332 BIONOMICS, ETC. [CH. 



It seems probable that it attacks the Dragonfly at metamorphosis, 

 placing either eggs or viviparous young in the positions already 

 described. The young mites cling to the Dragonfly without doing 

 it any harm, and are carried by it to other ponds or lagoons, where 

 some of them at any rate drop off. Thus the Dragonfly is used 

 as a means of dispersal by this peculiar Arachnid [34]. 



The Confervoid Alga Oedogonium is very abundant in stagnant 

 water or slowly-running streams. I have found it growing, not 

 only on rocks, wood, and even stones and soil, but also on various 

 animals which live in such localities, such as aquatic Hemiptera, 

 and also the larva of Aeschna brevistyla. As the latter takes two 

 years to mature, it usually retires for the winter, and becomes 

 very inactive. In the early spring (Aug.-Sept.) this larva can 

 be frequently taken around Sydney with a thick growth of 

 Oedogonium upon it. There is no question of parasitism here. 

 I found, by means of sections, that the Oedogonium does not 

 penetrate the cuticle of the larva, but simply grows on it as 

 it grows on everything else in such places. On one larva of 

 Aeschna there were no less than three species of Oedogonium, 

 fifteen species of Diatoms, and a large number of Vorticella. The 

 exercise of a little imagination can, of course, elaborate a theory 

 of symbiosis in a case like this. But, as this method of erecting 

 a scientific mountain out of a small natural mole-hill has already 

 been used to perfection by Kammerer [78], I shall content myself 

 with stating my opinion that such methods are not to be considered 

 scientific. Kammerer's Aeschna larvae could not avoid becoming 

 overgrown with Oedogonium in the filthy surroundings in which 

 he found them. But to give a list of "advantages," both on the 

 side of the alga and on the side of the larva, is quite another 

 question, and one that can be safely left for common-sense to 

 decide. 



Larvae in Brackish Water. 



Dragonflies are known to breed in coastal lagoons, where the 

 water is brackish. Osburn [HO], by a series of experiments, shewed 

 that the larvae could withstand a salinity up to a density of 1-01 

 (sea-water 1-026). At that point the larvae could live, but could 

 not undergo metamorphosis. At a density of 1-015 the larvae 



