Lyside is a fairly common large sulphur of the southern fringe of the US (from southernmost Florida to southernmost California) that sometimes strays further north. The bulging vein in the middle of the hindwing is distinctive. Compare with Statira Sulphur.

Big Pine Key, Monroe Co., FL 6/27/2010.

Highly variable, Lyside Sulphur is perhaps the second most common butterfly in the Lower Rio Grande Valley, after American Snout.

Santa Ana NWR, Hidalgo Co., TX 10/15/04.

Bentsen-Rio Grande State Park, Hidalgo Co., TX 6/6/04.

Looks like this one has a chunk bitten out of its hindwing.

Santa Ana NWR, Hidalgo Co., TX 10/15/04.

The rarely-seen dorsal (top) side, courtesy of a spider.