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. 
