x\o.474] DRAGON FLIES IN BRACKISH WATER 



397 



In the first place, a series of salinometer tests of the water in al] 

 the ponds in which dragonfly nymphs were found, was made. 

 Water from four such ponds on Nonnamesset Island and from 

 three on the mainland was tested and in none of them was the 

 density greater than 1.0015, while the average was about 1.0008. 

 These tests were made at a temperature of 72° Fahr. and as the 

 figures have not been reduced the actual density would be con- 

 siderably greater. It will be noted, however, that water of such 

 density contains but little salt in comparison with that of sea water, 

 which has an average density of 1.026. One pond examined had 

 a density of 1.015 at 72°, but, though dragonflies of several species 

 were seen about this pond, a careful examination revealed no 

 dragonfly nymphs living in the water and it is a safe assumption 

 that the adults came from less saline ponds in the vicinity. Even 

 if oviposition should take place under such conditions it is highly 

 probable, as will be shown by the experiments to be discussed, 

 that no eggs would develop. 



Next, the experiment of placing nymphs in saline solutions of 

 various densities was tried. Chiefly the nymphs of Lestrs un- 

 guicnlatus, an Agrionid, were used. These were taken from a 

 pond of the density of 1.0012. Those placed in water which was 

 entirely fresh showed no ill effects from the change, and the same 

 is true of those put into saline solutions of low density, u]> to about 

 1.003. Beyond this point the larvje showed increasing signs of 

 irritation. In solutions at 1.005, 1.0075, and 1.01 the nvmi)hs at 

 first wriL^glcd and swam violenllv, truMl to climb up the sides ..t the 

 aquaria and ()th(M'\vis(> uavc cvKhMicc ot much imiaiKni. but {\\v\ 

 apparentlv became mured to it alter a (hv\ or m) an.l h\ed a> well 



solutions were always fatal. In sea water at 1.02 flie\ h\(.l oiiIn 

 a few hours, and at 1.015 they showed every sign ot di -com tort 

 and invariably died within a dav or so. 



