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PACIFIC SCIENCE, VoL XX, April 1966 
crabs. Toad fish captured adults in run-off chan- 
nels. M. messor seized soldier crabs near a rocky 
point at Dunwich (W. Stephenson, personal 
communication). The three birds preyed on 
adults, usually during the army wandering phase 
of activity. It is noteworthy that, except for a 
slight increase in the walking rate of the crabs 
at the approach of a predator, they exhibited no 
escape reactions. This contrasts strongly with the 
behaviour of soldier crabs in the presence of 
human observers. Their reactions in such cir- 
cumstances are as follows: 
1. Burrowing. On being chased by humans, 
soldier crabs burrow in typical corkscrew 
fashion. 
2. "Fright reflex.” M. longicdrpus occasionally 
reacts to the presence of an observer by assum- 
ing a "frozen” posture in which the limbs are 
extended rigidly. The presence of normal preda- 
tors was never observed to elicit this response, 
common among brachyurans (Schone, 1961: 
474) 
3. Crouching. A crab in this posture lowers 
the body to the substratum and folds the flexed 
legs as closely as possible to the body. The eye- 
stalks are lowered. 
4. Interspecific threat. Occasionally, crabs 
which were being chased by the observer, raised 
the chelipeds and ran away from the observer. 
This ambivalent behaviour is apparently an at- 
tempt to . flee and to perform a threat display 
simultaneously. 
5. Displacement feeding. Displacement feed- 
ing differs from the autochthonous activity in 
that any material that is raised to the vicinity of 
the buccal opening is never introduced into it, 
and consequently no feeding pellets are formed. 
Two types of displacement feeding occur. 
a. A crab remains stationary, continuously 
raising and lowering the chelae which may or 
may not touch the substratum. No material is 
picked up, and the chelae are not lifted as high 
vertically as they are in normal feeding. 
b. Displacement feeding of the second type 
has a much more "nervous” appearance (in the 
sense in which this term is used by Gordon, 
1955). Walking with unusually rapid steps, a 
crab lowers the chelae and picks up material 
from the substratum in such quantity and with 
such lack of normal manipulative dexterity as 
to drop it immediately. The chelae continue 
their upward movement but do not reach the 
buccal opening. Furthermore, material not con- 
veyed to the mouthparts during normal feeding 
is picked up in this displacement feeding, for 
example, small pebbles and pieces of Zoster a 
two inches long. 
It is evident that these two types of displace- 
ment feeding are essentially similar, differing in 
their degree of disorientation to the environ- 
ment, rather than in their organisation. Such 
correlation between intensity of thwarting or 
conflict and the degree of disorientation of dis- 
placement activity is well documented (Arm- 
strong, 1950). 
Circumstances eliciting displacement feeding 
in M. longicarpns are: 
( i ) Simultaneous activation of the drives 
to retreat and to stay. Crabs were disturbed by 
the observer until they retreated, after which 
the observer remained motionless at a distance 
of approximately 10 ft. After a few seconds the 
crabs halted and, while either standing station- 
ary or approaching the position of the ob- 
server, began displacement feeding. As they 
approached, the rate of movement of the chelae 
increased. 
When the observer’s hide was placed in such 
a position that the crabs trekked around or 
past it, they began displacement feeding as they 
approached to within about 4 ft. Some indi- 
viduals spent as long as 3 minutes at a distance 
of less than 4 ft from the hide, displacement- 
feeding throughout that interval. Then they 
made their way past the bide and ceased dis- 
placement feeding at about 4 ft from it. 
( ii ) Simultaneous activation and thwart- 
ing of the escape drive. Soldier crabs were 
chased until they burrowed. When the excava- 
tion was as deep as the body of the crab, the 
crab was removed from it and placed on the 
surface beside it. The crab burrowed again, and 
was again interrupted. After about five repeti- 
tions of this interruption, some of the crabs so 
treated began displacement feeding. Many could 
not be induced to do so, but assumed the 
crouching posture described above. 
All the instances of displacement activity de- 
scribed above were induced by the presence of 
the observer or of the hide, or by interference. 
