February, '17] Hewitt : insect behaviour 87 



of the manure pile on the larvae I showed that the larval habitat in the 

 well packed pile was peripheral, and that excessive internal heat be- 

 came practically a larvicide. Recently, Copeman has shown that 

 practical use can be made of this principle and that close-packing of 

 the manure is all that is necessary to prevent the breeding of flies. 

 In a recent letter to me Copeman states that this method of control is 

 being taken up by the military authorities on an extensive scale both 

 in England and abroad. 



Just as high temperatures are effective in insect control so also low 

 temperatures have a like value, as is now well known. And the util- 

 ization of low temperatures in applied entomology offers a fruitful 

 field for further investigation. Further, in northern countries exact 

 knowledge concerning the relation of low winter temperatures, asso- 

 ciated as a rule with degrees of humidity, to the distribution of insects 

 is highly desirable. 



The activities of all insects are so closely related to temperature that 

 no study of their behaviour can be made without full consideration of 

 its effects. 



Closely linked with thermotropic reactions are the effects of hydro- 

 tropism, and particularly humidity. This is especially the case in the 

 effects of climate on insect distribution and migration. The theory 

 advanced by Ellsworth Huntingdon in his " Civilization and Climate, " 

 that as climates change nations either change with them or migrate 

 when the change is unfavorable to more suitable climatic environment, 

 is equally applicable to insect life. We must ever take into account 

 the effect of the" climatic stimulus on insect behaviour. 



In a recent suggestive paper Pierce has called attention" to the fact 

 that "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 approxi- 

 mately near 83 degrees F. and 65 per cent of relative humidity." 

 This author has also pointed out the practical applications of a knowl- 

 edge of the relation of climatic conditions to the control not only of 

 the cotton boll weevil, but also to such pests as the cattle tick and the 

 fall army-worm. Other cases are numerous. 



We have always to bear in mind that a tropisin may include reac- 

 tions to a stimulus existing in very diverse forms. This fact is well 

 demonstrated in the case of hydrotropism. The negative hydro- 

 tropism of the salt-marsh mosquitoes of New Jersey and San Francisco 

 is a reaction to water en masse, as is the positive hydj'otropism of 

 aquatic Coleoptera and Hemiptera. On the other hand, the reaction 

 of insects to humidity is a hydrotropism that may be brought about 



