MR. F. BALFOUR BROWNE ON? AQUATIC COLEOPTERA. 303 
lions was that two or three of these pools on the 12th of July 
contained each a few specimens of a rare species, Hydroporus 
longulus, Muls., and on the return of the water at the end of 
the month these same pools again all contained this species, 
while only one or two of the pools previously examined 
yielded it. 
I have not had such favourable opportunities of observing 
the dykes and ponds in this district, but I have noticed that 
when a pond or dyke becomes re-hlled after a drought, the 
same group of species reappears. For instance, where 
Hydroporus gyllenhalii , Schiodte, is the dominant species 
before the drought, it will as a rule be the dominant species 
when the water returns. A dyke, where for two seasons 
I have recorded Hydroporus melanarius, Sturm., has twice 
been dry for at least two or three weeks. 
These facts seem to suggest that the Beetle fauna does not 
disperse on the drying up of a pond or dyke, but remains 
somewhere in the vicinity, probably in the cracks in the mud 
or among the damp herbage at the sides. If this surmise 
is correct it will account for the fact that the number of species 
per collection does not increase in the dykes and ponds which 
contain water all through the dry season, but it will not account 
for the decided falling off in the number of species per collection 
in July, 1904, a phenomenon which I certainly expect was 
repeated in July, 1905. 
There are three hypotheses which might account for this 
falling off. One is that the Hydradephaga are more common 
in the dykes and ponds which tend to dry up in the summer ; 
an assumption which necessitates the further assumption 
that the Palpicornia are more common in the dykes and 
ponds which do not tend to dry up, since this latter group 
does not show a falling off in July. A second hypothesis is 
that the high temperature of the water in the summer-time 
kills off a number of individuals of the Hydradephaga but does 
not affect the Palpicornia ; while a third possibility is that 
there is an actual tendency on the part of the Hydradephaga 
to leave the water at the hottest season and seek shelter, 
perhaps in the damp earth in the vicinity of the pond. From 
