NOTES ON THE GENUS ERYTHEMIS 

 WITH A DESCRIPTION OF A NEW SPECIES (ODONATA) 



By E. B. ^\'ILLIAMSox 



This paper discusses in a general way the distrilmtion of the species of 

 the genus and some possible factors involved, recording specimens taken in 

 the Canal Zone, Panama, Colombia, and Venezuela;^ the genus is defined, 

 following earlier authors ; a key to the species, based on some characters 

 heretofore used and on some new characters, and therefore supplementary 

 to keys by Calvert and Ris, is given ; and a new species, the first to be rec- 

 ognized as such in this genus for many years, is described. The paper deals 

 only with imagos. 



A GenErai, Consideration of the Distribution of the 

 Species of Erytiiemis 



A reference to the literature, summarized by Ris in Coll. Zool. de Selys, 

 will reveal at once the wide distribution of the species of this genus. For 

 example, seven of the eight known species occur in the West Indies, yet none 

 is peculiar there, and of the seven species, five have each a range at least 

 from Mexico to Paraguay. These dragonflies have been generally successful 

 in passing over or around whatever obstacles the varied topography of trop- 

 ical America could offer to their dispersal. 



Dragonflies as larvae and adults are active and more or less independent 

 of any one particular source of food. The larvae are aquatic and are hence 

 dependent on some water supply. The adults are on the wing when they 

 seize their food, and hence can sustain themselves only under certain weather 

 conditions which are equally important for mating and ovipositing. In con- 

 formity with the complexities of multiform bodies of water under diverse 

 climatic conditions, we find equally complex adjustments of dragonflies to 

 meet varying conditions. Regions with pronounced seasons, involving 

 changes in precipitation or temperature or both, and dry or frigid regions, 

 have relatively fewer species of dragonflies than tropical regions with a more 

 uniform climate and an abundant water supply throughout the year. In 

 eastern North America the broad belt of dragonfly fauna bounded on the 

 south by the northernmost extension of such genera as Dythemis, Orthemis, 

 Brachymesia, and Neoneura, for example, and on the north by Cordulia, 

 Agrion, and the bulk of Somatochlora and Aeshna, for example, is probablv 

 confined to its area and protected from encroachments from the south and 

 north largely by the temperature factor in the climate. 



Directly associated with the adjustments of dragonflies to seasonal changes 

 is their dependence on the orderly sequence of these seasons. Hence, in any 



1 See first paragraph, Misc. Pub!., Mus. of Zoology, University of Michigan, No. 9. 



