Everes amyntula
Western Tailed-Blue

Family Description:

Caterpillar: The caterpillar can vary in its color, from brownish yellow to green, and is covered with fine white hair. It is marked with a red or green stripe running lengthwise along the back, and pinkish or reddish purple diagonal dashes on the side. It reaches an average length of 1/2 inch.
Adult: This is a small butterfly, with a wingspan of 7/8 to 1 1/8 inches, which bears a thin tail extending from the rear of the hindwing. The male butterfly is purplish blue on the upperside and outlined in black. The female is brownish on the upperside, outlined in black, and may have bluish shading at the bases of the wings. Underneath, both sexes are white and may be marked with black dots and a faint row of grayish dots along the outer edge of the wings. There may be a small orange spot on the outer edge of the hindwing from where the tail extends.

This species has a fairly extensive range, from Alaska south to southern California and the southwestern U.S., east across Canada to Ontario and across the northern U.S. to northern Wisconsin and Michigan’s Upper Peninsula. It occurs through much of Idaho.

It can be found in open areas, including meadows, open woodlands, forest edges, and chaparral.


Caterpillar: Caterpillars feed on the flowers and fruits of legumes, such as milk vetches (Astragalus spp.), other vetches (Vicia spp.), and peas (Lathyrus 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 through much of the range, but there may be several in the west and in southern California. Each caterpillar undergoes four stages of growth, or instars. Caterpillars overwinter in a physiological state called diapause, often in the seedpods of the host. The entrance hole to the pod is sealed with silk. Pupation may occur within the pod as well. Adults in most of its range generally fly from May through June, but adults may be seen as late as October in the southernmost parts of its range.

Males both perch and actively patrol for receptive females. Females lay eggs singly on host plant flowers, seedpods, and stems.

Idaho Status: Unprotected nongame species.
Global Rank: G5; populations are widespread, abundant, and secure.

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. http://www.npwrc.usgs.gov/resource/distr/lepid/bflyusa/bflyusa.htm (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.