Ophidicism is a sub-type of the paraphilia referred to as zoophilia. It involves a sexual arousal that hinges on the use of a reptile as a sex object for women.
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In this particular paraphilia, the woman inserts the reptile, usually a snake, into her vagina and uses it as a masturbatory aid while the snake tries to wriggle free. Apparently in Ancient Roman times, the snake was inserted head first, though most modern pornography depicts the snake being entered tail first. It is important to note that sexual intercourse with reptiles can result in salmonella poisoning.