WATER-SURFACE PLANTS AND ANIMALS. 219
two groups, those which live on the surface and those living
just under the surface. To the first group belong the
Whirligig-beetles Gyrinus natator, G. elongatus, G. marinus,
G. urinator, etc., and Orectochilus villosus, the last-named being
nocturnal in habit ; certain of the Rhynchota, known as Water-
measurers and Pond-skaters, e.g. Hydrometra stagnorum, Gerris
thoracicus, G. lacustris, G. argentatus, G. najas, G. paludum,
Velia currens, Microvelia pygmaea (=reticulata) ; and certain of
the Collembola or Spring-tails, Podura aquatica, Sminthurides
aquaticus and Isolomura palustris. To the second group belong
the larva; and pupae of many gnats, e.g. Culex pipiens, Aedes
punctor (=Ochlerotatus nemorosus), A. annulipes, etc., Theobaldia
annulata, etc., Anopheles maculipennis, etc. ; the larvae of the
flies Dixa amphibia and Stratiomys chamaeleon ; and the
Entomostracans Scapholeberis mucronata, S. aurita and Noto-
dromas monacha. The eggs of some of the gnats may also be
mentioned here as they float on the surface either singly or in
rafts.
As in the case of the plants there is, in addition to the more
typical water-surface animals just mentioned, a number of others,
which at various times form part of the same community.
Some of these can move about on the surface, e.g. certain species
of beetles such as Dianous coerulescens, Stenus guttula, etc., and
of spiders such as Pirata piraticus, Dolomedes fimbriatus, etc.,
but most of them belong to the underwater world. They
include the larvae of certain beetles, e.g. Hydrobius fuscipes ; the
young, if not the adults, of the Water-boatman Notonecta
glauca ; Entomostraca such as Cyclops (C. fuscus, etc.),
Simocephalus (S. vetulus and S. exspinosus) ; Mollusca such as
Limnea (L. stagnalis, L. peregra, etc.), Planorbis (P. corneus,
P. albus, etc.), Physa fontinalis, Ancylus fluviatilis and A.
lacustris ; Planarians such as Polycelis nigra ; Hydrozoa such as
Hydra viridis, H. vulgaris, etc. ; and probably various Rotifera
and Protozoa.
The foregoing enumeration of the principal water-surface
plants and animals is, of course, by no means exhaustive or
rigid. Even casual observation will show that forms other
than those mentioned, especially among the insects, may
occasionally be found remaining at the surface for considerable
periods and it is more or less a matter of opinion whether