348
P. 9. MEADOWS AXD J. I. UAMPBEU
but similar shaped grains, or different shaped but similar sized grains,
a few larvae were more discriminating than their fellows.
The third category consists of species in which animals are of two
types present in approximately equal numbers. We will consider two
examples of marine invertebrates and two of freshwater invertebrates,
before discussing the pertinence of Wellington’s (1957, 1964) extremely
interesting work on the western tent caterpillar Malacosoma
pluviale (Dyar) for aquatic environments. Individuals of the marine
planktonic copepod Centropages typicus are either persistently attracted
to a light source, or after continuous exposure become indifferent to
light (Johnson and Raymont, 1939). The other marine example comes
from a commensal relationship. The Mediterranean hermit crab
Dardanus arroaor often actively assists the sea anemone Calliactis
parasitica (Couch) to transfer from another surface to its own
molluscan shell. Ross and Sutton (1961b) distinguished between nonperforming hermit crabs-which do not assist the sea anemone, and
performing crabs that do. The former were usually male and the latter
female. The first freshwater example is from an early paper by Pearl
(1903) on Planaria. Individuals of the species he studied appeared to
be either very active or rather sluggish, but they differed in more than
this, for the active ones moved right through aggregations on
encountering them and took no notice, while the sluggish ones turned
towards aggregates when a short distance away, and joined them. The
second example comes from a detailed study by Clarke (1930) on the
light and gravity responses of Daphnia magm. Most individuals
were photonegative and geopositive while a small number were
consistently photopositive and geonegative (p. 126, loc. cit.).
One of the most detailed investigations on behavioural differences
between individuals has been undertaken by Wellington (1957,1964) on
the western tent caterpillar Malacosoma pluviale. Larvae emerging from
egg sacs are of two types. Type I individuals are active, move in straight
lines, and are capable of independent directed movements towards a light
source. Type I1 individuals are sluggish larvae and move very little.
The latter tend to sway about and turn frequently, exhibit no
directed movement, and if placed in a beam of light huddle together.
On the other hand, if one type I larva is added to a type I1 larval
aggregation, it leads all the type I1 larvae along with it as it moves
from place to place, until if the type I larva is taken away, the type I1
larvae huddle together again-illustrating a quite remarkable sequence
of behavioural reactions. Type I larvae reach distant food in the
laboratory more readily, and in the field disperse more rapidly than do
type I1 larvae ; they also move more over the leaves they are eating,
P. 9. MEADOWS AXD J. I. UAMPBEU
but similar shaped grains, or different shaped but similar sized grains,
a few larvae were more discriminating than their fellows.
The third category consists of species in which animals are of two
types present in approximately equal numbers. We will consider two
examples of marine invertebrates and two of freshwater invertebrates,
before discussing the pertinence of Wellington’s (1957, 1964) extremely
interesting work on the western tent caterpillar Malacosoma
pluviale (Dyar) for aquatic environments. Individuals of the marine
planktonic copepod Centropages typicus are either persistently attracted
to a light source, or after continuous exposure become indifferent to
light (Johnson and Raymont, 1939). The other marine example comes
from a commensal relationship. The Mediterranean hermit crab
Dardanus arroaor often actively assists the sea anemone Calliactis
parasitica (Couch) to transfer from another surface to its own
molluscan shell. Ross and Sutton (1961b) distinguished between nonperforming hermit crabs-which do not assist the sea anemone, and
performing crabs that do. The former were usually male and the latter
female. The first freshwater example is from an early paper by Pearl
(1903) on Planaria. Individuals of the species he studied appeared to
be either very active or rather sluggish, but they differed in more than
this, for the active ones moved right through aggregations on
encountering them and took no notice, while the sluggish ones turned
towards aggregates when a short distance away, and joined them. The
second example comes from a detailed study by Clarke (1930) on the
light and gravity responses of Daphnia magm. Most individuals
were photonegative and geopositive while a small number were
consistently photopositive and geonegative (p. 126, loc. cit.).
One of the most detailed investigations on behavioural differences
between individuals has been undertaken by Wellington (1957,1964) on
the western tent caterpillar Malacosoma pluviale. Larvae emerging from
egg sacs are of two types. Type I individuals are active, move in straight
lines, and are capable of independent directed movements towards a light
source. Type I1 individuals are sluggish larvae and move very little.
The latter tend to sway about and turn frequently, exhibit no
directed movement, and if placed in a beam of light huddle together.
On the other hand, if one type I larva is added to a type I1 larval
aggregation, it leads all the type I1 larvae along with it as it moves
from place to place, until if the type I larva is taken away, the type I1
larvae huddle together again-illustrating a quite remarkable sequence
of behavioural reactions. Type I larvae reach distant food in the
laboratory more readily, and in the field disperse more rapidly than do
type I1 larvae ; they also move more over the leaves they are eating,