14 PROTECTION OF PLANTS, 1915-16 



eaten this response disappears, showing that tropism reactions are some- 

 times dependent on the state of the body. 



Mr. Strickland of the Entomological Branch, Ottawa, observed 

 during the outbreak of the Western Cutworm Chorizagrotis auxiliaris 

 and Porosagrotis orthogonia in Alberta in 1914-1915, that their move- 

 ment in army-worm fashion was always in a definite direction — north- 

 westerh— in response to a phototropic stimulus. 



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

 to be either positively or negatively thigmotropic. Cockroaches 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 stimuli influencing the 

 movements of insects. Maggots orient themsehes 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 de- 

 position of eggs by most insects on certain plants is also the result of chemo- 

 tropism. 



Wheeler and Loeb give several examples of geotropism among insects. 

 They observed that lady-birds and cockroaches at rest placed themselves 

 on vertical rather than horizontal surfaces. 



Observations show that tropic reactions are very adaptive. Ants 

 and aphids are positively phototropic when they get wings; and honey bees 

 are periodically phototropic, thus leading to swarming. Ants, moreover, 

 are strongly thermotropic, thus securing for theif brood the optimum 

 temperature conditions. 



While tropic reactions are adaptive they are not always beneficial. 

 For example, moths are attracted to the flame and are killed, and many 

 insects in following the stimulus become a prey to their enemies. 



Death-Feigning Reactions. 



Mention should be made here of a valuable study of Death Feigning 

 Reactions in Tychius picirostris by Mr. E. M. DuPorte of the Biology 

 Department of Macdonald College. An account of the study appears in 

 the current number of the "Journal of Animal Behaviour." This death- 

 feigning habit is quite common among insects, and scientists are not 

 agreed as to the meaning of it. 



Mr. DuPorte shows that intensity of shock has no effect on the produc- 

 tion or duration of the death-feint; that the duration of the feint is variable 



