1903] WEBSTER: —DIFFUSION OF INSECTS 1^7 



none of these, nor in fact all of them combined, appear to influence our entomolo- 

 gical fauna to the extent that is probably exerted by the southwestern trend of dif- 

 fusion. The Transappalachian trend of diffusion appears not to exert any such 

 influences, and individuals of introduced species do not vary from those in their 

 ancient home in the eastern hemisphere. 



I have attempted to make my position more clear by the construction of a map 

 that shows the various trends of diffusion of which I have spoken. Since so little is 

 known of species occurring in Mexico, Central America, and the Antilles, and their 

 affinities to our own, it is folly to expect that such a map can be made more than 

 tentative, but it is a beginning. Criticism will provoke investigation, and, in any 

 case, some good must come of it and our knowledge be increased. It is not to be 

 supposed that the area covered by these lines is proportionate to the territory cov- 

 ered by the diffusion itself ; the lines only indicate directions. 



The Pacific Maritime trend of diffusion may include such species as inhabit 

 this coast or such as are forced down or drift down from the higher areas and estab- 

 lish themselves. Some, like Halisidota angnlifera in California, have a subalpine 

 form, like II. aim. The Chinch-bug also is not confined to the sea level, but prob- 

 ably finds the least resistance to its progress there. This trend of diffusion is indi- 

 cated on the map by the line A. The Atlantic Maritime trend is to be looked 

 upon as occupying a similar position on the east coast, and may be the avenue 

 along which the Pampean forms make their way from the level areas of South Amer- 

 ica into those of a similar character in North America. But there is probably the 

 same intermingling of tide water and subalpine forms as on the west coast. It must 

 be continually kept in mind that, though the two coasts are, in Central America, 

 often separated by comparatively short distances, yet the climate is as different as 

 might be expected to exist between localities hundreds of miles apart. This latter 

 trend is indicated on the map by the line C, and the Pampean range by a, because 

 insects that subsist on grasses, or such plants as are to be found in treeless areas, 

 will be more likely to push northward along the base of the rocky mountains and 

 then spread broadly to the eastward than will arboreal species that must hold more 

 closely to the wooded sections. Thus, the Diabroticas evidently hold to the north 

 from Mexico, while the Halisidotas must keep to the eastward. Both may spread 

 up the Mississippi River, as the country is both wooded and grassy. As I have 

 shown elsewhere, the Chinch-bug may have followed both the Pampean, a. a, or the 

 main trend, C. Of course the Mississippi River trend, b, lay open to it, but inun- 

 dations would probably have impeded its progress. The Tropical Subalpine trend 

 may be illustrated by the argentata system of the Halisidotas, and by other forms 

 of the high altitudes. The Antillean trend, D, is illustrated by the many tropical 

 species that have clearly entered Florida from the south, and not by the way of 



