The care of spiderlings requires multitudes of small containers, usually glass or plastic vials or baby-food jars of about the same size for ease of handling. Fill each jar halfway with moist (but not wet) vermiculite, or with a mix of sterilized potting soil and moist vermiculite.
The lid is punched with a few pinholes or closed with a wad of sterile cotton or foam to allow some air circulation while letting the substrate remain moist. Each container can house only one spiderling, so if the egg case holds three hundred eggs that develop fully into spiderlings, you will need three hundred containers plus the shelf space to house them securely. Often groups of containers are placed in larger plastic boxes to help control the humidity, which should run about 65 percent to 70 percent for most species.
As soon as a spiderling is placed in a container, it will make its first little burrow and expect food to wander by. Most spiderlings will take a pinhead cricket (the size just after hatching, before more than one or two molts) or a micro mealworm (usually the larvae of small flour bee-des), but very small spiderlings may need flightless fruit flies (Drosophila spp.). The size of the food must match the size of the spiderling and, of course, will have to change as the spider grows. Often spiderlings will attack the dead body or at least the abdomen of a freshly killed cricket if it is split open so juices are easier to get to.
Most spiderlings are fed three to four times a week at first, and their containers have to be cleaned of waste and dried insect husks a day after each feeding. Check the substrate at cleaning time to make sure it has not dried out too much. This means that every week you will be opening and closing each container three or four times (assuming you wait until the next feeding to clean the cage). Multiply this times three hundred containers each time, and you can start to imagine how much patience and time you need to raise spiderlings.
As the spiderlings grow (some individuals grow faster than others, even on the same diet, and different species also grow at different rates), they will have to be moved to larger containers appropriate to their size. This is one advantage of baby-food jars, which can house a young tarantula longer than smaller vials can. On the other hand, a spiderling is easier to see in a vial (remember, they are somewhat translucent and burrowed in), and it is easier to see possible problem infestations such as mites, fly larvae, and the occasional vagrant, blood-thirsty cricket (which can eat a spiderling if allowed to grow). Have you also thought about how much three hundred baby-food jars partially filled with substrate will weigh?
With luck, each spiderling will molt about six to ten times during its first year, about half as many times in the second year, and perhaps two or three times in the third. By year three the immature tarantulas look like the parents but are smaller, often with slightly different color patterns (such as the brightly banded abdomens of Avicularia species) and often with different leg shapes and a less bulky body. Only a few tarantulas look much like the adults after their first year of life.
They now can be transferred to the usual terraria, one per container, and treated much like adult tarantulas, allowing for their small size. The first time you try to raise tarantulas outside an egg case you probably will have many spiderling losses due to fungus: it grows in substrate that is too wet and not cleaned often enough. You may get infestations of either mites (which compete with spiderlings for food and may attack the spider as well) or tiny humpbacked phorid flies, which may lay their eggs on the spiderling and cause its death. Keeping spiderling containers balanced between the proper humidity and feeding conditions versus cleanliness is difficult and learned only by experience. Not everyone has the patience to raise spiderlings, so don’t be disappointed if you don’t succeed. Besides, what do you do with three hundred brownish tarantulas that won’t be salable for at least three years?