Have you seen anything strange in your water bins amongst the rotting stagnant stench of vegetation that you call Comfrey Liquid Manure? Does it look as though it is not of this earth? Extra Terrestrial perhaps like something you've seen before but can't quite place where? A 50s or 60s B movie called Invasion of The Body Snatchers perhaps? If the answer is yes don't panic they are not an alien species, they are the larvae of the Drone Fly (Eristalis Tenax) which is found in most parts of the world.

They lay their eggs near manure heaps or on the edge of stagnant water into which the larvae migrate. The larvae are commonly known as rat tailed maggots because of their breathing tubes which are attached to their arses which can be up to 6 inches (15cms) long to reach the surface of the water. They move to higher ground to pupate . The males hover 5- 10 ft above the ground while waiting for females, hence why you are more likely to see the larvae. The insects feed on nectar and pollen from flowers which  are ground up between the toothplates  at the tip of the proboscis as the fly inflates and deflates this with blood, the pollen being partially dissolved during this process. Some overwinter in caves and hollow trees and can be seen throughout the year.  http://en.wikipedia.org/wiki/Rat-tailed_maggot