assassin bug, common name for members of the family Reduviidae, one of the largest and most varied groups belonging to the order Hemiptera (suborder Heteroptera). Assassin bugs are generally brownish to black, medium-sized to large insects, with heads that are elongate and narrow compared to the thorax. The raptorial front legs are used for grasping prey. Most assassin bugs are found on foliage, and some occasionally enter houses. The majority of species are predaceous on other insects, but a few are bloodsucking and will bite humans if carelessly handled. The bite of some species is painless, while the bite of others is extremely painful, resulting from a venom produced by the bug, the effect of which lasts for months. A painful biter is the common, black, wheel bug (Arilus cristatus), easily identified by the semicircular crest resembling a cogwheel on the top of its prothorax. Another is the masked hunter (Reduvius personatus), often found in houses where it preys on bedbugs and other insects. The adults often bite humans around the mouth, hence its other common name, the kissing bug. In the Southwest assassin bugs of the genus Triatoma are common. Called conenoses or Mexican bedbugs, they also invade houses and may bite humans. In Central and South America certain species of this genus are the vectors for a highly fatal trypanosome disease known as Chagas' disease. Assassin bugs are classified in the phylum Arthropoda, class Insecta, order Hemiptera, family Reduviidae.

Assassin, European name for the member of a secret order of the Ismaili sect of Islam. They are known as Nizaris after Nizar ibn al-Mustansir, whom they supported as caliph; the European term Assassin is derived from the Arabic for "users of hashish." The members of the order were distinguished by their blind obedience to their spiritual leader (known in the West as the Old Man of the Mountain) and by their use of murder to eliminate foes.

The order was founded by Hasan ibn al-Sabbah when he gained control (c.1090) of the mountain fortress of Alamut, located S of the Caspian Sea. The order spread over Persia and Syria, gaining control of many strongholds, and it soon inspired terror throughout the Muslim world. Members were organized into strict classes, according to degree of initiation into the secrets of the order. The most important of the classes were the devotees, who sought martyrdom and were the instruments of assassination.

Hasan and the grand masters who ruled the order after him wielded great political power until the coming of the Mongols. Hulagu Khan attacked and destroyed (1256) their fortresses and massacred most of the Persian branch of the sect. The Syrian branch, with which the Crusaders came in contact, suffered a similar fate at the hands of Baybars, the Mamluk sultan of Egypt. Only scattered groups of the order survived; they are said to persist today, particularly in N Syria. Tales of the Crusaders and the writings of Marco Polo brought the Assassins and the Old Man of the Mountain into European folklore. The term assassin came into English and is used today to mean murderer, particularly one who kills for political motives.

Any of about 4,000 insect species (family Reduviidae) characterized by a thin, necklike structure connecting the narrow head to the body. Many species are common to North and South America. Ranging in size from 0.5 to 1 in. (13–25 mm), assassin bugs use their short, three-segmented beak to suck body fluids from their victims. Most assassin bugs prey on other insects; some, however, suck blood from vertebrates, including humans, and transmit diseases. One species, the large assassin bug, defends itself by accurately “spitting” saliva toxic enough to blind a human.

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