272 NATURAL HISTORY OF THE FARM 



syrphus-fly larvae impale them and suck their blood. This 

 destruction is wrought openly. But greater destruction is 

 often wrought by minute parasites that feed unobserved 

 on the internal tissues of the aphids. Their work is evident 

 mainly in the dead and empty aphid skins, each with a 

 round hole in its back from which a little winged parasite 

 has emerged when fully grown. 



Study 39. Insects at Work on Farm Crops 



This study may be made at any time excepting when the 

 vegetation is wet. The equipment needed will be lenses, 

 insect nets, and cyanide bottles or vials of alcohol to hold the 

 specimens of insects found, pending their identification. 



The program of work will consist of a trip into the field 

 for collecting and observing the insects that are at work 

 upon the crops. Many pests may be located by the dis- 

 colorations and deformations of plant tissues they produce: 

 curling of the tops, ragged outline of leaves, yellowing, etc. 

 A few, like the potato-beetle larvae, are so conspicuous in 

 color and position as not to be easily missed. Some, notably 

 aphids, chinch-bugs, etc., are in dense colonies; but most are 

 solitary and protectively colored, and difficult to see. The 

 grass and herbage is full of plant-bugs and caterpillars, that 

 one would not notice ordinarily, but that are readily found 

 by "sweeping" the leaves with a net. Then having found 

 out what to look for and where to look, specimens may be 

 observed at work upon the plant. Species working where 

 less easily discovered, as in the stems or fruits, or under- 

 ground on the roots, may be pointed out by the instructor. 

 The treating of biting insects with food-poisons, and of the suck- 

 ing insects with contact-insecticides, may be demonstrated 



The work may cover either the commoner insects of a 

 number of crops, or a more careful collation and comparison 

 of all the pests present on some one crop. 



