382 STUDIES IN THE LIFE-HISTORIES OF AUSTRALIAN ODONATA, 



branched tracheae in these gills suggests anything at all, it must 

 be that originally they were in some way organs of respiration. 

 In the Agrionidce we see them converted from their original use 

 into practically an aid to locomotion only. It is certain that tiie 

 loss of these gills, even in Diphlebia, does not affect the respira- 

 tion of the insect in any way, for two of the four exuviae I 

 possess have no gills left at all. I noticed several of ni}^ young 

 larvae without caudal gills only a few days after birth, so that it 

 is clear that they are easily broken off and lost. After an ecdysis 

 tliey are again replaced, though usuall}'- somewhat smaller than 

 a gill that has not been lost previously. The triquetral form 

 suggests a modification of some original structure capable of 

 receiving a large quantity of water for purposes of respiration. 

 I doubt whether even the wide lateral gills of Diphlebia, fed by 

 two large tracheae, are of any use at present for auxiliarj^ respira- 

 tion. The form of structure has probably persisted long after 

 its use has vanished. Be that as it may, the triquetral gill 

 ai)pears to be at present confined to the Calopterygidce alone. 



The larva of Diphlehia shows no signs of the paired abdominal 

 gill-structures found in Euphcea. Such structures could scarcely 

 be expected to persist in what, I think, must be regarded as a 

 highly successful and specialised branch of the Galop terygid 

 stock. In the imago, the reduction of cross-veins in the wings 

 has been carried out to such an extent as almost to have reached 

 the end achieved by the Agrionidce. One feels that there is 

 much to ponder over in the unerring insight which led de Selys 

 to give the name lesto'ides to this peculiar insect. There is more 

 than a superficial resemblance, especiall}^ if the comparison 

 be made, not with Lestes but with Argiolestes. Though more 

 than two antenodals still persist, only the first two are con- 

 tinuous into the subcostal region; so that the elimination of the 

 other three or four is at least half-accomplished. In the larva 

 itself we probably have the secret of the success of this type. Its 

 whole form, — the enormous strength of the legs, the huge labium 

 and in-set eyes — shows us at once that Nature has evolved a 

 successful type capable of holding its own against all its enemies, 



