Distance to nearest neighbours.
(a) Root-mean-square nearest-neighbour distance for midge swarms and randomly distributed particles, shown as circles and squares, respectively. The data follow the same trend for each data set, but is always larger for real swarms. (b) Standardised PDFs of dnn for measured swarms (upper) and randomly positioned particles (lower); note that the lower curves have been vertically offset for clarity. Each curve shows data for one swarm. The most probable dnn is smaller for the swarms than for the random particles, (see also (c), which shows the same plots on linear axes), but the swarms also show a much longer tail, indicating larger voids.