NA TURAL HIS TOR Y OF MA Y FLIES. 107 



or those parts of the rivers where the current 

 is least strong. Although these larvae are 

 poorly provided with the means of swimming, 

 they can manage by serpent-like movements 

 to make fairly rapid progress ; but their short 

 legs and long bodies make it difficult for 

 them to stop when the current draws them. 

 The water rolls them over and carries them 

 off, unless they can manage to attach them- 

 selves to a stone to resist its action. 



The first care of the young larvae is to con- 

 struct a house. These retreats consist of 

 tubular galleries, straight or slightly curved 

 and horizontal, dug out of the clay or mud. 

 They dig with their mandibles and fore-feet. 

 The diameter of the tube is a little larger 

 than that of their body, but nevertheless not 

 sufficient for them to attain their full growth. 

 As they grow they change their galleries, 

 which are generally very slight and of little 

 durability. In some places even the move- 

 ment of the water obliges them to change 

 more often still, for they do not like to be far 

 from the surface ; still, they must be in the 

 water. When the river has a variable height, 

 these insects have to change their habitation 

 frequently. Thus the turning on and off the 

 water for a mill stream must be a source of 

 great trouble and inconvenience to these little 



