"The Moth" joke

A man wandered into a doctor's consulting rooms and asked if could see the doctor. The receptionist was hesitant to let him in, especially as it was Christmas Eve and she was waiting to turn off the Christmas lights in the waiting room and go home; but he was very insistent. So the Doctor, having had completed all his consultations for the day and feeling in a' good will to men' mood, agreed to see him.
The man entered in a rather aimless manner and after some hesitation flopped into a chair and looked nervously around the room.
"How can I help you?" said the doctor.
"Well, it's like this" said the man. "I keep thinking I'm a moth".
"A moth?"
"Yes" the man replied. "I'm convinced that I'm a moth".
"Well I'm very sorry, but you're in the wrong place. What you need is a psychiatrist".
"That's what I've been thinking" replied the man.
"Well, as it happens, I know just the man". said the doctor "I'll give him a call and see if he can fix an appointment for you after the holiday."
The man agreed and the doctor made the appointment.
"Tell me" said the doctor "It must have been very apparent from the sign outside that I'm a general practitioner. So if you already know you need to see a psychiatrist, why did you come in?"
"Well" the man said in a resigned voice "The door was open and the lights were on. ....".