Crab Spider (Misumessus sp.), Newark DE. June 2018.
Well-camouflaged, crab spiders are typically ambush predators, lying in wait against a background that renders them nearly invisible to their prey (even in the ultraviolet spectrum, outside our vision range but within the range of an insect’s). The first two pairs of legs are long for the purpose of grasping prey before they realize their mistake, capable of taking down prey larger than itself.
This species managed to catch itself a Sarcophagid fly and immediately began retreat when I approached. On the underside of this oak leaf, the spider is nearly invisible.