There is a more indepth article on the survival tricks the eggs have developed over millions of years. See See 'Eggs and babies'. But to summarise, if the eggs hatched out all at once then if there was a problem (hot spell, predator, etc.) then the whole population would die out. So only a few do at once. The first main trigger is the egg needs light on them to ensure the new born Triops can swim to safety and is not say buried under mud! Second main trigger is that the egg needs to be wet, but not just any old 'wet' water. The egg detects the amount of dissolved minerals in the water. Fresh rain water is usually pretty pure, this indicates to the egg that the rains have just come and the temporary pool will hopefully last for the few weeks the Triops needs to grow to adulthood and lay more eggs. Water with minerals in it usually indicates that the pool has been around for a while and so could dry up soon. This is why people often discover a few weeks into raising Triops and performing water changes that new Triops come along! These eggs think fresh rain have come! The amazing thing is that each egg is different, they all appear to have varying degrees of triggering points. Some will not hatch out the first time around but will maybe the 2nd or 3rd time it's wetted. Other trigger points are that the egg needs the water to be warm enough. Too cold and it's not going to be able to grow fast enough. Also some eggs need to be dried out for a while. This ensures it will hatch out the next time it rains. This may be in a few weeks or even a few years! Triops eggs can keep for up to 10 years!