HOW INSECTS WORK IN GROUPS — SUDD 493 



a different angle, and it goes on trying new angles until it finds a line 

 along which the prey will move. If this doesn't work it may release 

 the prey and seize it again at a new position. These changes of position 

 are not based on any knowledge of the type of snag ; it is simply a ques- 

 tion of "trial and error." If after a short time the prey does not come 

 loose, the ant may abandon it. But if the difficulty in transport is not 

 caused by, say, a grass stem, but by another ant pulling in the opposite 

 direction, swings and changes of position may again result in finding 

 angles at which the ants are not opposed to one another. This seems 

 to be the way in which transporting groups are formed from deadlocks, 

 although there is possibly also a tendency for pulling ants to aline 

 themselves with the direction of movement once it has begun. 



DISORDER, SEARCH, AND ORDER 



These three examples have an underlying pattern in common, a pat- 

 tern of three phases — disorder, search and order (see fig. 1). The 

 gradual appearance of order in these tasks suggests that cooperation 

 is not due to the imposition of a master plan but arises through the trial 

 of many possibilities, those which are unsuccessful being abandoned. 

 The trials are judged by effects, and the medium of communication 

 between individuals which enables them to tell whether or not they are 

 cooperating, is not incidental signals — scent, sounds, gestures — but the 

 progress of the work itself. It is deeds that tell, not words. 



Termites almost certainly build their strange-shaped nests by the 

 same system. Professor P-P. Grasse has kept termites in the labora- 

 tory, and given them soil for building. At first they laid their pellets 

 of soil at random, but later they were attracted to places where pellets 

 had already been laid, so that pillars and walls were formed. When 

 these were 4-5 mm. high, the termites began to build in horizontal 

 sheets, joining one pillar to another. The progress of the work not 

 only was the link between the work of individual termites, but also 

 provided the cue for a change from vertical to horizontal building. 

 Grasse calls this stimulatory effect of work "stigmergy" (from 

 stigma — 'prick, stimulus,' and ergon — 'work'). 



SUCCESS BY RANDOM CHANGE 



Many of the movements in an animal's behavior are closely adapted 

 to some rather restricted function, for instance, the pairing of the sexes. 

 Here, since all males and all females of the same species are similar, the 

 problems involved in bringing the pairs from the random positions 

 in which they first encounter one another to the stereotyped position 

 in which mating is possible, are predictable, and can be solved by a 

 fixed program, a kind of countdown of standard movements and re- 

 sponses. This is provided by the courtship of many animals. 



