Glaucopsyche piasus
Arrowhead Blue

Family Description:

Caterpillar: The caterpillar is quite variable in appearance. It may be bluish green and marked lengthwise along the back with a lighter stripe and on the sides with pink. Another form is greenish white, marked with white and red dots and red side dashes. A third form is yellowish brown, marked with a back stripe and whitish red side dashes. Its average, full-grown length is 3/4 inch.
Adult: This is a medium-sized butterfly, with a wingspan of 1 to 1 3/8 inches; for a "Blue," it is quite large. The male is violet blue on the upperside and edged in brown; the brown may extend inwardly on portions of the wing. The female is dull blue and brown. Underneath, both sexes are gray to brownish gray and marked with black dots. The hindwing has a curved row of white "arrowheads", and an uneven white spot. The wings are edged with a black-and-white checkered border.

A western butterfly, it ranges from southern British Columbia and Alberta south through the western third of the U.S. It occurs through much of Idaho. Typically, it is an uncommon butterfly.

This species occurs in meadows, grassy fields, woodlands, and in coastal dunes and lowlands.


Caterpillar: Caterpillars eat the flowers and fruits of legumes such as milk vetches (Astragalus spp.) and lupines (Lupinus spp.).
Adult: Butterflies drink flower nectar.

The caterpillar is equipped with a honey gland, also known as a dorsal nectary organ, which emits a sugary solution agreeable to ants. The ants feed on the solution and in turn protect the caterpillar from predators. Also for protection, the caterpillar bears a pair of everscible tubercles or tentacles on the eighth segment. These tubercles are usually housed within the body, but when the caterpillar feels threatened by the approach of a potential predator, they can be pushed out to release a chemical which mimics an ant alarm pheromone. This scent causes the ants to become frenzied and aggressive, and the potential predator takes leave or is eaten by the ants. There is one generation of caterpillars each growing season. Each caterpillar undergoes four stages of growth, called instars. Pupae overwinter in a physiological state called diapause. Butterflies typically fly from March to early July.

Males actively patrol in search of females. Females lay eggs singly on host plants, primarily on flower buds.

Idaho Status: Unprotected nongame species.
Global Rank: G5; most populations are widespread, abundant, and secure. Certain populations in California, however, may be in jeopardy.

Ballmer, G. R. and G. F. Pratt. 1988. A survey of the last instar larvae of the Lycaenidae of California. Journal of Research on the Lepidoptera 27:1-81.

Ferris, C. D. and F. M. Brown. (eds.) 1981. Butterflies of the Rocky Mountain States. Univ. of Oklahoma Press, Norman, Oklahoma, USA, 442 pp.

Opler, P. A., H. Pavulaan, and R. E. Stanford. 1995. Butterflies of North America. Jamestown, North Dakota, USA: Northern Prairie Wildlife Research Center Home Page. (Version 05Nov98).

Opler, P. A. and A. B.Wright. 1999. A Field Guide to the Western Butterflies.   Second Edition.  Peterson Field Guide Series. Houghton Mifflin Company, New York, New York, USA, 540 pp.

Pyle, R. M. 1981. National Audubon Society Field Guide to North American Butterflies. Alfred A. Knopf, Inc., New York, New York, USA, 924 pp.

Scott, J. A. 1986. The Butterflies of North America. Stanford University Press, Stanford, California, USA, 583 pp.

Stanford, R. E. and P. A. Opler. 1993. Atlas of Western U.S.A. Butterflies (Including Adjacent Parts of Canada and Mexico). Published by authors, Denver, Colorado, USA, 275 pp.