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Hallo Willem-Jan,

I am now experimenting with reaction tubes:

- water in the tip;
- a plug of cotton wool on top, which touches the water now and then (I tilt the tube)
- the substrate with larva(e),
- and another plug of cotton wool to keep the animals from flying away after emergence, while providing the necessary ventilation.

If you don't have a reaction tube at hand, maybe you can find a small bottle like those in which herbs are sold.

I think it should work well with larvae from most habitats, at least. As long as they're mobile they can also choose their distance from the moist cotton wool.

However, don't underestimate the capacity of these diptera to pass though the cotton wool! I have no good solution, only pressing the wool tighter. The problem is, if you cover the tube completely, you often get molds eating your larva.

This method is much used for small ant nests (cf. http://www.akolab.com/fourmis/forum/index.php).

In the picture, there's an arrow pointing to a pupa of - I think - Xylota segnis.

Good luck, Louis