Is the interior of the microwave (where you put the food) sealed? I.e., is there any way that ants (or other insects) could get into it when it’s not running, but the door is closed?
Depends on the brand, I guess. My Panasonic is definitely not sealed. There’s three vent panels on the inside, with holes large enough for ants to get in.
There’s a fan which blows air around in there. I doubt the thing is sealed, or else the fan wouldn’t do much good.
Not sealed, if it was and you boiled water in the M.W. you would have explosive decomperssion upon opening the door.
We had a spider get inside the door of our microwave. In the glass part where there’s that screen. Creepy.
The eentsy-weentsy spider crawled in the microwave.
He knew he shouldn’t be there, but he thought he would be brave.
On went the oven, microwaves from every side
And the eentsy-weentsy spider became a bug deep-fried.
Thanks, guys! I kinda thought so.
Soooo. What happens with microwaves exiting the machine via the vent? I recall that there was a big fuss when they first came out about seals on the doors so that the microwaves didn’t escape. 
The vents are sheilded against radiation leakage in the same manner as the door. See here.
[I’d be more worried about coming home to this…](http://www.imdb.com/title/tt0047573/?fr=c2l0ZT1kZnxteD0yMH
xzZz0xfGxtPTIwMHx0dD1vbnxwbj0wfHE9dGhlbXxodG1sPTF8bm09b24_;fc=1;ft=122;fm=1)
It might be worth noting that microwaves are non-ionizing radiation meaning it isn’t the kind of radiation that will turn you into a mutant. That isn’t to say anyone should happily have their hand in an operating microwave oven as it will cook you but beyond heating you up a bit there should be no lasting effects from getting zapped by the occaional microwave.
The cooking area of the microwave is not sealed. The door is interlocked to prevent operation when open.
Any and all cracks, holes, etc. must be shorter than the wavelength of the microwaves to prevent leakage. A deplorable consequence is that all manner of varmits can get in to look for little bits of edibles.