Well like a small fly. I mean the ports on the computer dont really allow access to inside of the computer but the only way I can think is the exhaust - but if the fan is on even at normal speed, I dont think a fly would want to get in there.
I found a few dead bugs in a Powerbook G4 15" 1.67Ghz when I opened it up. The outside of the machine was immaculate, I was surprised and grossed out to find them there. Maybe they're drawn to the warmth? Or perhaps the buzzing and whirring coming from inside sounds like mating calls?
I am just trying to understand through what area do they get in. When I look at my MBP I can only think of the exhaust or through the keyboard opening under the keys.