230 
(phase 5) merge with the return phase (7) de- 
scribed below. Juveniles do not participate in 
army wandering. 
7. Return Phase 
The return phase begins when the armies 
cease to wander about the beach in a seemingly 
undirected fashion, and orient their progression 
in the direction of the upper levels of the shore. 
As they proceed, individuals tend to dissociate 
themselves from their fellows so that the armies 
lose their cohesion. The crabs continue walking 
until they reach the level of the beach at which 
they emerged. This return may be protracted for 
as long as 11/2 hr, but it may be as short as 
15 minutes. 
8. Aggressive Wandering Phase 
On completion of the return phase, the crabs 
dig holes in typical corkscrew fashion (Mc- 
Neill, 1926). Although McNeill believed that 
these burrowing actions "are probably unique 
among the Decapoda,” Tweedie (1950) has 
since noted that Dotilla mictyroides employs a 
similar method. Soldier crabs do not remain in 
these newly dug holes but re-emerge, dig more 
holes, abandon these in turn, and continue thus 
for the duration of the phase. At the same time 
they respond to encounters with one another. 
Threat displays between pairs of adult males 
result from these encounters. In each case, one 
of the pair of crabs involved was temporarily 
associated with a hole. Females were not ob- 
served to participate in such displays. Four types 
of encounter were observed: (i) walking over 
an occupied hole, followed by emergence of the 
occupant and a threat display between him and 
the intruder; ( ii ) physical collision of two crabs 
in the vicinity of a hole, followed by a threat 
display; (iii) threat display by an occupant at 
the entrance of a hole, to which another male 
in the vicinity responded with a threat dis- 
play; (iv) intrusion by a male into an occupied 
hole, or digging by a male so close to an occu- 
pied hole that its occupant emerged, followed 
again by a threat display. 
Soldier crab threat display is performed as 
follows: Rival males meet. Each raises the an- 
terior legs off the substratum so that he is bal- 
anced on the third and fourth pairs of walking 
legs, or even on the most posterior pair alone. 
PACIFIC SCIENCE, Vol. XX, April 1966 
Fig. 7. Camera lucida drawings of the four types 
of spoon-tipped seta on the mouthparts of M. longi- 
carpus. 
Those walking legs thus freed are extended 
laterally, as are the chelipeds. Attainment of the 
posture of maximum threat occurs when the 
plane of the thoracic sterna is vertical. Thus, 
the ventral surfaces of the crabs are brought into 
proximity. However, they do not touch, and no 
pushing occurs. The eventual winner may re- 
main poised in the position of maximum threat 
for up to a second after the loser has fallen 
away. Throughout the encounter the free walk- 
ing legs are vibrated rapidly, and the chelipeds 
are gradually raised vertically from their posi- 
tion of horizontal extension. Immediately after ! 
the display the winner exhibits the following 
characteristics: (i) maximum vertical extension 
of the chelipeds, (ii) maximum extension of 
the free walking legs, (iii) maximum exten- 
sion of the legs on which he is balanced, (iv) 
maintenance of the eyestalks in the raised posi- j 
tion, and (v) maintenance of position with 
respect to the hole. 
