Reduviidae sp. Assassin bug, on of natures predatory insects. assassin bug (family Reduviidae), number about 4,000 species of insects in the true bug order, Heteroptera, that are characterized by a thin, necklike structure connecting the narrow head to the body. They range in size from 10 to 25 mm (0.4 to 1 inch). An assassin bug uses its short, three-segmented beak to suck the body fluids from its insect victims.