Sounds to me like 1 of the Calliphorid flies or " dumb house flies ".
2.
Calliphorid flies disperse an average of before pupation and remain in the pupal stage for 3 20 ( or more ) days.
3.
Calliphorid flies have three larval instars lasting 421 days in all . " C . vicina ", a close relative to " C . livida ", has larval growth estimated to be at.