200 A YEAR OF COSTA RICAN NATURAL HISTORY 



such an extent as to suggest that they may not be very effi- 

 cient respiratory organs. Examination of the hind part 

 (rectum) of their intestine shows the presence of a few 

 longitudinal folds which thereby increase the surface of this 

 organ. In the other suborder of dragonflies (Anisoptera), 

 the larvae breathe by drawing water into the rectum where 

 numerous folds and a rich supply of trachese absorb the 

 needed oxygen. The folds and tracheae of our Thaumato- 

 neura and Argia talamanca larvae are by no means as highly 

 developed, but their presence is at least suggestive of a sim- 

 ilar, if less intensive, function. 



While examining the larvae and exuviae of Thaumatoneura 

 we found great numbers of diatom shells attached to them. 

 A single leg, which had been detached from an exuvia, was 

 sent to Professor Albert Mann, Custodian in charge of the 

 Diatom Collection of the U. S. National Museum at Wash- 

 ington, who wrote: "I should say that there were at least 

 500 diatoms [representing sixteen species and varieties^] 

 on the small leg of this larva. The most of these are forms 

 that grow attached by a transparent jelly-like stipe; but 

 several of them are free and free-moving, — for example 

 the different species of Navicula. The finding of these forms 

 on this aquatic larva is of importance because it helps to 

 throw light upon a difficult problem connected with the dis- 

 tribution of diatoms. It has always been more or less of a 

 mystery how isolated pools and bodies of water of recent 

 origin become stocked with a rather extensive diatom flora 

 and frequently within a very short period of time. Some 

 light has been thrown upon this distribution by supposing 

 these organisms are carried on the feet of wading birds, 

 but this method of transportation is plainly very inadequate. 

 If, however, we add to this transportation by aquatic insects, 

 many of which have the power of flying, we have a means of 



^ For list, see Entom. News, Vol. XXVI, p. 299, Philadelphia, July, 1915. 



