They turn into Soldier Flies. I purchased mine at
http://www.azdr.com/SearchResults.asp?Cat=66 and they came in nice little containers. These worms/larvae are REALLY very, very tiny. They are more for your smaller chameleons. Maybe I'll try to take some pictures of one on a penny or something to show you just how tiny they are later.
They wiggle A LOT and are consistently and constantly moving around, which is really attractive for catching the chameleon's eyes. I put a few in my peacock chameleon cages and they didn't seem much interested in them (but this is when they were first acclimating, so I'll try again), however, my new baby veiled and my leaf chameleons seem to like them. And your right, the calcium content is very high, which is nice for your growing baby chameleons. Another nice thing, is that you don't need to feed them (at least where I obtained them) and if they start to not look so plump anymore you add a few drops of water for them.
These Phoenix worms are really too small for large panther chameleons or larger chameleons, however, the flies are slightly smaller than a housefly, which means it might be an entertaining treat for your larger chameleons once they morph. They are perfect size for leaf chameleons. I have a tiny bowl inside my terrarium eco-system I made for my leaf chameleons so they can't get out into the dirt. I actually didn't see one of the worms change to a fly, but I looked inside the tiny bowl one morning and there he was! One of my leaf chameleons saw him too and snatched it right away lol. So I might experiment in placing 20 or so into a container inside one of my larger chameleon's cages and wait for them to turn into soldier flies and see how they like it. If anything else, I'm sure they would be relieved of the boredom

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As far as knowing when they are close to morphing, I have no idea. I've only had them a bit over a week. According to the company I purchased these from, the larvae lasts about 2 weeks. So, based upon that, I'm guessing two weeks after purchase they might morph to flies. They also come in extra small, small, medium, and large sizes. I wanted a larger size when I ordered, but this particular company only had extra small and small sizes available at the time. So those are what I have. I suppose a larger size larvae would definitely have a shorter time prior to morphing. The largest I've seen out of mine was about 1/4 inch long and the rest were smaller. The woman at AZDR actually called me when they were out of the large and she said she would put as many as she could find that were larger in my order (which was very nice for her to call and take time out to find larger larvae's by hand! I was impressed with the service).
Another nice thing... they seem to last a long time. Each of my containers (I bought 3) has 150 larvae in it. It really seems to go a long way and I still have worms in each of my 3 containers.
So, that has been my experience so far with phoenix worms. I will take some and experiment morphing them into flies for the larger chameleons and see if they enjoy trying to catch them as they fly

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