toilet drain

hi,Iam new to forum,doing a basement finish in my new house .The house
is 1 year old.When we had house built,had rough in added to basement.
the project is going good,but when I cut into the 4 inch pvc pipe in
floor to mount toilet,the pipe was full of flying insects and bugs.
quickly temp re-glued pipe back on.Is this normal or is something not
connected under basement floor?
thanks trl

Pour a 5 gallon bucket of water down the drain a couple of times. You're most likely hooked up to the building drain and the water will help clear out the sewer gnats you're seeing. After you've done this have someone flush a toilet upstairs and listen to the 4" pipe you've just poured the water down. You should hear the water from the toilet above through the pipe.

Good luck