Acmon Blue (Plebejus acmon) is a small butterfly, with a wingspan of 0.75 to 1.125 inches (although in our area, it rarely gets above 1 inch). It can be seen from March to October. Numbers peak in July. Host plants include native Buckwheats (Eriogonum), as well as several plants in the Pea family (Fabaceae): Lupines (Lupinus), Acmispons, Trefoils (Lotus), and Milkvetches (Astragalus). Adults tend to stay close to the host plant.
Like Gray Hairstreak, Acmon Blue has a mutualistic relationship with ants (both species benefit). The ants protect the caterpillars from predators in exchange for the sweet “honeydew” the caterpillars secrete from a nectary gland. Roughly three-fourths of butterflies in the Lycaenidae family have a relationship with ants, but not always mutualistic.
It is found primarily along the west coast of North America, from Washington to Baja California. There are scattered sightings from further east.
Butterflies usually mate in a back-to-back position, joined together at the tip of the abdomen (see the Mylitta Crescent photographs for a rare exception). In some species, females mate only once, but for most species, they mate multiple times, albeit with a gap of several days in between. The reason has to do with how the male passes on his sperm. Sperm is not deposited directly. Instead, the male takes some time to form a spermatophore, which occurs inside the female. Some matings last only 20 minutes, others can last a whole day or more! A spermatophore consists of a tough outer envelope, an inner matrix of fluids, and a bolus of sperm. While the exact number varies across species, a male may pass on as much as 13% of his total body weight through the spermatophore during mating. It is large enough to block the female's reproductive tract, so she can't mate again for a few days till she has broken down the outer shell of the spermatophore and fertilized her eggs. Only then is she ready to mate again.
This may explain why, for many species, males are observed much more frequently than females. Males hang around in the open every day, searching for receptive females. For a few days after each mating, a female does not appear except to feed. And the average life of an adult butterfly is 3 weeks. "A few days after each mating" can add up to half its life.
