“The venom of the P. nigriventer spider is a very rich mixture of several molecules,” says Dr. Kenia Nunes, a physiologist at the Medical College of Georgia who is currently studying the odd side effect. “These molecules are called toxins, and then we have various toxins in this venom with different activity. Because of this, when a human is bitten by this spider, we can observe many different symptoms including priapism, a condition in which the penis is continually erect.”
http://en.wikipedia.org/wiki/Brazilian_wandering_spider
