One day I noticed three strange caterpillars in the tank too and I do not know where they came from.
The only thing I can think of is that the Emperor Moth I had in there some time ago laid eggs and these have hatched out.
They remind me of the old time robbers who wore a mask over their eyes. I think the fruit fly was hitching a ride.
I am keeping them and recording their progress to see what happens.
There is just one slight problem!! The mantis, although a quarter of its size, keeps on trying to eat it!!
The caterpillar is much too clever and keeps on swaying his body away from the mantis.
I am sorry but some of these pictures are not very good but it will give you a good idea of what is happening.