Note: Tarantulas of the western hemisphere possess barbed, irritating, detachable hairs on the back of the abdomen which may be kicked off as projectiles when the spider is threatened. It is thought that the presence of these defensive hairs may explain the limited aggressiveness of New World tarantulas and the minimal effect (such as redness or swelling) of their venom on humans. Tarantulas of the eastern hemisphere, however, lack these irritating hairs, tend to display greater aggressiveness, and are more likely to inflict painful bites with venom that may sometimes cause severe systemic reactions.