292
P. 9. MEADOWS AND J. I. OAMPBELL
ments, or that live at the sediment surface, have preferences which in
general agree with their distribution. The larvae of the mayfly Hexagenia can only burrow easily in mud-their normal environment
(Lyman, 1943), while larvae of the chironomid Nilodorurn brevibucca
Freeman prefer the same type of sediment to sand (McLachlan, 1969),
and larvae of Chironomus riparius Meigen prefer algae to a gravellsand
mixture (Edgar and Meadows, 1969). The Nilodorum larvae were
much more likely to migrate from unfavourable than from favourable
sediments, which is also true of the behaviour of the hemipteran
Arctocorisa (Popham, 1941) and of larval chironomids (Edgar and
Meadows, 1969). The preferences of mayfly (Ephemeroptera and
stonefly (Plecoptera) nymphs living at the sediment surface have
also been investigated, and they mirror the species usual habitats
(Madsen, 1968, 1969). Analogous species differences that agree
with their natural distributions have been described for two crayfish
(Bovbjerg, 1952b). Finally, caddis fly larvae (Trichoptera) are very
particular as to how and with what they make their cases (Hanna, 1961;
Hansell, 1968), although these are perhaps rather special instances of
habitat selection. In all these instances, laboratory preferences agree
in general with field distribution, however this is not always true
(Cummins and Lauff, 1969, 10 species of insect larvae and nymphs and
a snail; Bovbjerg, 1970, two crayfish species Orconectes; Gale, 1971, a
bivalve, Sphaerium; see Section VI). As in the sea, we know almost
nothing of the way animals may respond to changes in thixotropy
and packing of sediments (Webb, 195813, 1969) except for an interesting
study by Wallace (1958) on the movement of nematodes in wet sand
(see section on interstitial animals, p. 296).
The depth of sediments is likely to vary as much in fresh waters as
it does in the sea, and the distribution of burrowing animals will be
influenced by these variations if they avoid very shallow sediments or
prefer a particular level within a deeper one. Preferences of this nature
have been demonstrated experimentally. The depth to which mayfly
nymphs will burrow is limited by a layer of sand below mud-their
preferred sediment (Lyman, 1943), while the larvae of Chaoborus
punctipennis prefer the top 3-5 cm of sediments (LaRow, 1969).
LaRow’s study is interesting because it seems to be the only one in
which vertical migration within a sediment has been experimentally
investigated. Under continuous light Chaoborus larvae are dispersed
between depths of 0-6cm during the day but also migrate slowly
upwards, so that at sunset they are within 1 cm of the sediment
surface. If the light is left on after this time they move slowly down
again but if it is turned off, many of them become planktonic. Particularly noteworthy is LaRow’s conclusion that the larvae rise to the surface
P. 9. MEADOWS AND J. I. OAMPBELL
ments, or that live at the sediment surface, have preferences which in
general agree with their distribution. The larvae of the mayfly Hexagenia can only burrow easily in mud-their normal environment
(Lyman, 1943), while larvae of the chironomid Nilodorurn brevibucca
Freeman prefer the same type of sediment to sand (McLachlan, 1969),
and larvae of Chironomus riparius Meigen prefer algae to a gravellsand
mixture (Edgar and Meadows, 1969). The Nilodorum larvae were
much more likely to migrate from unfavourable than from favourable
sediments, which is also true of the behaviour of the hemipteran
Arctocorisa (Popham, 1941) and of larval chironomids (Edgar and
Meadows, 1969). The preferences of mayfly (Ephemeroptera and
stonefly (Plecoptera) nymphs living at the sediment surface have
also been investigated, and they mirror the species usual habitats
(Madsen, 1968, 1969). Analogous species differences that agree
with their natural distributions have been described for two crayfish
(Bovbjerg, 1952b). Finally, caddis fly larvae (Trichoptera) are very
particular as to how and with what they make their cases (Hanna, 1961;
Hansell, 1968), although these are perhaps rather special instances of
habitat selection. In all these instances, laboratory preferences agree
in general with field distribution, however this is not always true
(Cummins and Lauff, 1969, 10 species of insect larvae and nymphs and
a snail; Bovbjerg, 1970, two crayfish species Orconectes; Gale, 1971, a
bivalve, Sphaerium; see Section VI). As in the sea, we know almost
nothing of the way animals may respond to changes in thixotropy
and packing of sediments (Webb, 195813, 1969) except for an interesting
study by Wallace (1958) on the movement of nematodes in wet sand
(see section on interstitial animals, p. 296).
The depth of sediments is likely to vary as much in fresh waters as
it does in the sea, and the distribution of burrowing animals will be
influenced by these variations if they avoid very shallow sediments or
prefer a particular level within a deeper one. Preferences of this nature
have been demonstrated experimentally. The depth to which mayfly
nymphs will burrow is limited by a layer of sand below mud-their
preferred sediment (Lyman, 1943), while the larvae of Chaoborus
punctipennis prefer the top 3-5 cm of sediments (LaRow, 1969).
LaRow’s study is interesting because it seems to be the only one in
which vertical migration within a sediment has been experimentally
investigated. Under continuous light Chaoborus larvae are dispersed
between depths of 0-6cm during the day but also migrate slowly
upwards, so that at sunset they are within 1 cm of the sediment
surface. If the light is left on after this time they move slowly down
again but if it is turned off, many of them become planktonic. Particularly noteworthy is LaRow’s conclusion that the larvae rise to the surface