The western tailed-blue (Cupido amyntula) is a Nearctic non-migratory butterfly that commonly takes flight during the spring and summer seasons of March–July. They can be best classified as having a presence of a tail on their hindwing, an upper surface that's bluish while the under surface is chalky-white with occasional black spots, and an orange spot toward the base of their tail. Their wingspan ranges from 0.875 to 1.125 inches (2.2 to 2.9 cm). Larvae feed on various plant species including Astragalus (several species), Lathyrus (several species), Oxytropis, and Vicia (several species). Adults feed on various things such as flower nectar, horse and coyote manure, urine, and mud.

During mating, the male and female C. amyntula join tips of their abdomen and the male passes sperm to the female to fertilize its eggs in which they typically lay up to 3 eggs on a single host plant flower or young seed pods.

Females are characteristic of a brown wing with blue coloring at the base. Once fertilized, females can lay up to 80 eggs per ovariole. Eggs hatch in about 4-5 days and develop from instar to pupa in about 14-25 days. Adults then emerge from pupae in 11-14 days.

Male C. amyntula typically perches and patrols between shrubs, hillsides, and gully mouths throughout the day in search of a receptive female to mate with. Males can also be distinguished by a blue upper surface.

The caterpillars of the C. amyntula vary from brownish-yellow to green and are covered in fine white hairs marked with a red or green stripe running along the back and diagonal pinkish and reddish-purple dashes on the side. They typically grow an average length of 1/2 an inch.