DOI: 10.5061/dryad.kprr4xh7d
title: Trophic allometry in a predator that carries corpses of its prey

description: Ant-snatching assassin bugs carry a ‘backpack’ of ant corpses as an
 antipredator strategy. From photographs, we quantified the relative size
 and number of ants in these backpacks. We found a trade-off between size
 and number of carried ants, suggesting that trophic allometry has
 implications beyond energy acquisition, potentially affecting camouflage.
description: We searched for photographs of
 Acanthaspis and Inara individuals in online public image repositories
 (Deviant Art, Flickr, iNaturalist; search term: “assassin bug”) and
 selected those (i) in high resolution (i.e., enough to precisely visualize
 the extremities of the bug and ants) and (ii) that showed the assassin
 bugs in a side view (lateral photograph). In each selected photo, we quantified our two
 operational variables: PPSR and the number of prey. As PPSR is
 dimensionless, we measured the size of the predator and the prey in pixels
 using the ImageJ software since in the photographs there was no scale to
 know their real size. The predator size was measured by its body length
 (from the tip of the rostrum to the tip of the abdomen). We assumed that
 all ants on a given backpack have the same body size. We also used the
 head length (from the mid-point of the anterior clypeal margin to the
 mid-point of the posterior margin) as a proxy for prey size.
 In all selected
 photographs, at least one head was in a front view and, if there was more
 than one ant in this position, we calculated the mean head
 length. We used a mathematical approximation to estimate
 the number of ants present in the predator’s backpack as it was not
 possible to count them precisely from the photographs. First, we
 calculated the volume of a single ant based on Tschinkel (2013), which indicates that the gaster volume of
 Solenopsis represents approximately 57% of its total body volume. We used
 this proportion for all ants, because this fine allometric
 characterization has rarely been estimated for other genera. Then,
 considering the ant's gaster as a spheroid (i.e., an ellipsoid with
 two equal semi-diameters) (Tschinkel, 2013), we measured its length (GL)
 and width (GW), and used the formula of spheroid volume
 to quantify it. We also
 considered the predator’s backpack as a spheroid. To calculate the total
 volume of the backpack, we measured its length (BL) and height (BH), and
 used the same formula of spheroid volume. After calculating the two volumes (of the
 backpack and of an ant), we used the maximally random jammed (MRJ)
 parameter of 0.637 (Donev et al., 2004) to estimate the percentage of the backpack
 volume occupied by ants. Then, we divided the resulting occupied volume
 by the volume of an individual ant to estimate the total number of ants.
   To test the relationship between the number of
 carried ant carcasses and the relative size of predators and their prey,
 we fitted a Generalized Linear Model with Poisson error distribution. PPSR
 was included as the predictor variable and the number of ants in the
 backpack (rounded to next integer) as the response variable. Statistical
 analyses were performed in the R environment (R Core Team,
 2021).
description: The only program required is R.