They are fungus gnats, their maggots eat the roots of plants, faster that the plant can heal itself, leaving your mj open to all sorts of problems around the roots, and in the end killing your mj, you will have to search up on google a organic remedy, when i had the problem i put a slice of potato in the top of the soil to try and trap them, but i could never get them all, i tryed tobacco on the top soil, killed my plant, hydrogen paroxide is meant to help but involves drying out your plant and its not guarenteed, (my plant died) lost a whole crop to these mother f***ers, i changed everything. Have a look on google, you may have more luck than me, please if you find somthing that works let me know