1904 - 5 .] The Plankton of Tliingvallavatn and Myvatn. 1141 
always reach the bottom in great parts of it, the greatest depth 
being only 2-3 m. 
It must he emphasised that during the great volcanic eruption 
in 1729 the lava ran into the lake ; the greater part of this became 
exsiccated, and people thus laboured under the impression that the 
water burned like oil. Great quantities of the water were trans- 
formed into vapour, and all living organisms probably became 
extinct. The shape of the lake was altered, islands arose, and in 
other localities the water inundated the meadows and old islands 
disappeared. 
At the present moment animal life is extremely rich. The fine 
mud, which covers the lava bottom, is filled by myriads of larvae 
of gnats, especially Chironomus and other diptera ; at certain seasons 
the water teems with enormous quantities of the skins of the pupa 
and dense clouds of gnats arise. From these clouds the lake 
has derived its name, Myvatn signifying the lake of gnats. These 
gnats have never to my knowledge been scientifically studied. 
Some of my samples contain nothing but skins of Chironomus 
pupae, but undoubtedly stinging gnats ( Culex , Simulium) may also 
be found in or near the lake. 
All travellers have pointed out how difficult the sojourn is near 
or on the lake in hot summer days on account of the mosquitoes, 
which attack people as well as horses. 
In shallower bays, especially in those in which the water is 
tepid owing to hot springs near the shore, a great many molluscs, 
•especially Limncea , Succinea, and Planorbis, are found. 
Myvatn is an extremely fine trout lake, the trout being present 
in enormous quantities. The same is also the case with Thing- 
vallavatn. One of those peculiarities, which makes Myvatn an 
extremely interesting locality, is the innumerable masses of 
birds, especially ducks, which breed on its shores or along the 
inlets ; various travellers and naturalists have given extensive 
reports with regard to this bird life. 
The temperature of Myvatn is quite different from that of Thing- 
vallavatn ; still, it must be remembered that the observations made 
in the two lakes unfortunately do not date from the same year. 
The lake is icebound until 28th May, then the tp. in only 6 
weeks reaches its max., 121° C. (15th July). This tp. we also find 
