400 ANIsTUAL REPORT SMITHSONIAN INSTITUTION, 1955 



to exodus is provided by daylight, but the invasion of the environ- 

 ment cannot exceed the low statary level unless an active brood pro- 

 vides the essential stimulative increment. During the morning the 

 outgoing rush is diverted into trailmaking, capturing and transport- 

 ing booty. It is normally depressed at midday in a kind of general 

 "siesta effect," perhaps attributable to a combination of radiation, 

 heat, and desiccation (Schneirla, 1949). IVlien this temporary leth- 

 argizing effect disappears early in the afternoon, the exodus again 

 bursts forth, but now advances rapidly along one or more of the 

 established chaimels (Sclineirla, 1938, 1944b), gaining such headway 

 that no normal or accidental extrinsic or intrinsic change can halt it 

 more than temporarily. Theoretically, the beginning of a raid in 

 the morning marks the start of an emigration, but trail conditions and 

 internal colony conditions are not propitious for a continued exodus 

 until late afternoon. Then, when failing light at dusk brings raid- 

 ing activity to a halt, foragers drain into the trails and actually aug- 

 ment the force of the exodus already under way. Wlien this exodus 

 has been completed by resettlement in a new place, the colony remains 

 moderately quiet in the bivouac until daylight again acts upon it. 



These statements are derived from studies of the terrestrial Eciton 

 species. It is probable that the fundamental principle of brood 

 stimulation as a hyperenergizing factor also applies to the subterranean 

 species, doubtless with certain secondary variations in the mechanism. 



The nomadic "instinct" in these terrestrial army ants thus finds 

 its basis in the dynamic, reversible, and repetitive relationship be- 

 tween reproductive processes and social behavior. Since the stimula- 

 tive effect of the broods is the driving factor in the process, the 

 Eciton cyclic pattern rests first of all upon the queen (pi. 2) — source 

 of the immense broods. This curious individual is much larger than 

 her sterile sisters, the workers, and wingless throughout life (Gallardo, 

 1920; "VVheeler, 1925). The queen of Eciton hamatum is capable of 

 producing more than 40,000 eggs in a single batch, all of them laid 

 within about one week midway in each statary phase. In the hurchelli 

 queen, which produces even larger broods, Hagan (1954) has found 

 a total of about 2,400 ovarioles, each judged capable of producing 20 

 or more eggs in a series. Since every brood is produced as a unit 

 and is laid within a few days, its members pass through their successive 

 developmental stages and reach full maturity almost in step. These 

 are basic facts for the occurrence of the cycle. 



Since the turning points in the colony activity cycle are correlated 

 in a relatively precise manner with major changes in brood develop- 

 ment, the pacemaker function must somehow depend upon whatever 

 sets the queen at regular intervals from a resting condition to a fur- 

 ther prodigious feat of egg production. The idea of an attmiement 



