Is aggregated oviposition by the blow flies Lucilia sericata and Phormia regina (Diptera: Calliphoridae) really pheromone-mediated?

When female blow flies Lucilia sericata and Phormia regina (Diptera: Calliphoridae) oviposit in aggregations on a carrion resource, even-aged larval offspring reportedly develop faster, and fewer are parasitized or preyed upon. The benefits of aggregated oviposition equally affect con- and heterospecific larvae sharing a resource. The benefits imply that female blow flies engage in coordinated, pheromone-mediated oviposition behaviour. Yet, repeated attempts to identify oviposition pheromones have failed invoking doubt that they exist. Simply by regurgitating and feeding on carrion, flies may produce attractive semiochemicals. If flies were to aggregate on a carrion resource in response to feeding flies rather than ovipositing flies, then the semiochemical cue(s) may be associated with the salivary gland. Working with female L. sericata and P. regina and using liver as a surrogate oviposition medium, we test the hypotheses, and present data in their support, that (1) gravid or non-gravid females ovipositing and/or feeding on liver enhance its attractiveness to gravid and non-gravid females; (2) females respond to semiochemicals from feeding heterospecific females; (3) females respond equally well to semiochemicals from feeding con- and heterospecific females; (4) macerated head tissues of females applied to liver enhance its attractiveness; and (5) females feeding on liver, but not just residing next to it, enhance attraction of other flies.  We conclude that oviposition site-seeking females do not respond to an oviposition pheromone. Instead, they appear to co-opt semiochemicals associated with feeding flies as resource indicators, taking chances that resources are suitable for oviposition, and that ovipositing flies are present.