STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 59 



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 scientific data obtained in recent years enable us to 

 speak more definitely regarding 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 efiiciency. With the boll weevil these points lie approxi- 

 mately near 83°F., and 65 per cent, relative humidity." 



Ewing has found that a constant temperature of go^F. prevents 

 the development of Aphis avencB, and that the optimum temperature 

 for the production of the wingless agamic forms is about 65°F. 



The larvae of the common house-fly are killed at a temperature of 

 io5°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 wheat midge. 



Gardeners and florists have long observed that red spiders and most 

 species of thrips are more abundant, and hence more injurious, under 

 warm dry conditions. 



The investigations of Bachmetjew show that humidity is an im- 

 ortant factor modifying the effects of temperature, and that the 



