The butterflies in the family lycaenidae are particularly fascinating to me because many of them have associations with ants. The most familiar lycaenids, at least in Europe, are the blues, coppers and hairstreaks, representatives of which are shown below.
The lycaenids are one of the biggest families of butterfly, with many different species. However, they tend not to be very common, and are often overlooked. It seems likely that the association of lycaenid butterflies with ants has led to them speciating faster than other groups of butterflies and expoloiting ecological niches that other butterflies can't use. However, their dependence on ants may be one reason for their general rarity, because they often need the combination of a particular food plant and particular ant species in any area in which they are to survive. So it is probably no coincidence that the lycaenid butterflies feature more than any other insect family on lists of endangered species.
The types of interaction that lycaenid butterflies have with ants are highly variable. Some get along fine without ants, but seem to be able to appease anst when they come across them so that they donot become their prey. Many, and perhaps most, lycaenids have a mutaulsitic relationship with ants, providing food rewards from specialised glands in return for protection from predators and parasites. An example of this type of relationship is that shown by the Common Imperial Blue butterfly, Jalmenus evagoras. Yet other lycaenids are parasites - feeding from ant colonies but giving nothing or very little in return. One such is the Alcon blue butterfly Maculinea alcon.