kL4l3ITAT SELEOTION BY AQUATIO INVERTEBRATES
291
furthermore penultimate and ultimate instar nymphs maintained
over two different backgrounds moulted to adults that were the
same colour as the background (Table Ib). A study of the shed skins
and killed adults from the latter two experiments made it clear
to Popham that the lighter backgrounds had in some way inhibited
the process of pigmentation. There is one similar example. In the
species Asellus aquaticus L., dark animals are mostly photopositive,
medium coloured animals are usually photonegative, and light animals
are always photonegative (Janzer and Ludwig, 1952). Popham’s and
also Janzer and Ludwig’s experiments are of great importance
particularly if they can be substantiated with other species. They also
imply that the previous experience and previous environment of an
animal population, and morphological or physiological differences
between animals, can affect habitat selection. But these topics will be
discussed more fully in Sections VII and VIII.
As in the sea, planktonic invertebrates are likely to maintain their
position and migrate vertically in the water column using light and
pressure as environmental clues. There is evidence that planktonic
Crustacea respond t o gravity (McGinnis, 1911; Dice, 1914; Clarke,
1930,1932) but there appears to be no information on their pressure
responses. The possible interrelationships of light, gravity and
pressure responses in regulating vertical migration in fresh waters is
considered in great detail by Cushing (1951) and Hutchinson (1967)
and it is not proposed to discuss the matter further. One should consider, however, whether it is really possible to distinguish an animal’s
response to gravity from its response to pressure in water, particularly
since the latter varies very sharply with depth. There are few investigations of the gravity responses of benthic freshwater invertebrates
(Walter, 1906 and Kanda, 1916b for references).
Bottom living animals in fresh waters may well prefer contact with
solid objects (thigmotaxis) as well as preferring certain types of bottom
such aa gravel and sand, and we will now examine these hypotheses,
Mayfly and dragonfly nymphs are thigmotactic and cling to stones or
plants depending on their normal habitat (Curtis Riley, 1912 ; Lyman,
1945; Wautier and PattBe, 1955), while caddis fly larvae respond in
the same way to their tubes. Asellus aquaticus is strongly thigmotactic
and will aggregate under a clear sheet of glass in the light part of a
dish even though it is usually photonegative (Janzer and Ludwig,
1952). These examples show that thigmotactic behaviour is likely to
occur widely amongst animals living on the bottom of lakes and rivers
and to be an important factor in their choice of habitat.
Freshwater invertebrates that burrow or build their tubes in sea-
291
furthermore penultimate and ultimate instar nymphs maintained
over two different backgrounds moulted to adults that were the
same colour as the background (Table Ib). A study of the shed skins
and killed adults from the latter two experiments made it clear
to Popham that the lighter backgrounds had in some way inhibited
the process of pigmentation. There is one similar example. In the
species Asellus aquaticus L., dark animals are mostly photopositive,
medium coloured animals are usually photonegative, and light animals
are always photonegative (Janzer and Ludwig, 1952). Popham’s and
also Janzer and Ludwig’s experiments are of great importance
particularly if they can be substantiated with other species. They also
imply that the previous experience and previous environment of an
animal population, and morphological or physiological differences
between animals, can affect habitat selection. But these topics will be
discussed more fully in Sections VII and VIII.
As in the sea, planktonic invertebrates are likely to maintain their
position and migrate vertically in the water column using light and
pressure as environmental clues. There is evidence that planktonic
Crustacea respond t o gravity (McGinnis, 1911; Dice, 1914; Clarke,
1930,1932) but there appears to be no information on their pressure
responses. The possible interrelationships of light, gravity and
pressure responses in regulating vertical migration in fresh waters is
considered in great detail by Cushing (1951) and Hutchinson (1967)
and it is not proposed to discuss the matter further. One should consider, however, whether it is really possible to distinguish an animal’s
response to gravity from its response to pressure in water, particularly
since the latter varies very sharply with depth. There are few investigations of the gravity responses of benthic freshwater invertebrates
(Walter, 1906 and Kanda, 1916b for references).
Bottom living animals in fresh waters may well prefer contact with
solid objects (thigmotaxis) as well as preferring certain types of bottom
such aa gravel and sand, and we will now examine these hypotheses,
Mayfly and dragonfly nymphs are thigmotactic and cling to stones or
plants depending on their normal habitat (Curtis Riley, 1912 ; Lyman,
1945; Wautier and PattBe, 1955), while caddis fly larvae respond in
the same way to their tubes. Asellus aquaticus is strongly thigmotactic
and will aggregate under a clear sheet of glass in the light part of a
dish even though it is usually photonegative (Janzer and Ludwig,
1952). These examples show that thigmotactic behaviour is likely to
occur widely amongst animals living on the bottom of lakes and rivers
and to be an important factor in their choice of habitat.
Freshwater invertebrates that burrow or build their tubes in sea-