HABITAT SELECTION BY AQUATIa INVERTEBRATES
287
but by interacting positive rheotaxis and positive thigmotaxis (clinging).
The current responses of various insect nymphs and larvae have been
investigated by Edington (1968), Madsen (1969) and in great detail
by Ambuhl (1959). Edington placed baffles into a small stream which
locally reduced the current flow ; this induced the larvae of the caddis
fly Hydropsyche instabilis (Curtis) to move to where the current was
faster. Ecological experiments of a similar nature are simple and are
likely to be very informative, so it is surprising that they have not
been undertaken more frequently. The wide range of insect nymphs
and larvae tested by Ambiihl fell into three groups, those that avoided
a strong current, those that preferred it, and those that behaved
indifferently. With Allee (1914) he felt that the results of his experiments
did not strictly accord with the ecology of the species although they did
illustrate the importance of a single factor. We know less of the current
responses of other invertebrates. The leech Dina microstoma Moore is
strongly rheopositive (Gee, 1913) but little attempt was made to link
this to its normal environment (c.f. Herter, 1928). Allen (1923) and
Bovbjerg (1962a) suggested that natural aggregations of stream-dwelling
bivalves and a snail, respectively, might be related to current responses
and Bovbjerg presented evidence in support of this. How do the
current responses of stream-dwelling forms differ from those of forms
that live in lakes and ponds? Stream forms show abnormal behaviour in
still water (Madsen, 1969), but there is more detailed evidence than this.
Firstly, Allee (1912) compared the current responses of AseZZu communis
collected from streams with those collected from ponds, and found
that a higher proportion of the former were rheopositive. In
passing, it would be interesting to know whether these results could be
repeated with animals from the same field population that had been
kept in the laboratory either in still water or in a suitable current for
some while previously. Secondly, in a comparison of the responses of two
crayfishes, Bovbjergh (1962b) showed how the stream-dwelling species
Orconectes propinquus (Girard) maintained its position in currents more
successfully than did the pond-dwelling form Cambarus fodiens (Cottle).
In an analogous study by Edington (1968), the net-spinning larvae of
the caddis fly Hydropsyche instabilis which are found in exposed sites in
rapids, built their nets more easily at high current speeds than did the
larvae of Plectrocnemia conspersa (Curtis), a species that is characteristic of more sheltered sites. The current responses of stream-dwelling
forms, therefore, are well adapted to their own environment and seem
to play an important role in habitat selection.
Anaerobic conditions appear to inhence habitat selection in one of
287
but by interacting positive rheotaxis and positive thigmotaxis (clinging).
The current responses of various insect nymphs and larvae have been
investigated by Edington (1968), Madsen (1969) and in great detail
by Ambuhl (1959). Edington placed baffles into a small stream which
locally reduced the current flow ; this induced the larvae of the caddis
fly Hydropsyche instabilis (Curtis) to move to where the current was
faster. Ecological experiments of a similar nature are simple and are
likely to be very informative, so it is surprising that they have not
been undertaken more frequently. The wide range of insect nymphs
and larvae tested by Ambiihl fell into three groups, those that avoided
a strong current, those that preferred it, and those that behaved
indifferently. With Allee (1914) he felt that the results of his experiments
did not strictly accord with the ecology of the species although they did
illustrate the importance of a single factor. We know less of the current
responses of other invertebrates. The leech Dina microstoma Moore is
strongly rheopositive (Gee, 1913) but little attempt was made to link
this to its normal environment (c.f. Herter, 1928). Allen (1923) and
Bovbjerg (1962a) suggested that natural aggregations of stream-dwelling
bivalves and a snail, respectively, might be related to current responses
and Bovbjerg presented evidence in support of this. How do the
current responses of stream-dwelling forms differ from those of forms
that live in lakes and ponds? Stream forms show abnormal behaviour in
still water (Madsen, 1969), but there is more detailed evidence than this.
Firstly, Allee (1912) compared the current responses of AseZZu communis
collected from streams with those collected from ponds, and found
that a higher proportion of the former were rheopositive. In
passing, it would be interesting to know whether these results could be
repeated with animals from the same field population that had been
kept in the laboratory either in still water or in a suitable current for
some while previously. Secondly, in a comparison of the responses of two
crayfishes, Bovbjergh (1962b) showed how the stream-dwelling species
Orconectes propinquus (Girard) maintained its position in currents more
successfully than did the pond-dwelling form Cambarus fodiens (Cottle).
In an analogous study by Edington (1968), the net-spinning larvae of
the caddis fly Hydropsyche instabilis which are found in exposed sites in
rapids, built their nets more easily at high current speeds than did the
larvae of Plectrocnemia conspersa (Curtis), a species that is characteristic of more sheltered sites. The current responses of stream-dwelling
forms, therefore, are well adapted to their own environment and seem
to play an important role in habitat selection.
Anaerobic conditions appear to inhence habitat selection in one of