IN' THE NORFOLK BROADS DISTRICT. 
6 7 
becoming unsuitable, the Beetles again begin to congregate 
in the more favourable places, so that in September, although 
more species are in the imago condition than were so in the 
spring, the range of distribution is about the same as it was 
in May. Thus the places where the species are found in the 
autumn are their normal habitats, and there they will again 
appear in the spring. 
Now this process by which a species adds to its distribution 
area at one period of the year and is as it were driven in 
upon its centre at another is very interesting, but where do 
all the individuals come from which fill all the summer 
stations ? 
In various families in the animal kingdom, but especially 
among insects, one hears of or sees every now and again some 
particular species appearing in abundance. Every now and 
again some Butterfly or Dragon-fly or Beetle suddenly appears 
in hundreds or thousands, as if from the sky, and again almost 
as suddenly disappears, or rather, becomes normal again in 
its numbers. If one collects Water Beetles systematically 
month by month, one finds not uncommonly some spot where 
a particular species occurs in abundance. In some cases this 
same spot may be visited month by month and the same 
species will be found there more commonly than anywhere 
else, but in other cases — and this is much more usual — this 
spot which was crowded with a particular species last 
month will not yield more than a few' individuals of it this 
month. 
It seems to me that this phenomenon is identical upon 
a small scale with what we may call the plagues of insects, 
wdiich from time to time force themselves upon the notice 
of the casual observer. Whereas, how’ever, such plagues are 
due to abnormal reproduction, and, fortunately, only occur 
occasionally, owing, as is usually said, to some upset in the 
balance of nature, the former phenomenon is the normal 
result of the reproduction of the species. The “ plagues ” 
of insects are probably useful in renewing the distribution of 
the particular species from the plague centre. The excessive 
numbers of individuals put food at a premium, and death or 
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