238 



SCIENCE. 



[N. S. Vol. IX. No. 216. 



germs of Asiatic cholera and typhoid fever 

 by flies ; the demonstration claimed by 

 Finlay of the carriage of a mild type of 

 yellow fever by mosquitoes ; the suggestion 

 by Hubbard that the ' pink eye ' of the 

 South is spread by Hippelates ; the well- 

 recognized fact among the Europeans of the 

 Fiji Islands that without a veil a serious 

 native eye disease will spread through the 

 medium of gnats ; the suggestion 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 Taba- 

 nus and Stomoxys — all indicate an impor- 

 tant and very injurious function of insects 

 practically unsuspected until comparatively 

 recent years. It is, in fact, a rapidly in- 

 creasing field of investigations, the possi- 

 bilities of which cannot be accurately estab- 

 lished at the present time. It is, however, 

 not a field which should be left entirely to 

 the medical bacteriologist ; the entomolo- 

 gist 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 conclu- 

 sions. For example, it is a fact probably 

 unknown to the medical men who may be 

 strongly impressed by the suggested car- 

 riage of typhoid germs by flies, that the 

 house fly, so common in our dining rooms, 

 does not breed in and rarely 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 occur- 

 rence a departure from the normal food 

 habits of the adult. 



Enough has been shown, however, to em- 

 phasize the potentiality of this phase of 

 insect injury. 



BENEFITS 

 AS DESTEOYEES OF INJUEIOUS 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 in- 

 sects. In fact, to the student of insects who 

 has followed out even a single chain of these 

 inter-relationships the thought must neces- 

 sarily come that upon its organic environ- 

 ment, 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. 

 Temperature is the great factor which con- 

 trols the geographical distribution 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, themselves affected by 

 other living causes in an almost endless 

 chain, sometimes, to all appearance, dwarf 

 even temperature as a controlling factor. 



There is not a species of insect that has 

 not its natural enemies in the guise of other 

 insects ; there is not one of these other in- 

 sects which has not its own insect foes. 

 From a single species of Bombycid moth, 

 the larvse of which fi-equently damage 

 forests in Europe to an alarming extent, 

 there have been reared no less than sixty 

 species of hymenopterous parasites. From a 



