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Any ideas? Taken in South Africa in a small subtropical stream

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There is a key to families of larvae in Manual of Afrotropical Diptera (free download). My first guess was Empidoidea, but perhaps some other Orthorrhapha.

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My first guess was a Limoniidae, for it has a fully developed head capsule and dorsal and ventral pseudopods. It reminded me a bit of Lipsothrix species, larvae of those live in deaf wood in brooks. But the anal segment with the spine is unlike Lipsothrix I know. There are some Limoniidae known with a spine but the genera have a reduced head capsule.

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Stumbled upon a drawing of the larva of Asioditomyia japonica (Ditomyiidae) which comes a bit close to your larve.
It is depicted as fig. 98 in Krivosheina, 2012. Howver, in her key on Palaearctic nematocera larva she writes on Ditomyiidae that Ditomyia and Asioditomyia has the penultimate segment dorsally with 2 long acute sclerotized projections, carrying spiracles, and this is a discriminating feature to separate the genera from Symmerus sp. So, does your larva has 1 or 2 spines on the penultimate segment?

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and photo 40 with Ditomyia sp from the same book.

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Perhaps foolishly, I had excluded Ditomyiidae as they have previously been recorded from terrestrial situations (fungi and wood). I suppose this might have been an accidental visitor to the stream.

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Thank you for the discussion