312 ENTOMOLOGY 



electrically-operated machine, invented by Mr. John Hays Hammond, 

 Jr., is a box containing the mechanism and mounted on three wheels. 

 Two of these are geared to a driving motor and the third, on the rear 

 end, can be turned by means of electro-magnets in a horizontal plane. 

 A pair of five-inch condensing lenses on the front end look like large 

 eyes. If a portable electric light, as a hand flashlight, be turned on 

 in front of the machine this will immediately move toward the light and 

 will follow the light all around the room in complex manceuvers at a 

 speed of about three feet per second. Upon shading or switching 

 off the light the "dog," as it is called, can be stopped at once, but will 

 resume its uncanny movements as soon as the light reaches the "eyes" 

 of the machine in sufficient intensity. The orientation mechanism 

 possesses two selenium cells, one behind each "eye," which when influ- 

 enced by light effect the control of sensitive relays, analogous to the nerv- 

 ous system of a moth. These relays operate electro-magnetic switches, 

 which control the driving motors and the steering wheel. 



The principle of this mechanism has been applied to the "Hammond 

 dirigible torpedo." 



Thermotropism. Ants are strongly thermotropic; they carry their 

 eggs, larvae and pupae from a cooler to a warmer place or vice versa, 

 and thus secure optimum conditions of temperature. Caterpillars and 

 cockroaches migrate to regions of optimum temperature. 



In thermotropism it appears that the direction of heat rays has 

 little or no effect as compared with differences of intensity. 



Tropisms in General. Other kinds of tropisms are known, for 

 example, tonotropism, or the control of the direction of locomotion 

 by density, and electrotropism (gahanotropism) ; not to mention any 

 more. 



All these phenomena are responses of protoplasm to definite stimuli 

 and are almost as inevitable as the response of a needle to a magnet.. 



The tropisms of the lower organisms have been experimented upon 

 by many skilled investigators, whose results furnish a broad basis for 

 the study of the subject in the higher animals. Even in the simplest 

 organisms, behavior is the resultant effect of several or many stimuli 

 acting at once, and the precise effect of each stimulus can be ascertained 

 only by the most guarded kind of experimentation; while in the higher 

 animals, with their complex organization, including specialized sense 

 organs, the study of behavior becomes intricate and cannot be carried 

 on intelligently without an extensive knowledge of the behavior of uni- 

 cellular organisms. The properties of protoplasm are the key to the 



