February, '17] ENTOMOLOGISTS' DISCUSSIONS 161 



as the fly-free date and the wheat sowing date October 28. What is 

 your fly-free date for Manhattan? 



Mr. G. a. Dean: October 4. 



Mr. a. D. Hopkins: Well, there too we miss it only by a day. 

 You cannot calculate across the country much closer. These theoreti- 

 cal dates will approximate the actual dates and thus serve as indices to 

 the proper date for any place in the area in which winter wheat is 

 sown, including Canada. The law of latitude, longitude, and altitude 

 enables us to give an approximate date for sowing wheat anywhere 

 from southern Canada to Texas, and we will be glad to help those of 

 you who are investigating the Hessian fly to apply this law to your 

 local problems. If you will give me the determined fly-free date, lati- 

 tude, longitude, and altitude for any place in your state, I will explain 

 how to find the theoretical fly-free dates for any other places in the 

 state. Then if you will take these dates as a basis for comparison 

 with previous or subsequent records of actual fly-free dates and let me 

 know later on what you find it will help us both, I think. There is 

 special need of local observations on such details. Here is an oppor- 

 tunity to carry out, on quite an extensive scale, investigations to deter- 

 mine the practical value of the law as applied to the Hessian fly. 



With a known date of the disappearance of the fly at the northern 

 range of winter wheat in Canada, we would have from four to seventy- 

 two days in which to inform the farmers in the United States when to 

 sow wheat at places south of the Canadian base. The actual date will 

 vary, of course, in different localities, due to local conditions, but such 

 constant departure will indicate the intensity of the local influence 

 and the number of plus or minus days required to connect the theoret- 

 ical date. If, on the other hand, it is a seasonal variation, as that re- 

 sulting from a drought, farmers can be instructed to wait until it rains 

 before sowing wheat. 



Mr. T. J. Headlee: How does longitude come in? I understand 

 quite well the matter of altitude and latitude, but when you deal with 

 longitude what is the factor? There is a principal factor that you 

 measure, and following that, where do you establish your base line? 



Mr. A. D. Hopkins: At any longitude. It makes no difference on 

 what meridian you start. A given periodical event in the spring will 

 be four days later 5° east and four days earlier 5° west and the reverse 

 in the fall. 



Mr. T. J. Headlee: What is the factor you are measuring? 



Mr. a. D. Hopkins: I do not attempt to explain that. I accept 

 the facts as I find them. Specialists in other branches of science will 

 have to explain the controlling factors. The fact that there is a varia- 

 tion due to longitude has been determined in Europe as well as in this 



