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D.—ZOOLOGY 65 
species but other species as well. In the French vineyards a bug * 
attacks the vines, but the damage is done mostly after a clean up which 
entails the destruction of a weed,® which is the preferred food. ‘At 
Dartford in Kent, where henbane and belladonna are grown on a large 
scale for the sake of their alkaloid bases, it has been found that, whereas 
in some years as much as 80 per cent. of damage is done to the former 
crop by the maggot (of a fly) the latter remains unaffected, although in 
close proximity. . . . When henbane is absent, belladonna proves quite 
attractive.’ This was Cameron’s observation upon the Belladonna Leaf- 
miner.6 The Cotton Boll-worm, the caterpillar of a moth,’ a pest of 
various crops, including maize, cotton, tobacco, lucerne, etc., which is 
a very general feeder in many countries, shows a definite preference 
in some for maize over cotton, and one of the methods of control in the 
cotton-fields is to interpose rows of maize, upon which the moths lay 
their eggs in preference to ovipositing upon the cotton. 
In all the above examples, and they could be multiplied many times, 
it is evident that choice plays a part in determining the food of the insect, 
and I think it is possible to show that choice also plays its part in con- 
nection with the habitat occupied by the water-beetles. In the first 
place, the carnivorous water-beetles are not restricted to any type of 
habitat by the nature of the food, since both adults and larve will consume 
any living organism they can catch and which can be penetrated by their 
mandibles, and the adults will also eat any dead material of a similar soft 
nature. I have kept in captivity many species in both these stages and 
have never had any difficulty in feeding them. In the second place they 
are not restricted by soil conditions or by the nature of the vegetation, 
since they can be kept in captivity under conditions very different from 
those in which they are normally found. Peat pool species live quite 
well in my cement-lined aquaria in hard water and even breed there. 
A brackish pond species ® rarely found inland has bred in one of these 
aquaria for three years. I have kept for some years one of the small 
running-water species ® in tumblers in which the water was never changed. 
All these examples, and many others could be given, indicate that 
neither soil nor plant environment have a direct effect upon the water- 
beetle communities, so that, if the struggle for existence is what segregates 
the species, the active factor must be the internecine strife of the animal 
population. There is undoubtedly a large amount of preying of one 
_ organism upon another, but unless some species of water-beetles are more 
palatable than others or unless the life-history of different species differs 
to such an extent that the larve of one species are killed off at a time 
4 Nysius senecionis, Schill. (Lygeide). 
5 Diplotaxis erucdides. 
6 A. E. Cameron, ‘ A Contribution to a knowledge of the Belladonna Leaf- 
miner, Pegomyia hyoscyami, Panz., its Life-history and Biology,’ Ann. Appl. 
Biol., vol. i, 1914, pp. 43-76. 
7 Heliothis obsoleta. 
8 Hygrotus parallelogrammus, Ahr. 
9 Bidessus minutissimus, Germ. 
