larvae are disturbed, they shake the front part of their body, calling attention to these spots, intimidating predators, and discouraging further attack. Second, the brightly colored two-pronged osmeterium (normally kept tucked under the mid dorsal region of the thorax) can be made to release a bright, unpleasant-smelling liquid. Adult males frequently congregate at mud puddles or along streambanks.

Questions: Does the percentage of melanic to yellow female vary from population to population, and what environmental factors influence any such changes? Is the dark form mimetic and related to the protection derived from other swallowtails (e.g., Pipevine or Spicebush Swallowtails)? How protective are the eyespots against predation by native birds? What is the chemical nature of the odor-producing osmeterium?

The Butterflies of Iowa, by Dennis W. Schlicht, John C. Downey, and Jeffrey C. Nekola