HOW INSECTS WORK IN GROUPS — SUDD 491 



SUCCESS— A KEY STIMULUS 



Among the truly social insects the ants are perhaps the most varied 

 in their behavior. One of the wonders of the ant world is the nest of 

 the tailor ants. These ants live in the Tropics, in a contmuous range 

 from North Queensland to West Africa, and always build their nests 

 in trees. Unlike the many other tree-dwelling ants, their nests are 

 made by drawing living leaves together to form envelopes which they 

 secure with silk threads produced by their own mature larvae. In 

 West Africa the French zoologist A. Ledoux has shown that leaves 

 are bent to form nests in two ways: Either two nearby leaves are 

 drawn together and their edges held in a tissue of silk, or a single 

 leaf can be rolled up to form a tube. 



The rolling-up of leaves to form the second type of nest is most 

 interesting. The leaf is not rolled up in a logical way by a group 

 of ants collecting at its apex and pulling it back under the leaf-blade. 

 On the contraiy, ants begin pulling at any point around the leaf mar- 

 gin, and they pull singly, not in groups. These first efforts are mostly 

 abandoned, and some ants leave the leaf altogether, others merely move 

 to another point on it — particularly to places where the leaf is already 

 bent, either naturally or experimentally. Soon some ants succeed in 

 bending the leaf edge. Because of the arrangement of veins in the 

 leaf this is more likely to happen at the tip of the leaf than at its sides. 



Throughout the process ants let go of the leaf and move about on 

 its surface before they settle again, and these ants are attracted to 

 places where bending is well advanced, so that they add themselves 

 to the most successful groups. In this way the efforts of the ants are 

 gradually concentrated at promising sites, usually the tip of the leaf, 

 which are drawn down under the leaf blade. As the successful party 

 moves down the leaf, ants pulling at the sides are drawn in too. 

 Finally, when the leaf is doubled back, ants appear carrymg larvae 

 and close up the gaps with silk. How they are called in at this point 

 is not known. 



There are a good many similarities here to the case of the jack 

 pine sawfly. Although ants are in general attracted to one another, 

 those which are beginning to pull leaves do not aggregate in this way. 

 The groups of ants which bend leaves form only as the work of bend- 

 ing progresses, just as the feeding groups of the sawfly did not form 

 unless some larvae were feeding. Ant groups, like the sawfly groups, 

 formed where there was evidence of success at the job in hand. 



PULLING THEIR WEIGHT 



The existence of cooperation has been most debated in the trans- 

 port of prey by ants. Many ants are carnivorous and take insect 

 prey back to their nest to feed their growing brood. In some species, 



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