40
M. J. WELLS
the nest from areas above highwater mark into which the animals did
not normally penetrate. These authors believed that return to the nest
was guided by “ aerial dissemination of some guiding substance ” on the
following grounds: (1) that while the route out from the nest was
normally irregular, the return was nearly always a straight line (i.e.
the animal did not follow its outward track) ; (2) where the nest had
more than one entrance, individuals did not always enter the way
they came out ; (3) the returns when displaced to unfamiliar territory,
already mentioned; (4) the animals’ only “ home ” in air; underwater,
they appear to be lost; ( 6 ) homing sometimes involves complete
reversal of the animals’ otherwise regularly negative heliotropism ;
(6) individuals displaced into the area of another nest appear lost,
rarely enter the new nest, and when they do, re-emerge rapidly.
I n this very thorough study, Arey and Crozier tried also to reproduce
field conditions in the laboratory, but though their animals continued
to live and feed, they did not form nests or respond to artificial tides by
the simultaneous emergence and withdrawal that is so marked a
feature of Onchidium behaviour in the field. Arey and Crozier’s explanation in terms of an aerial guiding substance emanating from the nest is
probably wrong, as two of their own observations suggest. One of
these is that a homing Onchidium removed and replaced on the substrate continues home, while one replaced on a sheet of glass laid on the
substrate does not (it becomes negatively heliotropic and this may
carry it away from the nest). The other is that interference with
the entrance to the nest by chipping bits away upsets the animals,
which wander about over the wreckage when they get there instead of
entering the nest. Both observations argue against an airborne guide.
Moreover, cutting off the oral lappets which are in contact with the
ground disorients the animals; removal of the tentacles does not.
Altogether it would seem more likely that the animals respond to a
gradient generated by their own repeated excursions from the nest, the
main arguments against this being that they do not obviously react to
recent trails and that they apparently home successfully when placed
close to the nest but outside their normal area of operations.
Whatever the explanation, it seems that the capacity to return home
is lost if the animal is kept away from its nest for any length of time.
Individuals kept in the laboratory for 24 h failed to return home when
replaced in their normal foraging ground. While this forgetting is
perhaps explicable in terms of physiological state (it would appear, on
the face of it, that the animals home when replete), their former
specificity is most unlikely to be genetic ; one might compare it with the
learning of nest smell by hymenopterans.
M. J. WELLS
the nest from areas above highwater mark into which the animals did
not normally penetrate. These authors believed that return to the nest
was guided by “ aerial dissemination of some guiding substance ” on the
following grounds: (1) that while the route out from the nest was
normally irregular, the return was nearly always a straight line (i.e.
the animal did not follow its outward track) ; (2) where the nest had
more than one entrance, individuals did not always enter the way
they came out ; (3) the returns when displaced to unfamiliar territory,
already mentioned; (4) the animals’ only “ home ” in air; underwater,
they appear to be lost; ( 6 ) homing sometimes involves complete
reversal of the animals’ otherwise regularly negative heliotropism ;
(6) individuals displaced into the area of another nest appear lost,
rarely enter the new nest, and when they do, re-emerge rapidly.
I n this very thorough study, Arey and Crozier tried also to reproduce
field conditions in the laboratory, but though their animals continued
to live and feed, they did not form nests or respond to artificial tides by
the simultaneous emergence and withdrawal that is so marked a
feature of Onchidium behaviour in the field. Arey and Crozier’s explanation in terms of an aerial guiding substance emanating from the nest is
probably wrong, as two of their own observations suggest. One of
these is that a homing Onchidium removed and replaced on the substrate continues home, while one replaced on a sheet of glass laid on the
substrate does not (it becomes negatively heliotropic and this may
carry it away from the nest). The other is that interference with
the entrance to the nest by chipping bits away upsets the animals,
which wander about over the wreckage when they get there instead of
entering the nest. Both observations argue against an airborne guide.
Moreover, cutting off the oral lappets which are in contact with the
ground disorients the animals; removal of the tentacles does not.
Altogether it would seem more likely that the animals respond to a
gradient generated by their own repeated excursions from the nest, the
main arguments against this being that they do not obviously react to
recent trails and that they apparently home successfully when placed
close to the nest but outside their normal area of operations.
Whatever the explanation, it seems that the capacity to return home
is lost if the animal is kept away from its nest for any length of time.
Individuals kept in the laboratory for 24 h failed to return home when
replaced in their normal foraging ground. While this forgetting is
perhaps explicable in terms of physiological state (it would appear, on
the face of it, that the animals home when replete), their former
specificity is most unlikely to be genetic ; one might compare it with the
learning of nest smell by hymenopterans.