486 PROCEEDINGS OF THE ACADEMY OF [Oct., 



correlation between paleness of wing-veins and dryness of climate,^^ 

 which seems to receive some support from Argia mcesta?^ and Enal- 

 lagma civile ;^'^ but, on the other hand, appears to be negatived by- 

 specimens of Enallagma prccvarum from many of the same localities 

 which furnish E. civile. ^^ 



Whatever of a more exact character we learn in the future of the 

 dependence of these insects on climatic conditions, we must conceive 

 of the latter as operating in a manner which may be compared to the 

 beating of the waves upon a shore. A higher temperature and a 

 more copious rainfall, together or singly, advance upward to a greater 

 elevation or northward to a higher latitude, making possible the exist- 

 ence of certain species in the larval state where they were previously 

 unknown. The next year, or after several years, these favorable con- 

 ditions retreat down the mountain slopes or southward along the 

 coastal plains, and the species whose existence they permitted dis- 

 appear from certain localities for a longer or shorter period of time 

 until the necessary conditions are again established.^^ To demon- 

 strate the correctness of this view such continuous observations at a 

 number of stations as were mentioned above (page 479) are essential. 



Relations of the Odonate Fauna to Vegetation Areas. 



Dr. Charles C. Adams, whose recent researches have been directed 

 chiefly to the detection of the relations of faunse to their physiographic 

 surroundings, has suggested to me to endeavor to correlate the dis- 

 tribution of these insects with that of vegetation areas, the latter to 

 serve as indices of the general physical features of the country. In 

 this attempt I have employed Dr. Karl Sapper's vegetation maps of 

 Central America,^^ locating the various places at which the Odonata 

 have been collected in his zones and tabulating the distribution of 

 the species accordingly. The results, save in a few instances to be 

 mentioned shortly, have been unsatisfactory, as the great majority 

 of species appear in several columns of the tabulation. Thus of 133 



^* Biol. Cent.-Amer. Neurop., p. 235. 



''^L.c., p. 361. 



^''L.c, pp. 110, 380. 



" L.c, p. 380. 



*^ Some remarks by Mr. S. E. Meek {Puhlicat. Field Columb. Mus. Chicago, 

 V, pp. xxvi-xxvii, 1904), on the effects on the fish fauna of fluctuations in bodies 

 of water on the northern part of the Mexican plateau, may also be appUed to 

 Odonate larvae. 



*^ In Das Nordliche Mittelamerika (map dated 1895), 1897, and Mittelameri- 

 kanische Reisen und Studien (map dated 1900), 1902. There is also a larger 

 scale map for Guatemala only, dated 1894, in Petermann's Mittheil., Ergan- 

 zungsband XXIV, 



