After a teen stole her purse and iPhone while she waited for the N train at Fifth Avenue, 29-year-old shoe designer Sayaka Fukuda arrived home to find that her assailant had used the device to e-mail a photo of himself. Fukuda tells the Post, "I checked my e-mail and saw something in my outbox. I opened the attachment, and there he was with his black and red knitted [Spider-Man] hat." After combing through mug shots, cops zeroed in on 18-year-old Daquan Mathis. But before they made the collar, Fukuda initiated a correspondence with the suspect, telling him, "Thank you for your picture. I sent them to the cops, you're going to get arrested soon." To which Mathis allegedly replied, "I will kill you! I know where you live, I know where you work. I'll send people." Before he could make good on his threats, the NYPD arrested him Thursday; sources say he confessed to two other crimes, including another robbery on an N train in which he stole an iPod at gunpoint. Of course, this isn't the first time a mugger's been busted by the very gadget he stole.