Like all true insects, mealworms go through 4 distinct stages in their development. The beetles lay the eggs that hatch very small worms that grow into larger worms that grow into pupas that grow into beetles....

I raise my worms in a 4 drawer storage container. (sold at at target or Walmart) They are clear plastic so let light in and can stack if you use more than one set.  In the bottom drawer I keep all the beetles in about 2" of bran meal.  Once a month I sift the beetles and place all that comes through the sieve into a new drawer with 2" of fresh bran. (always use human grade bran to avoid infesting your culture with mites or other insects.) I then give the beetles fresh bran.  The bran and casings sifted off the beetles is full of eggs and small worms. Within a week or two you should see small worms crawling through the culture.  They are VERY small at this point.  Within a month to 6 weeks, they are about ½" long.  This small size is the size my birds prefer. I then place a folded piece of newspaper on top of my culture. The worms tend to crawl into the paper.  Harvesting is then as simple as picking up the newspaper and shaking out the worms.) By the time they are 2 months old they are bigger than my birds like and I let whatever is left at that point develop into larva and then beetles.  They do this at about 2-3 months. I have found that higher temps, and more light both speed the development of all stages of the insect

I offer wedges of cabbage for moisture to all the drawers once or twice a week..  Moisture is a two edges sword.  Too little and their growth slows, too much and rot slows the culture.  I compromise by putting in a wedge of cabbage (causes less rot than other things I have tried) and then not adding more until the cabbage is either gone or dried out for 2 days. In this way I can be fairly sure that the bran has a chance to completely dry out before I add more moisture.

To increase the nutritional value of the worms, I also place cuttlebones and insect essentials on top of my cultures right before harvesting worms.  As the worms eat the cuttlebone, their guts are filled with this rich calcium source which is in turn fed to the birds.

I have found this way of raising worms to be very successful. It allows me to have the various stages in separate drawers which decreases the cannibalism that seems to be a problem without strict moisture controls.  It also allows me to go to a drawer of worms and sift out however many I need and know they are all about the same size.

Happy worm farming!