272 NATURAL HISTORY OF THE FARM 



syr]:)hus-fly larvae iiu])alc tlicin and suck their Ijlood. This 

 destruction is WToug:ht openly. But greater destruction is 

 often wrought by minute parasites that feed unobser\^ed 

 on the inteniiil tissues of the aj^liids. Their work is evident 

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

 round hole in its back from wliich a little winged parasite 

 has emerged when fully growTi. 



Study 39. Insects at Work on Farm Crops 



Tliis study may be made at any tijne exce])ting 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 foimd, 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 defonnations 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 difBcult to see. The 

 grass and herbage is full of plant-bugs and cateriDillars, 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. 



