Sap beetles are black and shiny with four irregular spots on the forewings. Note the antennae are clubbed and the wings don't fully cover the abdomen. Photo by Joyce Gross, UC-Berkeley.
In addition, I've got reports of a few different flea beetles in corn. Corn flea beetles are common to see early in the season. Corn is the preferred host plant where the adults scrape along upper and lower corn leaves. They will also feed on grasses. The feeding itself isn't an economic problem, but they can vector a bacteria that causes Stewart's wilt. Foliar insecticides are not economically justified after V5.
Corn flea beetles are small, shiny and black. Note the enlarged hindleg femurs used for jumping. Photo by Mike Quinn, TexasEnto.net.
Another flea beetle out and about lately is the redheaded flea beetle. They are similar in size to the corn flea beetle, but more oval in shape and have a red head (funny how entomologists come up with common names!). They can feed a several weeds, corn and alfalfa.
The redheaded flea beetle also has enlarged hindleg femurs for jumping. Photo by Lewis Veith.