A praying mantis is a type of insect that can easily be mistaken for being a moving leaf.
While Fox Mulder was climbing a tree, one day in his childhood, he noticed a praying mantis, walking towards him. He initially mistook the creature for being a leaf and, after a long duration, realized what the mantis actually was. Repulsed by the insect, he screamed.
In 1995, Mulder, now an FBI agent, recounted this story to his partner, Special Agent Dana Scully, explaining that the incident was the reason he now hated insects. Before Mulder revealed the true nature of the thing he had first thought was a leaf, Scully immediately guessed that it had been a praying mantis. Mulder confirmed her suspicion - referring to his encounter with the creature as a "praying mantis epiphany" - and claimed that his scream had not been a girlie one, but the scream of a person being "confronted by some before unknown monster that had no right existing on the same planet [he] inhabited". He then asked Scully whether she had ever noticed how the head of a praying mantis resembled that of an alien, but she was more curious whether he was sure that his scream had not been a girlie one. (TXF: "War of the Coprophages")