DIPTERA OF MINNESOTA. 55 



a long band on the ground, two, three, four or niorc inches wide, 

 nearly half an inch deep and sometimes ten feet or more long. Some 

 observers claim to have seen bands from forty to one hundred feet 

 long, five or six inches in width and one inch deep. This peculiar 

 column advances, slowly, at the rate of about four inches in five 

 minutes ; those in the rear working toward the front over the tops 

 of the others. On account of this peculiarity the species which have 

 this habit are spoken of as the "Sciara Army Worm." The Germans 

 refer to it as "Heerwurm." What is the purpose of these migra- 

 tions in every case is not known ; it cannot always be to obtain food, 

 because they are frequently full grown when the journey is under- 

 taken. 



Fig. 41. Sciara. Original. The lines below fly indicate natural size. 



Another peculiar trait of Mycctophilids is the luminosity, a rare 

 thing amongst Diptera, shown by certain larval forms of the genus 

 Sciara. Some members of this genus, too, are said to make galls 

 on plants. 



There are probably nearly i.ooo species of this family known, 

 and others not described. Economically they are of some importance. 

 Hopkins has shown that Epidapns scabies is the cause of potato scab; 

 the larvae of Sciara mali "destroy the interior of apples," particularly 

 those that have already been injured by the Codling Moth. The fond- 

 ness of some of the species for edible mushrooms has already been 



