The Arrow-shaped micrathena (Micrathena ssagittata), also known as the Arrowhead orbweaver, is an odd-looking bug. It belongs to the group of spiders that weaves a flat, more or less circular web.
This particular individual, a female, spent most of her time upside down, hence the ventral view of the photo. The top has much more yellow. One can see that it is a female because the points are absent (or reduced) in males, and the pedipalps (you have to look very closely) are not broadened near the ends for transfering sperm during mating. Also, the epigynum (through which the sperm is received) is visible at the front of the abdomen.
This species is native to the eastern United States and is found as far south as Panama. The spider in this photo was at its web about three feet above the ground between leaves of a pokeweed.