I have a query about these ants we saw in our local park in Madrid, Spain. The ants themselves seemed fairly standard, with one nest hole, and a long line of foraging ants heading out to a nearby area and coming back with various tasty tidbits. As can be seen from the photo the ants were a number of different sizes.
However, amidst the ants were several white creatures (in the photo centre left, bottom right and the tail of one top left).
These looked nothing like ants, but appeared to be coexisting with them peaceably. They were going in and out of the ant hole, and were ignored by the ants. One or two travelled along the foraging line, but didn't appear to do any actual foraging. They were quite quick too, about as nimble as the ants.
Any information as to what these might be would be gratefully appreciated. My guesses would be a) some precursor ant stage (pupa or some such) b) other insect living symbiotically with the ant or c) I have no more ideas.
Your second guess is correct! The other insects in your photograph are in fact silverfish, cohabiting with the harvester ant Messor barbarus. There are many species of such myrmecophilous (or "ant-loving") Zygentoma around the world, with 16 occurring in the Iberian Peninsula alone. Unfortunately, without a clearer image, it is difficult to provide you with a more precise ID (several different species have been observed in Messor nests in particular), but based on a superficial diagnosis, they most likely belong to the genus Neoasterolepisma.
Silverfish are among a wide variety of other arthropods (including beetles, crickets, spiders, millipedes, even cockroaches) known to inhabit the nests of ants, either commensally or parasitically, and like other myrmecophiles, silverfish have evolved very successful strategies for avoiding detection by their ant hosts. While looking nothing like the ants, as you say, these scaly symbionts are able to blend in with the rest of the colony by rubbing against "callow" or immature workers and adopting the unique chemical profile that sister ants use to recognize one another. In this way, the silverfish enjoy easy access to shelter and resources within the nest and can intermingle freely with foragers on the outside (no doubt pilfering some of the tasty tidbits you observed being brought back to the harvester's granaries). Of course, this chemical disguise is only temporary, so the silverfish must also rely on another characteristic adaptation--speed--if their cover starts to fade, hence their nimble-footedness around the workers.
While some myrmecophilous silverfish are apparently highly specialized, demonstrating a preference for a single ant species, most are generalist interlopers, using their peculiar knack for chemical mimicry to infiltrate the subterranean strongholds of a variety of different ants. Another widespread Mediterranean silverfish species, Proatelurina pseudolepisma, for example, was found in the nests of a total of 13 different species of ants! Those of Messor barbarus, incidentally (the same species in your photograph) happened to be the most popular.
Hope this helps!
Alexandra Westrich & the AntAsk Team