DRONE-FLIES. 199 



until the tail is two inches in length, and 

 graduates to a thread-like point. If these grubs 

 were thrown into deep water they would be 

 drowned, being suffocated from want of air, but 

 in ditches, where they are usually found, they 

 can crawl along in the mud by means of very 

 small legs on the thorax and abdomen, and 

 ascend the sloping bank until they reach the 

 needful air. Respiration is carried on by means 

 of a double air-tube within the tail. When at 

 its full expansion these tubes lie parallel to 

 each other, but when the tail is retracted the 

 tubes fall into two coils at the base, where it 

 issues from the body of the grub truly a mar- 

 vellous piece of mechanism for such a lowly 

 creature. The most noisome black mud is the 

 favourite habitat of this rat-tailed maggot, as it 

 is called, and to it we owe a deep debt of grati- 

 tude, since, repulsive as it may appear to our eyes, 

 its life-work is to purify such foul places as would 

 pollute the air we breathe ; it feeds and luxuriates 

 upon that which is full of the germs of fever and 

 mortality to us, and then, when full grown, it buries 

 itself in the ground to come forth in due time as a 

 bright-winged fly. 



