Perhaps the youngsters of a fly, cabbage maggot.

Cabbage root maggot. There are traps for the adults and beneficial nematodes for the larvae available to home gardeners. If they are not already in the soil, row covers will help.

Here is a link that might be useful: Cabbage Root Maggot

Thank you; that's it!. Row covers it shall be -- when the yellow rocket blooms. Regards, Peter B.

There is also a turnip root maggot; same genus (Delia), different species. I suppose that either fly will attack turnip or cabbage, but I'm not sure. (There is also a species of Delia that attacks carrot family roots.) Since I grow brassicas in pots, row covers seem a lot of work. Either of these two root maggots are my worst enemy. At least you can see the caterpillars or aphids as a gradual process; these maggots are invisible until within a single day your brassica can suddenly wilt & die without any warning whatsoever. Unlike many brassicas, turnips & radish have enough "decoy root" to recover (I am a brassica & radish "greens" guy; if I get clean roots, that is just icing on the cake).

I found the same thing that you describe inside a pumpkin...the pumpkin had only one small tiny brown spot on the skin but something got inside because I found little tiny maggot things...