Computational exploration of treadmilling and protrusion growth observed in fire ant rafts
Fig 1
(A-C) A top view of the same experimental raft is illustrated at the (A) start, (B) middle, and (C) end of a roughly 106 min duration. To visually illustrate treadmilling, a set of structural ants at the perimeter is selected every 22 minutes. These ants are then image-tracked as they flow inwards due to network contraction and the geometry defined by these ants is traced by a distinctly colored and numbered outline. The set of ants labeled “2” in (B), for example, corresponds to the same set of ants labeled “2” in (A), but roughly 53 min later. The label “1” represents the oldest set of ants while “6” represents the newest. The shrinking of these contours indicates retraction of the raft structure, while the existence of new layers indicates outwards expansion. Periods of raft expansion and coinciding protrusion emergence (A,C) were interrupted by interstitial spans of decreased activity and less eccentric morphologies (B). All scale bars represent 10 ℓ where 0 mm is the approximate average body length of 1 ant. See S1 Movie for unannotated video. (D) A schematic visually illustrates the four concurrent mechanisms of treadmilling: (1) structural raft contraction at a global rate , (2) transition of structural ants to freely active ants in the bulk at a nominal rate δ, (3) transport of the free ants on top of the raft with a mean persistence length lp, and (4) binding of free ants back into the structural network at the edges of the raft at nominal rate α. The schematic is taken from Wagner, et al. (2021) [14]. The freely active layer is offset from the structural layer for illustrative purposes only, as it resides directly on top of the structural network in real ant rafts. Furthermore, note that the freely active layer, while shaded continuously, is comprised of dispersed ants while the structural layer is relatively homogeneous and condensed.