Preflight inspection revealed the presence of a number of non-revenue passengers on board the aircraft. Wasps had entered one wing through a vent and were building a nest inside it. The weight of a paper nest and a few dozen wasps wouldn't affect the flight properties of the aircraft, but that vent would give them access to the cockpit. It might negatively impact flight safety if I were to share the cockpit with a number of disgruntled poison-injecting stinging creatures.
I'm not afraid of nor allergic to wasps, but a pilot shouldn't try to do everything herself. I believe in CRM. I called maintenance. The guys smiled tolerantly, as though I had somehow caused the wasps' presence through shear bumbling pilotude. They promised to see to the problem. True to their word, they had evicted the wasps by the time the preflight paperwork was complete. One lone straggler staggered out during the runup, and the rest of the flight was wasp-free.
The next day, I noticed a wasp-coloured object lodged deep inside the opening to the stall horn. I didn't think it would affect the function of the stall horn, so I left it, intending to tell maintenance later that there was still a dead wasp in there. I flew the airplane, then before the next flight noticed that the object had moved closer to the wing opening. I took the cap of a pen and poked it inside, hoping to dislodge the stripey corpse. It moved, and not just because I was poking it with a pen. After a bit more poking and manipulation, I turned the wasp head-side out and it crawled out through the opening in the leading edge of the wing, and flew away. Apparently it had endured at least two flights in there. Tough little critter.