Ant tending

Myrmecophily is the mutualistic association of ants with other organisms.  The larvae of many species of Lycaenidae (family of blues, coppers, and hairstreaks) are tended by ants.  The relationship between the ants and butterfly larvae varies depending on the species, but it is usually mutually beneficial, where the ant protects the larva from predators and parasites, and the larva provides a sugary substance high in amino acids that the ants love.  In the Pacific Northwest, ants are commonly associated with larvae of silvery blue (Glaucopsyche lygdamus), Boisduval's blue (Plebejus icarioides), and echo blue (Celestrina echo) butterflies, among others.
Following are photos of thatch ants (Formica sp.) tending Puget blue larvae (Plebejus icarioides blackmorei), a subspecies of the Boisduval's blue.

(1) ant "tickling" the larva with its antennae
(2) larva protruding organs that produce a nectary substance
(3) ant consuming "nectar"

Series of photos on the left show ants "herding" mature larva into a tunnel at the base of a lupine plant.  Photos on the right show the larva in the hole the next day, slightly curled up, possibly preparing to pupate.  I uncovered the larva to take the photo, and replaced the moss and sticks when I was done.

Following photos show unidentified ants tending larvae of the federally endangered Fender's blue (Plebejus icarioides fenderi), another subspecies of the Boisduval's blue.  Both photos were taken at the base of a lupine plant.

Silvery blue larvae (Glaucopsyche lygdamus) usually feed on lupine flowers and are colored pink and purple, compared to Boisduval's blues which feed on leaves and stems and are colored green.

Silvery blue larva on lupine flower stalk

Silvery blue larva tended by several unidentified ants