GENERAL 



19 



(thus reversing his sickle), turns about and cuts another 

 swath back across the same field. The cross strokes of the 

 swath are always concave to rearward, as swinging in an 

 arc, necessitates. It follows that going and return swaths 

 have opposed concavities. A rather beautiful herringbone 

 pattern results from this, made in a single line of progression, 

 back and forth, redoubled, wavering at vein barriers and 

 becoming regular again when new leaf areas are entered 

 but continuous from the beginning to the end of the mine; 

 and in it one may read the complete record of the miner's 

 travels. 



Fig. 10. A leaf of ragweed, collapsed where mined by the moth, Tischeria 

 ambrosifoliella. 



The effect of the mining-operations on the plant varies 

 with the character of the leaf, as well as with the extent and 

 nature of the injury to the tissues. Thus the firm leaves 

 of the oaks stand up well under the attacks of a great variety 

 of miners, while the soft leaves of garden herbs, often col- 

 lapse rather quickly after even moderate injury. The 

 structural strength of the parts surrounding the mine deter- 

 mines this as is well seen by comparing the mines of two 

 species of Lithocollitis, — belonging to the group that, in the 

 late larval instars, normally makes tentiform mines. The 



