222 NEW YORK STATE MUSEUM 
their sustenance between the upper and lower epidermis of one leaf, the 
jarvae tunneling apples and other fruits, or the borers in the trunks of trees, 
The practical farmer or fruit grower soon learns that another class of 
insects is not affected by such internal poisons as have been mentioned. 
Each season several reports are received at my office stating that paris 
green has no effect when sprayed on trees infested with aphids or plant 
lice. The simple reason for this is, that these forms belong to a large 
class known as sucking insects, because they extract the fluids upon 
which they live through a delicate beak inserted into the tissues. Con- 
sequently particles of internal poisons lying upon the surface of a food 
plant have no effect, as the tip of the tiny beak is beneath the surface when 
the insect draws its fill, It is not difficult to recognize these sucking 
insects by their work, and one soon becomes familiar with their general 
aspect. They never devour portions of plants. The most common 
evidence of injury is the withering or wilting of either entire leaves or 
limited portions. A small lens will readily show whether the injury is due 
te a biting or sucking insect. Some of the more common species falling 
in this latter class are all plant lice, scale insects and the larger forms 
like the squash bug, the four-lined leaf bug and their allies. 
Operation of contact insecticides. Asa rule it is best to protect 
a plant by applying paris green or other poison to its foliage, provided 
the insect is one that devours. Otherwise, recourse must be had to con- 
tact insecticides, that is, to substances that will kill an insect when 
brought into contact with it. None of those commonly used are so 
deadly to insect life, that they will kill by simply touching the victim, at 
a single point. They operate by closing or choking the breathing pores 
or spiracles, along the sides of the caterpillar, or they may exert a para. 
lyzing influence through these orifices. Therefore it will be seen at once 
that success will be in proportion to the thoroughness with which the 
treatment is performed. Insects not hit by the contact insecticide are 
unharmed, those barely touched, if affected at all, may recover, Only 
those well covered with the substance succumb. It is by no means easy to 
secure the desired results, even with the aid of a good spraying apparatus. 
Some insects take flight so quickly that it is practically impossible to hit 
many with a spray. Among these are the small leaf hoppers which are 
sometimes very injurious to certain plants, and about the only way they 
can be killed is by catching them on sticky fans or screens carried 
between the rows. Many plant lice secrete a woolly substance which 
serves to protect them from the action of insecticides, The drop of 
honey dew on the young pear psylla andthe hard covering of many 
scale insects serve the same purpose. ‘The attacks of some plant lice 
cause the leaves to curl and thus they are protected from deadly sprays. 
