A black widow spider.
black widow definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
an American spider (Latrodectus mactans), the female of which has a glossy black body with an hourglass-shaped red mark underneath, and a poisonous bite which is normally not fatal: so called because the female sometimes eats its mate
black widow definition by American Heritage Dictionary
A poisonous New World spider (Latrodectus mactans), the female of which produces extremely toxic venom and has a black shiny body with red markings.
Origin: From the fact that the female eats its mate.