1919.] NATURAL SCIENCES OF PHILADELPHIA. 61 



southern Arizona (Baboquivari Mountains — neo-mexicana and mac- 

 ulipennis maculipennis) . The genus is apparently absent, within 

 this area, from the east central states, excepting certain portions 

 of Illinois, and in the eastern states it has a very limited range, 

 being absent from almost the entire Piedmont and more elevated 

 regions, only a few records from above the fall-line in North Caro- 

 lina, Georgia and Alabama being known. As one passes north- 

 ward along the eastern coast the area of distribution narrows, 

 until, in Virginia, it is merely the narrow coastal and estuarine 

 marginal region. Beyond question a number of the species of 

 the genus range over much of northern Mexico, but the lack of 

 data from that region is so great that we can do no more than 

 mention the probability of their occurrence. 



All of the species are partial to grassland, tall salt marsh grass, 

 grassy fresh marsh, prairie grassland, grass patches in park-like 

 woodland, the clump type of bunch grass in arid steppe and sand- 

 hill regions and even grassy pockets on steep rocky slopes. While 

 frequently taken on other vegetation, the presence of grass will 

 be found to be the influencing factor, and the campestrian envir- 

 onment is the favorite situation. 



Zonally the genus Mermiria is an inhabitant of the Lower Aus- 

 tral, Sabalian and Tropical Florida Zones in the eastern states, 

 narrowly encroaching on the Upper Austral in the northeast, 

 while in the western and central United States it occurs in the 

 Upper and Lower Austral Zones, entering areas of the lower part 

 of the Transition Zone in western Texas (Davis Mountains) and 

 southern Arizona (Santa Catalina Mountains) and possibly in 

 the northwestern United States (Montana and North Dakota). 



When tested out for a probable center of distribution we find 

 that far the greater proportion of the factors, to be considered 

 in locating such a center, give central Texas as the center of the 

 generic dis-emination and differentiation. This is the region of 

 greatest differentiation, dominance in individuals, location of 

 synthetic forms (i. e., hivittata and maculipennis, neo-mexicana 

 and alacris), maximum size of individuals, convergence of lines 

 of dispersal and of least dependence on a restricted habitat.^ 



^ Our quotation of these criteria should not be taken as an indication that we 

 are convinced of the propriety of using all of them for such work. We are fol- 

 lowing the general practice in so using them, but at least one, i. e. area of max- 

 imum size of individuals, can hardly be given equal importance with the others. 

 The advent of species of insects into a favorable habitat, removed from their 

 normal natural enemy control, often produces colonies of maximum sized indi- 

 viduals. 



