beings: rat tailed maggot
The eggs are laid by strongly polluted water, nowadays these a manure pits. The rat-tailed maggots feeds on organic particles, algae and most often on the bacteria that lives in the manure pit. As there is abundant bacteria and no preditors that can survive in a manure pit, the rat-tailed larva will thrive strongly.
http://www.waterwereld.nu/dronefly.php