1918.] NATURAL SCIENCES OF PHILADELPHIA. 105 



Till^-ard's earlier views (1915) as to the respiration of the 

 Zygopterous larvae are stated as follows: "Firstly, I shall deal 

 only with the suborder Anisoptera, i. e. those dragonflies whose 

 larvae breathe by means of rectal gills, and shall not consider the 

 somewhat different problem presented by the Zygoptera, wJiose 

 larvce breathe by means of caudal gills." (The italics are ours.) 



Calvert (1915) says, "From the various data which have been 

 brought together here, it seems reasonable to suppose, at least 

 until much more exact experiments show the limitations of each 

 mode of respiration for different stages and for different species of 

 Zygopterous larvae, that the general body surface, the caudal pro- 

 cesses, the rectal epithelium, certain spiracles and in a few species 

 lateral external abdominal tracheal gills, all contribute to satisfying 

 the needs of the organism for oxygen." 



Summing up this historical treatment of the possible functions 

 of these parts in respiration we find that three methods for the 

 taking in of oxygen by the larva are described, (1) by the caudal 

 tracheal gills^ — (2) by the rectal gills— (3) by the surface of the 

 body (including spiracles). 



Each of these will be discussed in turn and reasons for considering 

 them heretofore as entering into the phenomena of respiration, 

 will be given. 



I. The caudal tracheal "gills." These, three in number, situated 

 on the caudal end of the abdomen, have a structure which seems 

 admirably adapted for a respiratory function. Normally, they 

 are present on the larva throughout its life, but are, at the time of 

 transformation, left behind on the exuvia in their normal position 

 and relation. Their presence, however, does not seem to be necessary, 

 because larvae with one, two or all of them missing, live apparently 

 normally and transform into the adult in the usual manner. Hence, 

 if they do act as respiratory organs, it must be onlj^ in a secondary 

 manner. The}' do, however, aid the larva in swimming, acting as 

 fins or as rudders. When at rest the larvae have the tendency to 

 move the abdomen in a wave-like fashion from side to side, the 

 "gills" in this case possibly acting only as mechanical devices for 

 the stirring up of the water and hence in bringing water in which 

 the content of oxygen is richer near the larva. 



If they have (as many hold) a respiratory function, how can 

 the larva continue to live and reach maturity, when they are 

 completely removed? This seems to entirely eliminate them (or at 

 least to cast much doubt on their role) in a consideration of 



