1908.] natural sciences of philadelphia. 463 



Relations of the Mexican-Central American Odonate Fauna 



TO THOSE of other ArEAS. 



The study of the species of Odonata found over large parts of Mexico 

 has shown that, to the northward, many of them occupy also con- 

 siderable portions of Texas, New Mexico, Arizona, and California 

 (in the last named possibly to San Francisco), although their north- 

 ern boundary line has not been determined in any of these States.'* 

 Therefore, all species, subspecies or varieties found in Mexico and 

 Central America and which may extend also into these four States of 

 the United States, but not beyond them, nor into the West Indies 

 nor South America, have been considered as endemic. With this 

 explanation, which applies to all the tables in this paper, the general 

 relations of the Odonate fauna may be learned from an inspection of 

 Table 1. 



A further analysis of the relations of the fauna is given in Table 2, 

 wherein, passing from north to south and, in Mexico, from plateau 

 to lowlands, the decrease in the northern element and the increase in 

 the southern element is clearly shown. Even on the Mexican plateau, 

 excluding its highest portion,^ as the most northern and most elevated 

 section of the present faunal district, the southern element, measured 

 by the number of the "exclusively South American" species, is almost 

 as strong as the northern, represented by the "exclusively Northern 

 American" species'' — a striking fact when the narrow land connection 

 with South America is contrasted with the very much wider union with 

 the United States, and the geological history of the plateau is borne 

 in mind. It is of further interest to note that while 15 exclusively 

 Northern American species are found on the plateau, 14 Northern 

 American species are found in Mexico exclusive of the plateau, the 

 corresponding figures for the exclusively South American species 

 being 14 and 50. These differences are in agreement with Gadow's 



* The existing data for about 40 Mexican species would seem to show that 

 their northern boundary line may correspond with the upper hmit of the Upper 

 Sonoran of Merriam (Map in Bull. 10, U. S. Dept. Agric, Div. Biol. Surv., 1S9S) 

 in California, Arizona and New Mexico; but not east of the last named, as in 

 Texas these species are not yet known as far north as the upper limit of Mer- 

 riam's Lower Sonoran. 



* The Distrito Federal embraces much of the highest portion of the Mexican 

 plateau, having an elevation of 7200-8000 ft. or 2200-24.50 m., and has been 

 fairly well examined as regards its Odonate fauna, which numbers 21 species, 

 etc. Outside of Mexico and Central America, 6 of the 21 occur exclusively in 

 Northern America and 2 of the 21 exclusively in South America, so that here 

 the southern element is weak. 



* Contrast on this feature Bates, Biol. Centr. Ainer. Coleop., I, pt. 1, p. vi, 

 and W. Horn, Deut. ent. Zeitschr., 1897, pp. 161-2. 



