58 ECONOMIC ENTOMOLOGY 



Again, most moths move away from sunlight but move toward a 

 lesser light such as electric or oil lamps. Davenport explains this 

 difference by saying that " butterflies are attuned to a high intensity of 

 light, moths to a low intensity." Loeb explains the circling of moths 

 and other insects about a light. The stimulus orients the insect by its 

 more intense action on the muscles next the light, and the insect then 

 moves toward the light. 



Loeb states that caterpillars of the brown tail moth as they emerge 

 from hibernation in spring are positively phototropic, but after they 

 have eaten this response disappears, showing that taxic reactions are 

 sometimes dependent on the state of the body. 



"Swaine finds that the destruction of piled logs by the wood-boring 

 larvae of the sun-loving Monohammus can be prevented by forming a 

 dense shade over the logs by means of brush. In his study of the army 

 cutworm {Euxoa auxiliaris) in Alberta, Strickland found that the larvae 

 are negatively phototropic and hide beneath the soil till about four or 

 five o'clock in the afternoon when they come to the surface to feed. 

 With the weaker light they become positively phototropic and a 

 general migration in a westerly direction takes place. When food is 

 scarce hunger may overcome their aversion to sunshine with the result 

 that the larvae come above ground, but they still display a modified 

 negative phototropism and migrate in a northwesterly direction. 

 These facts are of practical value in controlling outbreaks of this insect 

 (Hewitt)." 



Insects are very responsive to the stimulus of heat, i.e., they are 

 thermotactic. 



Some insects respond to the stimulus of touch or contact, and 

 are said to be either positively or negatively thigmotactic. Cock- 

 roaches are in the habit of squeezing into narrow crevices, and Loeb 

 mentions the case of a moth Pyrophila which also has the same habit. 

 Chemical substances and foods also act as stimuH influencing the 

 movements of insects. Maggots orient themselves with regard to 

 their food and then move toward it, the orientation being the result of 

 unequal chemical stimulation of the muscles of the two sides of the 

 body. The deposition of eggs by most insects on certain plants is also 

 the result of chemotropism. The house-fly and many piercing insects 

 such as the biting flies and mosquitoes are repelled by phenol and other 

 coal tar products. 



