80 
Applied Entomology 
2. Contact poisons comprise a very wide range of chemical sub¬ 
stances which ai’e used either as sprays, washes, fumigants, or soil 
dressings with the object of killing injurious insects by means of surface 
contact with the latter. They may be divided into :— 
a. Soaps and various alkalies. 
h. Pyrethrum and alkaloids of tobacco and other plants. 
c. Various phenol compounds and mineral oils. 
d. Rosin, and lime and sulphur compounds. 
e. Poisonous gases such as hydrocyanic acid and bisulphide of 
carbon. 
It may be added that there is a very wide field of research open for 
an experimental study of the physiological action of these substances. 
Does the egg, larva, pupa, or perfect insect furnish the best opportunity* 
for their application ? Further, if a compound is effective against one 
species of insect it by no means follows that it is equally effective for 
many others. A leaf-feeding larva may be killed by poisoning the 
plant it feeds upon, but sucking insects, which subsist upon the nutritive 
juices drawn from the interior of the plant, can usually only be dealt 
with by means of contact poisons. In some cases these insecticides kill 
by means of direct surface contact by “burning” the skin. In other- 
instances they affect the respiratory apparatus by forming a coatirrg 
over the body, thus closing up the apertures of the spiracles. Whether 
they ever enter the respiratory systenr in other tharr the gaseous state 
is a doubtful point. On account of the small size of the spiracles, the 
extremely elaborate closing and guarding apparatus with which the 
latter are provided, and the narrow calibre of the tracheal branches, it 
seems doubtful whether insecticides (in the form of liquids or powders) 
could ever enter the respiratory system in sufficient quantities to be 
effective. Maxwell-Lefroy (1906, p. 81) states that kerosene emulsion 
acts partly by mechanically closing the spiracles thei’eby inducing 
asphyxiation ; possibly it also has a poisonous effect on the tissues of 
insects when its vapour is gradually absorbed through the tracheal system. 
Dewitz (1912, p. 352) is inclined to believe that contact insecticides, 
both in the liquid and powder form, have a fatal effect on the sense 
organs of insects. He quotes the statement of Fujitani (1903, p. -I?) 
that the active agent of pyrethrum is a nerve and muscle poison to 
which fishes and insects are very sensitive. The physiological action 
of soil dressings is likewise not understood. The whole subject of the 
physiological effect of insecticides has never been adequately studied; 
a much more exact knowledge of the subject is necessary before many 
