Pearl crescent, Phyciodes tharos
With wings spread the butterfly is about one and one-half inches. The name comes from a white "pearl" or chevron on the trailing edge of the hindwing's underside.
The underwing of the pearl crescent showing the white "pearl" or chevron
There is another very similar butterfly -- the northern crescent. The two are hard to tell apart and were once considered the same species. The northern crescent lacks the white chevron on the hindwing and occurs more commonly in northern New England and Canada. However, I took the following photo today along with the pearls and it looks to be lacking the chevron, so many both species are flitting around our yard.
Northern crescent, Phyciodes selenis
(lacks the white chevron on the hindwing)
The pearl and northern crescents belong to a very large family of butterflies -- the Brushfoots. Now that name sounds straight out of The Hobbit. Apparently about one in three butterflies worldwide is a brushfoot - about 6,000 species in all. You might recognize some other members of this family: great spangled fritillary, question mark, mourning cloak, as well as ladies, admirals, and monarchs. These butterflies all look very different. What brings them together into a common family is, well, their tiny brushy or hairy forelegs on the caterpillars. David Wagner, in his fabulous guide, Caterpillars of Eastern North American, gives the key family feature a more technical description: "the presence of a minute filiform seta near the base of the scolus on A9." I prefer "brushy feet," although caterpillar legs or so small you'd need a strong lens to pick out the hairs, let alone finding the little caterpillars at all.
Keep an eye out for the Brushfoot butterflies. They surely visit your yard too.