Introduction: The Lycaeides melissa butterfly, also known as the Melissa Blue butterfly, is a member of the second largest family of butterflies (Lycaenidae) with about 6000 species worldwide making up 40% of butterflies in existence (1). It is found in western North America from Canada to Mexico. However, the mutualistic relationship of interest occurs between the Lycaenid caterpillar and several species of ants (2). When the eggs of the L. melissa hatch, the caterpillars eat for two to three weeks, form a chrysalis and pupate for eight to eleven days, and finally emerge as a butterfly. As a butterfly, the L. melissa lives for a period of one to two weeks, during which they will mate and lay eggs (2). The ant goes through a four part life cycle from egg, to larva, to pupa, and finally to adult.
Relationship Description: The relationship between the L. melissa caterpillar and several species of ants involves the caterpillar producing a "honeydew" that the ant feeds on from the pores of its body (4). In return the ant protects the caterpillar from other predatory ants as well as other species. This relationship is not obligatory; however, it is beneficial for both species in that the caterpillar is protected and the ant receives nourishment. Due to the fact that this relationship is not obligatory, it is more accurate to list this as a cooperative association rather than a purely mutualistic relationship. The association is not obligatory because the ant can receive its nourishment from other resources and the caterpillar does not have to be protected from predation. The caterpillar's life expectancy is greatly increased with the protection of the ants (4). This relationship is common to other species in the Lycaenidae family of butterflies, and there are several species of ants involved in this type of relationship (3).
Cost/Benefit Analysis: In this particular situation, the relationship may be slightly described as commensal. This is because, while both species do benefit from the relationship, the caterpillar has a much greater benefit in that its chance of survival is greatly increased. The ant is also greatly benefitted because it requires much less energy expenditure when feeding from the caterpillar than having to forage for food. And while this is helpful for the ant, it also is at risk when having to defend the caterpillar from predators. In a sense, the risk may be higher for the ant due to having to ward off the caterpillar's enemies, but the ant receives free nourishment for performing its duties. The only cost of the caterpillar is its cost of production of the honeydew it excretes from the pores on its skin.
1.) Fiedler, K. 1996. Host-plant relationships of lycaenid butterflies: large-scale patterns, interactions with plant chemistry, and mutualism with ants. Entomologia Experimentalis et Applicata 80(1):259-267 doi:10.1007/BF00194770
2.)Venkatesha, MG. 2005. Why is homopterophagous butterfly, Spalgis epius (Westwood) (Lepidoptera: Lycaenidae) amyrmecophilous? Current Science 89 (2): 245-246.
3.) Pierce, N., Braby, M., Heath, A., Lohman, D., Mathew, J., Rand, D., & Travassos, M. (2002). The ecology and evolution of ant association in the Lycaenidae (Lepidoptera) Annual Review of Entomology, 47 (1), 733-771 DOI: 10.1146/annurev.ento.47.091201.145257