90 THE EEPORT OF THE No. 36 



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 

 larva; come above ground, but they still display a modified negative phototropism 

 and migrate in a north-westerly 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. 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 themselves with regard to their food and then move 

 towards 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. 



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 taxic* reactions are very adaptive. Ants and aphids 

 are positively phototaxic when they get wings ; and honey bees are periodically 

 phototaxic, thus leading to swarming. Ants, moreover, are strongly thermotaxic, 

 thus securing for their brood the optimum temperature conditions. 



Relation of Insects to Temperature and Humidity. 



Two important factors influencing the life of insects are temperature and 

 humidity. Their general regulatory action has been known for a long time^, but 

 scientiflc data obtained in recent years enable us to speak more definitely regard- 

 ing the behavior of insects toward the varying temperature and humidity of 

 their environment. 



Pierce in his studies of the Cotton Boll Weevil and other forms says : '' A 

 careful study of the records of any species, charting for the time required for 

 each activity and the temperature and then similarly for the humidity will disclose 

 temperature and humidity points of maximum efficiency. With the Boll Weevil 

 these points lie approximately near 83 deg. F. and 65 per cent, relative humidity." 



Ewing has found that a constant temperature of 90 deg. F. prevents the 

 development of Aphis avence, and that the optimum temperature for the production 

 of the wingless agamic forms is about 65 deg. F. The larvs of the common 

 House Fly are killed at a temperature of 105 deg. F., and the close-packing 

 of manure is sufficient to prevent the breeding of flies. 



With regard to changes in humidity insects vary somewhat widely in their 

 reactions. For example, moist air is favorable to most aphids and hastens the 

 development of the larva of the Hessian Fly. On the other hand dry seasons 

 favor the development of the Chinch Bug and the Wheat Midge. 



The investigations of Bachmetjew show that humidity is an important factor 

 modifying the effects of temperature, and that the metabolic activities of insects 



*The term taxic is now more commonly used than tropic, when applied to the move- 

 ments of animals under the action of stimuli just referred to. 



