JUAN VI N AS— THE WATERFALLS 199 



dragonflies. Yet It is likely that such a habit will be found 

 to exist In some other members of this group of Insects, 

 especially In the tropics of the Old World, where topographic 

 and climatic conditions similar to those of Costa Rica pre- 

 vail. In countries where frost occurs low temperatures may 

 prevent the survival of Odonate larvae In waterfalls of small 

 volume, where the water Is spread out In a thin sheet as Is 

 the case in those falls which Thaumatoneura Inhabits. On 

 the other hand, the force exerted by a large volume of falling 

 water may be too great to permit Odonate larvae to dwell 

 therein. 



In addition to the two waterfalls here described we found 

 Thaumatoneura at only one other — a small cascade in the 

 bottom of the Reventazon valley. We were puzzled by not 

 finding these Insects also at other streams under generally 

 similar conditions. An explanation of their absence from one 

 of these, the little Rio Naranjo, Is suggested In our diary: 

 "At the Naranjo I saw a number of lizards from two in- 

 ches to a foot In length and these possibly keep down the 

 number of insects. At our two waterfalls [I. e., the 'nearer' 

 and the 'farther'] we have seen no lizards." 



A structural peculiarity of the larvae of Thaumatoneura 

 and of Argia talamanca which greatly interested us when we 

 came to study them after our return to the United States 

 was the condition of the three so-called "caudal tracheal 

 gills" with which the abdomen of all members of their 

 suborder (Zygoptera) terminates. In the majority of spe- 

 cies thus far known, these " gills" are thin, expanded and leaf- 

 like; supplied with branching air-tubes, or tracheae, they are 

 believed to absorb, through their thin walls, oxygen from 

 the air which is mixed with the water In which they live and 

 the tracheae carry this oxygen to all parts of the body. The 

 larvae of Thaumatoneura and of Argia talamanca have these 

 "gills" relatively and absolutely thickened and narrowed to 



