Oviposition in fire-loving fly-species

Besides Microsania and Hormopeza (Empididae), Hypocerides nearcticus now is the first phorid fly that has been documented to swarm on freshly burnt sites where smouldering wood was still present. Interestingly, a few H. nearcticus were caught in a Malaise-trap on a burnt site in Sweden, three weeks after a burn in an old-growth pine forest. This represents the first Palaearctic record of the genus (Ulefors et al. 2001). However, there were no smoke plumes or hot spots present when the trap was installed (Viklund, 2008, personal communication).

As individuals aggregate in small swarms where copulation takes place frequently, a role of the freshly burnt site as a “mating point” for the sexes seems obvious. Like proposed for M. australis, visual orientation to conspecifics may cause formation of swarms. The concentration of relevant olfactory stimuli may limit the extension of the swarm by confining the swarming range close to the burnt substrate.The entering of openings in burnt wood and soil by individual flies and copulas may serve for oviposition. The observed short inspections of a cleft may enable a fly to check if the underground substrate is appropriate for oviposition. Important olfactory cues might be odours that are specific for different stages of combustion or for the plant species that has been burnt. Furthermore, the flies could check the humidity inside the cavity. The latter suggestion is supported by the observation of thousands of H. nearcticus swarming around the base of a big Eucalyptus tree, where humidity was raised by quenching water. Obviously, these flies are strongly attracted by the increased humidity, which might be an important quality factor for a promising oviposition site. Nothing is known about the larval stages of H. nearcticus. Breeding on fire-adapted fungi like described for M. australis may also be true for this species. Indeed, many phorids breed in moist decaying matter and there are some species whose larvae feed on fungi and, therefore, are pests of cultivated mushrooms (Johal & Disney 1994). As discussed above, H. nearcticus seems to prefer moist substrate for oviposition. As moisture is an important factor for the development of fungi, this might be an adaptation for the exploitation of post fire fungi.

Oviposition in Anabarhynchus hyalipennis hyalipennis

Up to now, there have been no indications for pyrophilous behaviour within the family of Therevidae. Consequently, this is the first documentation that a therevid fly is strongly attracted to hot spots, ash and smoke on freshly burnt areas.

It seems obvious that we have observed only ovipositing females of A. hyalipennis hyalipennis because the insertion of the abdomen into the ash accompanied by vibration-like movements can only be interpreted as deposition of eggs. The purpose of hiding most probably is an attempt to minimize the risk of being captured by a predator during oviposition. Alternatively, the ecological meaning of oviposition near structures sticking out of the ash thereby casting shade could be to make sure that the larvae could develop in a favourable microclimate. It is not known which stimulus may cause the attraction of A. hyalipennis towards the ash-covered soil around burnt logs. As ash, smouldering wood and hot spots were present in all localities, neither olfactory cues nor detection of infrared radiation as described for some pyrophilous beetles and bugs (Evans 1964; Schmitz et al. 2008; Schmitz et al. 2000; Schmitz et al. 2002) can be ruled out. We have examined A. hyalipennis for infrared organs. However, special infrared organs could not be found (Klocke D, 2008, unpublished data). Therevid larvae are predators, feeding on insect larvae and annelids but preferring Coleoptera larvae. They are found mainly in sand or soil close to the surface (English 1950).

As hot spots on smouldering superficial roots were present in all localities, it can be assumed that at least a part of the roots are killed by the fire, serving as a food for a variety of wood feeding insect larvae. Consequently, larvae of A. hyalipennis would find a lot of prey at these spots. Hence, the pyrophilous behaviour of A. hyalipennis can be interpreted as coevolutionary adaptation to the habitats of other pyrophilous species whose larvae feed on decaying wood or other organic burnt substrates. Immediate oviposition after the fire guarantees a considerable advance in larval development compared to other predatory larvae which may emerge later.