Heterospecific aggregations and foraging associations have been observed between different species, from apes to birds to insects. Such associations are hypothesized to result in a mutually beneficial relationship entailing benefits that are not apparent in conspecific groupings. Therefore, the objectives of the present study were to investigate 1) how 3 blowfly species, namely, Calliphora vicina, Calliphora vomitoria, and Lucilia sericata, aggregate according to species, and 2) if developmental benefits are linked to heterospecific aggregation. For objective (1), larvae of 2 species were placed between 2 conspecific aggregates, each with a different species (i.e., a binary choice test). After 20 h, the positions of all larvae were determined. On average, 98% of the maggots added later settled together on one of the 2 pre-existing aggregations, demonstrating a collective choice. The aggregation spot with C. vicina was preferred against others, indicating different attractiveness of different species. To relate this behavior to its benefits (objective ii), C. vicina and L. sericata larvae were raised from first instar to adult in con- and heterospecific conditions, and their development time, mortality rates, and morphometrics were measured. Thereby, mutual and asymmetric consequences were observed: specifically, there were significant increases in size and survival for L. sericata and faster development for C. vicina in heterospecific groups. These results indicate that the predilection for heterospecific association leads to mutual developmental benefits. This heterospecific aggregation behavior may be a resource-management strategy of blowflies to face carrion-based selection pressure.

Necrophagous blowflies (Diptera: Calliphoridae) such as Calliphora vicina, a cold-tolerant species, and Lucilia sericata, a warm-adapted species, are pioneer carrionbreeder. Although these two species have different temperature preferences, larvae aggregate actively and often feed simultaneously on carrion. The hypothesis to be tested was that L. sericata benefits from the association with C. vicina at lower temperatures (i.e., 15°C) and that C. vicina derives greater benefits from this association at higher temperatures (i.e., 28°C). Therefore, both species were raised at these two constant temperatures from first instars to adults under three different conditions: monospecific low-density, monospecific high-density, and heterospecific high-density. The time until larval migration, surface area of puparia, and survival rates were determined for each condition. Differences between these fitness-related traits were found between species, temperatures, group densities, and species compositions. At 28°C, C. vicina larvae bred in heterospecific groups migrated significantly earlier and in higher numbers than that under same density conspecific conditions, with a lower mortality rate. At 15°C, both species benefited from high-density heterospecific associations, expressed by faster development and larger puparia. In conclusion, necrophagous larvae benefited from heterospecific aggregations at suboptimal temperatures by adapting their migration time to that of the faster species. Since temperature changes throughout the day and over the year, the beneficiary of such a collective association also changes. The costs involved and deviations to the temperature-size rule highlight the complexity of the carrion ecosystem.

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