446 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 13 
they are, enmeshed in the trachac close to the spiracles, their function 
is no doubt respiratory. Moore has pointed out that the spiracles are 
the weakest link in the insect’s armor against contact insecticide. 
He has also shown that the heavy vapors of petroleum and the other 
volatile contact insecticides are the most efficient. These heavy gases 
will condense on the walls of the trachse. There may be some relation 
between these various observations but more work must be carried 
out before conclusions can be drawn. 
Moore’s work on the physical properties of contact insecticides is a 
valuable contribution to our knowledge of petroleum insecticides. His 
definition of wetting and spreading will eliminate confusion. 
A knowledge of the chemistry of petroleum may make it possible to 
find compounds fatal to insects and not injurious to plants. Unfortu¬ 
nately oils penetrate plant tissues about as easily as they penetrate into 
the insect. The history of petroleum sprays has been a development of 
methods to protect the plant from the injury caused by the commercial 
oils. The emulsion has made it possible to dilute the oil with water. 
The early mechanical mixtures gave trouble. The soap emulsions have 
proved satisfactory where soft water is available. The invention of 
the miscible oil has made it possible to commercialize the soap emul¬ 
sion. Unfortunately the miscible oil formula is not adapted to the 
heavy oils. We have found in California that a natural crude oil of 
about 24° Be is the most efficient spray to control diaspine scales. 
This oil, coming direct from the wells, contains volatile and heavy 
fractions. The heavy fractions are no doubt valuable in that they 
control the rate of evaporation of the lighter and more toxic fractions. 
A blend of distillate and lubricating oil gives the same result. The 
high price of cresol soap has made the cost of miscible oils rather high 
compared to the home-made emulsion, that is if the cost is compared 
on the basis of petroleum content. The latest improvement has been 
along the line of colloidal emulsions. These can be made with any oil 
and being chemically inert will mix with hard water, lime sulphur 
solution, arsenates, etc. By using a mixture of different colloids as an 
emulsifier good spread and penetration can be obtained. 
These emulsions have all been developed with the idea of protecting 
the plant from oil injury. Professor George P. Gray made a funda¬ 
mental advance when he discovered that the unsaturated petroleum 
compounds were, as a class, far more injurious to plants than the 
saturated series. 
Modern synthetic chemistry is making available hundreds of thou¬ 
sands of compounds not only derived from petroleum but from other 
sources. Some of these are undoubtedly toxic to insects. The scarcity 
of petroleum with its resulting high cost may make it profitable to hunt 
