ARMY ANTS — SCHNEIRLA 389 



rapid advance of its members and soon drains in the other radial 

 expansions. Thereafter this growing mass holds its initial direction 

 m an approximate manner through the pressure of ants arriving in 

 rear columns from the direction of the bivouac. The steady advance 

 in a principal direction, usually with not more than 15° deviation to 

 either side, indicates a considerable degree of internal organization, 

 notwithstanding the chaos and confusion that seem to prevail within 

 the advancing mass. But an organization does exist, indicated not 

 only by the maintenance of a general direction but also by the occur- 

 rence of flanking movements of limited scope, alternately to right and 

 left, at intervals of 5 to 20 minutes depending upon the size of the 

 swarm. 



Tlie huge sorties of hurchelli in particular bring disaster to prac- 

 tically all animal life that lies in their path and fails to escape. Their 

 normal bag includes tarantulas, scorpions, beetles, roaches, grass- 

 hoppers, and the adults and broods of other ants and many forest 

 insects; few evade the dragnet. I have seen snakes, lizards, and 

 nestling birds killed on various occasions ; undoubtedly a larger verte- 

 brate which, because of injury or for some other reason, could not run 

 off, would be killed by stinging or asphyxiation. But lacking a cut- 

 ting or shearing edge on their mandibles, unlike their African relatives 

 the "driver ants" these tropical American swarmers cannot tear down 

 their occasional vertebrate victims. Arthopods, such as ticks, escape 

 through their excitatoi*y secretions, stick insects through repellent 

 chemicals, as tests show, as well as through tonic immobility. The 

 swarmers react to movement in particular as well as to the scent of 

 their booty, and a motionless insect has some chance of escaping them. 

 Common exceptions, which may enjoy almost a community invul- 

 nerability in many cases, include termites and Azteca ants in their 

 bulb nests in trees, army ants of their own and other species both on 

 raiding parties and in their bivouacs, and leaf -cutter ants in the larger 

 mound communities ; in various ways these often manage to fight off 

 or somehow repel the swarmers. 



The approach of the massive hurchelli attack is heralded by three 

 types of sound effect from very different sources. There is a kind of 

 foundation noise from the rattling and rustling of leaves and vegeta- 

 tion as the ants seethe along and a screen of agitated small life is 

 flushed out. This fuses with related sounds such as an irregular 

 staccato produced in the random movements of jumping insects knock- 

 ing against leaves and wood. This noise, more or less continuous, 

 beats on the ears of an observer until it acquires a distinctive meaning 

 almost as the collective death rattle of the countless victims. When 

 this composite sound is muffled after a rain, as the swarm moves 

 through soaked and heavily dripping vegetation, there is an uncanny 

 effect of inappropriate silence. 



