August, 'OS! JOURNAL OF ECONOMIC ENTOMOLOGY 



255 



sparingly in southern New Hampshire, if at all. That the "sum of 

 effective temperatures" is an important factor is not disputed, but 

 evidence accumulates that the ''critical point" (in the sense of 

 phenology) from which the ''thermal constant" should be computed 

 varies with groups and species of animals and plants. For example, 

 melons and egg-plants are grown in eastern Massachusetts, but cannot 

 be matured successfully at Durham, N. H. They are planted in 

 Massachusetts about :\Iay 20 to '25 and in New Hampshire about June 

 ], and mature in :\Iassachusetts about September 1. The effective 

 temperature over 43°F. for Boston for the three months is 234'3°F. 

 and for Durham, N. H.. 206PF. The effective temperature in May 

 and September is of no value in this connection. That these plants 

 cannot be grown in New Hampshire is due to the fact that there is 

 not sufficient "effective temperature" over 60°F., which is the tem- 

 perature above which these plants must be grown, or their "critical 

 point." Boston has 801°F. over 60° in summer, while Durham, N. 

 H., has only 525°, or lacks 35% of the requisite effective temperature. 

 Other examples will be given below. 



Second. But even if the "effective temperatures" be accumulated 

 above the true "critical points" instead of over 43°F., still there are 

 numerous cases in which there is a sufficient positive temperature for 

 the development of species in southern New Hampshire which are 

 not known to breed there or in eastern Massachusetts commonly. 

 Some other law must therefore determine the limitation of these 

 species to a more southern clime. 



Third. It is well known that the main question in the introduc- 

 tion of horticultural varieties northward is one of "hardiness." Many 

 varieties will fruit and mature at latitudes where they cannot grow 

 on account of lack of hardiness. Probably as large a number are dis- 

 • qualified for northern growth on this account as by the shortness of 



the season. 



If the southern spread of some species is controlled by the heat 

 of summer, which is undoubtedly the case, why should not the di- 

 rect opposite be true, and why may not the northward spread be con- 

 trolled bv the cold of winter ? 



The following species have been studied with reference to these 

 objections and as t.^ whether the influence of minimum temperatures 

 offers anv explanation of their northern limits. 



The Harlequin Cabbage Bug {Murgantia histriouica) has migrated 

 from Mexico around the Atlantic coast to Long Island, N. Y., and up 

 the :^Iississippi Vallev to southern Ohio, where it occurred in five 

 counties bordering the Ohio River in 1895.^ ^ In 1899 the temper- 



iBulletin 68. Ohio Agricnlturul Experiment Station, p. 36 (1896). 



