We tried Phoenix worms - since we have so many critters, our first order was 1000. Half arrived black; dead; or dying.
I haven't been using the grubs as feeders much lately, as I was also finding quite a few undigested grubs coming out.
But I still like using the soldier flies that the Phoenix worms pupate into as stimulating flying treats for my chams. I doubt whether they offer much nutritional value, and you can't gutload or supplement them, but they are great for stimulating lethargic eaters and for giving your chams some tongue exercise (they become quite vigorous hunters at the sight of these flying bugs).
Anyway, in my experience the hard, black phoenix worms that you were finding are the ones that are busy pupating into flies. If you leave them for a few days, eventually a fly will emerge.
What I usually do is keep the open tub of Phoenix worms inside a much larger closed (but vented) tub. The flies will escape out of the worm tub and fly around the larger tub, and then I just open the tub into my cham's cage. The soldier flies are actually relatively slow and cumbersome fliers, so they're not difficult to catch by hand if one or two escape by accident. And also, even though they look nasty and wasp-like, they're totally harmless.
I kept getting these looks like "What do you expect me to do with those brown specks in my dish?" ... or "Do you have a magnifying glass so I can see what is in my dish?"... or "I asked for food - not appetizers!"
I had an interesting experience with my most recent batch of Pheonix worms. The previous few tubs I've had, have had the same tiny little worms that you describe. But the last tub had really big grubs in them. They looked the same as the other Pheonix worms, just much bigger. And it wasn't simply that the worms were in a different (later) stage of life: the earlier small batches had pupated into quite tiny flies, whereas this later batch turned into quite large flies.
Anyone know of different size strains of Phoenix worms?
