Aggregation in an heterospecific population of blowfly larvae: social behaviour is impacted by species-specific thermal requirements and settlement order

Abstract

Larvae of several blowfly species grow on carcasses and actively aggregate together. They face harsh developmental conditions resulting in a strong pressure to reduce development time: this is achieved either through thermoregulation or aggregation. We investigate how these two developmental strategies are modulated within heterospecific groups. In a first experiment, larvae of two species with different thermal requirements were deposited simultaneously on a thermal gradient. This resulted in the formation of two monospecific groups, each located at the species-specific thermal preferendum. However, when Calliphora vomitoria (Linnaeus) larvae were placed first, the later arriving Lucilia sericata (Meigen) larvae attracted the whole group to its own thermal preferendum. In the reverse experiment, half of the replicates resulted in single dense heterospecific groups observed at temperatures ranging from C. vomitoria to L. sericata preferendum. The other half of the replicates resulted in loose groups spread out on the thermal gradient. These results highlight the emergence of collective decisions ranging from thermal optimization to heterospecific aggregation at suboptimal temperatures. They demonstrate that species settlement order strongly affects self-organization processes and mixed-species group formation. We conclude that thermal optimization and heterospecific niche construction are two developmental strategies of carrion fly larvae. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Keywords: aggregation; allee effect; carrion ecology; mixed-species groups; mutualism; niche construction.

Figure 1.

Cumulated numbers of larvae found on each temperature zone after 19 h spent in the thermograde during one-step process experiments. Forty larvae of each species were simultaneously placed on the thermal gradient at the beginning of the experiments. (a) Cumulated numbers of larvae found on each temperature zone. Blue bars represent the cumulative observations of C. vomitoria (CV), green bars L. sericata (LS), and grey histograms the sum of the two species. Height replications were performed, giving a total of 640 initial larvae. (b) Distribution of larvae per replication. Blue bubbles are for CV, green ones for LS. The surface of bubbles is proportional to the number of larvae that were counted in each area. (Online version in colour.)

Figure 2.

Cumulated numbers of larvae found on each temperature zone after 19 h when 40 C. vomitoria (CV) larvae were placed first and followed 4 h later by 40 L. sericata (LS) larvae (CV-LS). (a) Cumulated numbers of larvae found on each temperature zone. Blue bars represent the cumulative observations of CV, green bars LS, and grey histograms the sum of the two species. Height replications were performed, giving a total of 640 initial larvae. (b) Distribution of larvae per replication. Blue bubbles are for CV, green ones for LS. The surface of bubbles is proportional to the number of larvae that were counted in each area. (Online version in colour.)

Figure 3.

Placement of larvae on the thermal gradient after 19 h when 40 L. sericata (LS) larvae were placed first and followed 4 h later by 40 C. vomitoria (LS-CV). (a) Cumulated numbers of larvae found on each temperature zone. Blue bars represent the cumulative observations of CV, green bars LS, and grey histograms the sum of the two species. (b) Bubble chart representing the distribution of larvae per replication. Blue bubbles are for CV, green ones for LS. The surface of bubbles is proportional to the number of larvae that were counted in each area. ADI are reported on the left side for each experiment. (Online version in colour.)

Figure 4.

Summary flowchart of the placement of larvae on the thermal gradient under all experimental conditions. Blue areas represent the location of CV, green bars LS, blue/green dashed areas the aggregation of the two species and bubbles spread larvae. The average temperatures corresponding to each location are reported in °C for each experiment: please refer the text and former figures for exact values and distributions. (Online version in colour.)