Zooming in on the Hangers On

My garden compost bin has been producing clouds of Drosophila. Before it cooled off I had to stand back when I opened the lid to avoid being covered in flies. Over the weekend I took a closer look. They turned out to be a mixture of two species, Drosophila immigrans and Drosophila hydei, both cosmopolitan tramp species. 

Looking closer, most of the D. hydei were carrying a good load of mites. The good folks of the Facebook Soil Biodiversity UK group were able to identify these for me as Macrocheles spp. These are predatory mites which feed on nematodes, small worms, fly eggs and larvae and other tiny arthropods. They are strongly associated with ephemeral, nutrient-rich substrates such as leaf litter, decaying plant matter, animal dung, carrion, and animal or insect nests. A common behaviour in this group is phoresy - the mites attach to mobile carriers such as flies to travel from one temporary food source to another. Adult females are the usual dispersal stage, and phoresy is typically non-feeding transport rather than parasitism. Their reliance on transient resources makes rapid colonization and dispersal central to their life history: when a resource is exhausted, phoresy lets populations reach the next suitable habitat. 

Looking closer still and the mites were carrying a load of sac-like objects. They seem to be nematode dauer larvae, hitching rides from one compost heap to another. A dauer larva (from the German dauer, meaning "endurance" or "persistence") is a stress-resistant non-feeding third-larval stage that some nematodes enter when environmental conditions are unfavorable. Instead of progressing to adulthood, the worm halts development at this specialized stage. Dauer larvae do not feed and slow down their metabolism dramatically. They can survive for several months (up to four times longer than a normal lifespan), enduring harsh chemicals, desiccation, and extreme temperatures. Dauers actively seek out new environments; once they encounter favorable conditions (such as fresh food or a host), they exit the stasis, moult and resume normal reproductive development. The anti-slug nematodes you can buy are actually dauer larvae and another example was a dauer larva frozen in Siberian permafrost 46,000 years ago, when Neanderthals still walked the Earth, which was revived successfully (A novel nematode species from the Siberian permafrost shares adaptive mechanisms for cryptobiotic survival with C. elegans dauer larva. (2023) PLoS Genet 19(7): e1010798 https://doi.org/10.1371/journal.pgen.1010798).

And this is without looking for parasitic wasps utilizing the flies, mites and possibly even nematodes in my compost heap. Who knew my discarded banana skins were so interesting?