426 A YEAR OF COSTA RICAN NATURAL HISTORY 



supposed them to be the nests of a potter wasp but I saw 

 no insects near them. A few of the nests were preserved 

 dry in a vial and long afterward some fragments of adults 

 were obtained among the broken nests. These fragments 

 indicate an insect two-fifths of an inch long with a slightly 

 clouded fore wing one-quarter inch long. They are certainly 

 those of a potter wasp, of the genus Eumenes, and possibly 

 of the species E. placidus originally described from Panama. 

 Both sexes are represented in these fragments; the male 

 possesses a curved claw-like hook at the tip of each antenna 

 while this hook is lacking in the female. Most of the body 

 is black with some reddish-yellow markings, and is pitted 

 over the surface; some parts, like the face and hind (meta-) 

 thorax, have short white hair, glistening in some lights and 

 invisible in others. The abdomen is attached to the thorax 

 by a relatively long slender stalk which is enlarged rather 

 suddenly in its hind half. The potter wasps are solitary 

 in their habits, that is, each spherical vessel represents the 

 work and offspring of but one female. Yet some of the adults 

 closely resemble certain social wasps of the genus Polybia, 

 a case similar to that described on page 226. 



Hovering over the water and then over a high bank was 

 a flock of dragonflies which moved thus to and fro for at least 

 two hours until sunset. When I first saw them the flock 

 was composed chiefly of the widespread tropical American 

 species Miathyria marcella, but as sunset approached this 

 species was gradually replaced by two species of Macrothemis 

 {pseudimitans and hemichlora) and two of Brechmorhoga 

 {vivax and prcecox) and yet there was no apparent break in 

 the continuity of the flock. The cause of these flocks is not 

 clear; I saw many of them in Guanacaste and in the Banana 

 River region. There seemed to be no special abundance of 

 food in the places where the flocks were seen. Two females 

 of Miathyria marcella belonging to this day's flock which 



