THE ECONOMIC STATUS OF INSECTS AS A CLASS, 557 



eye disease will spread through the medium of gnats. The suggestiou 

 by Symond of the agency of fleas in the spread of the bubonic plague; 

 the demonstration of anthrax bacilli in malignant pustules in human 

 beings, caused by the bite of Tabauus and Stomoxys — all indicate an 

 important and very injurious function of insects practically unsus- 

 pected until comparatively recent years. It is, in fact, a rapidly 

 increasing field of investigations, the possibilities of which can not be 

 accurately established at the present time. It is, however, not a field 

 which should be left entirely to the medical bacteriologist; the ento- 

 mologist should have a share. The life histories and habits of the 

 insects concerned in the damage should be thoroughly understood, since 

 it is not impossible that otherwise the medical investigators may find 

 themselves arriving at perhaps unwarranted conclusions. For exam- 

 ple, it is a fact probably unknown to the medical men who may be 

 strongly impressed by the suggested carriage of typhoid germs by flies, 

 that the house fly so common in our dining rooms, rarely breeds in 

 and seldom visits human excrement, while those other kinds of flies, 

 which do so breed, are rarely attracted to articles of food used by 

 human beings. In the crowded and unnatural conditions of army 

 camps, however, and especially where cavalry regiments are stationed 

 so that there are great amounts of horse manure, the house fly may 

 breed in such enormous numbers as to render of very likely occurrence 

 a departure from the normal food habits of the adult. 



Enough has been shown, however, to emphasize the potentiality of 

 this phase of insect injury. 



Benefits. 



as destroyers of injurious insects. 



The economic bearings of insect enemies of insects are very great, 

 and perhaps this is, all things considered, the most important of the 

 beneficial function of insects as a class. 



In the eternal warfare of organism upon organism, in the perpetual 

 strife of species, one preying upon another and that upon a third, the 

 complications of relations of forms which determine the abundance of 

 one species and the scarcity of another are nowhere more marked than 

 among the insects. In fact, to the student of insects who has followed 

 out even a single chain of these interrelationships the thought must 

 necessarily come that upon its organic environment, and especially 

 upon its relations with its living neighbors of the animal kingdom, 

 depend the chances of a species not only for increase, but for survival 

 almost to no lesser degree than upon its inorganic environment. Tem- 

 perature is the great factor which controls the geographical distribu- 

 tion of life, and temperature is at the back of all these apparent living 

 first causes which control the abundance of a species in a given region, 

 provided we trace them far enough. Yet these living causes, them- 



