LIFE-HISTORY OF THE DRONE-FLY 261 



developed in the drone-fly (Eristalis) larva, a scavenging 

 creature that does immensely valuable service in render- 

 ing stagnant water and decaying substances innocuous. 

 This larva, the ' rat-tailed maggot/ though only two- 

 thirds of an inch in body-length, can extend its tail from 

 half-an-inch to four inches in length, drawing it out to 

 a mere colourless thread, and at the least alarm 

 shutting it up in a flash. The mode of feeding no less 

 than the breathing of this animal shows a most complex 

 form of adaptation. The organic debris on which it 

 lives is highly innutritions, and a special collecting 

 apparatus has arisen to enable the bee-fly larva to 

 make good the lack of quality in its nutriment. This 

 apparatus is a strainer opening inwards, placed between 

 the mouth, and the gullet, and dividing the gullet 

 into upper and lower chambers. The larva roots 

 about with its flexible muzzle, and, having loosened 

 a quantity of debris, gulps down a mouthful. This 

 passes easily through the strainer and is sucked 

 into the upper chamber. Then the water is forced 

 back, filtered through the fringes, and discharged 

 through the mouth, whilst the food is left stranded on 

 the strainer. The process is repeated again and again. 

 When a sufficient quantity has accumulated it is 

 converted into a bolus and swallowed. The action of 

 the fringes is curiously like that of the baleen-plates 

 of a whale. 



When this rat-tailed larva is about to pupate, 

 it leaves the water, enters damp earth, and conceals 

 itself beneath a cocoon made of the larval skin, dis- 



