Each praying mantis egg case will hatch about 100-200 tiny mantises, all at once. In order to hatch they'll need several weeks of warm weather, so they can "sense" that summer (and pest insects for food) has arrived.
Once hatched, praying mantis begin feeding on small insects, such as aphids, spider mites, lacewings, and any other insect smaller than them. Later on, they'll continue advancing up to larger and larger prey. By summer's end, praying mantis can reach several inches in length. In the fall, females produce more eggs, deposited in a frothy secretion that hardens to protect the eggs from predators and severe winter climates. Egg cases are attached to twigs, leaves, fences, etc. Several egg cases may be laid before cold winter finally sets in. This new generation of praying mantis will hatch when warm weather returns, to repeat the process.
When hatching, the young crawl from between tiny flaps in the cases and hang from silken threads about 2" below the case. After drying out, the long-legged young disperse into the vegetation, leaving no evidence of their appearance. This happens within an hour or two, and it's very difficult to know hatching has occurred unless the elusive, well camouflaged young are found. (The egg case does not change appearance in any way.)
If you'd like to see when the mantis have hatched, place the egg cases in a paper bag, fold the top and seal shut with a paper clip or clothes pin. Place the bag on a window sill in direct sunlight. Periodically open the bag carefully, and when you see tiny mantids running around inside, take them outside and sprinkle them throughout the garden. Be patient - sometimes it takes up to eight weeks of warm weather for them to hatch.
Watching the babies fall out of the sack and crawl around was so fun and entertaining for us. They just kept coming and coming. Sadly we haven’t seen any since the hatch but hope they are out doing their job. Can’t wait for next spring & do it again !!