April, ’23] 
MOORE: NEED OF CHEMISTRY BY ENTOMOLOGISTS 
175 
use of the one are unfavorable for the use of the other. Bordeaux 
mixture will have more influence on the toxicity of lead arsenate than 
of calcium arsenate. 
The arsenic usually classed as water soluble may be in the form of 
a very soluble or a slightly soluble compound. It may even be colloidal. 
These differences are generally ignored but are important since they 
influence the amount of injury to the foliage. 
The value of lime sulphur in the control of scale insects has recently 
been questioned. Certain chemical reactions produce calcium poly¬ 
sulphide while a different series of reactions result in its decomposition. 
By having conditions favorable for the formation of the polysulphide 
and unfavorable for its decomposition, good lime sulphur may be made 
without the use of heat. Under similar conditions the sulphur 
may be kept in the form of a polysuphide for longer periods of time than 
is possible with ordinary lime sulphur. May not such a preparation 
have greater insecticidal value? 
Slight and what may appear trivial differences in the chemical con¬ 
stitution of the insecticide may cause decidedly different results. A 
potassium soap may be more injurious to foliage than a sodium soap 
altho made from the same oil and having the same alkalinity and the 
same water and glycerine content. How often do we see the - recommen¬ 
dation of soap without any specification as to its composition? 
In cattle dips the mere change from the arsenite to the arsenate is 
accompanied by a change in its insecticidal value altho the same amount 
of arsenic is still present in solution. 
Even the arrangement of the groups in the molecule of an organic 
compound may make a great difference in its effects. M-nitro-p- 
toluidine is slightly more injurious to bean foliage than p-nitro-o-tolui- 
dine, but the latter compound is more toxic to Epilachna borealis Fab. 
Both of these chemicals have the same groups, differing only in the 
position of the radicals in' the benzene ring. 
Conclusion 
These few problems picked at random from the field of insecticides 
will serve to show that the economic entomologist should be well ground¬ 
ed in chemistry. Having completed the general courses in physics 
and chemistry what further work should the student of economic 
entomology take in the limited time available for these subjects? It 
should be borne in mind that the object sought is, not the making of a 
•chemist but rather, the production of an entomologist with sufficient 
