What insect has a black head and thorax with a raised red abdomen?

it has 6 legs and 2 very long antennae. i do not think it was a spider, although the antennae were long enough that they resembled legs. about 1 or 1.5 cm long. its body is very narrow. the most distinctive feature was the red abdomen which curved upwards. i saw it outside in northern new jersey. any idea what it could be?