Yes, they can and will hatch if they leave the female flea’s body intact. Considering that a female flea will lay about 50 eggs a day, there are probably always eggs ready to hatch on your critter, so even if you were successful in smashing the egg(s) inside the female, there were a LOT more that you didn’t squish already in the animal’s coat hair, or on your couch, carpet, or bed.
Why it is so weirdly satisfying to smash them is one of life’s little mysteries. When I have had this issue with my zoo, I can often hear myself mumbling swift death wishes to each tiny invader.
You probably know this already, but food grade diatomaceous earth is the safest and best long term flea control available. It’s super cheap ($1.49 for 1 lb at my health food store), and easy to use. Just dust your critter and anywhere the animal sleeps or wanders. It doesn’t kill the fleas like a traditional chemical pesticide…the tiny particles penetrate the hard shell of the flea and they dehydrate fairly quickly. Best to not make giant clouds of it around the eyes and nose while applying.
Although I haven’t tried it, I know folks that also mix the powder with a minimal amount of water to make a paste and feed it to their pets on a spoon for internal parasites as well (worms).
For bad infestations, I use one Comfortis tablet. It is as natural as you can get in the veterinarian prescribed oral flea controls. It’s derived from organic RUM, of all things, so it is safer than those super popular brands that are made of a toxic soup. One tablet lasts a whole month, and I don’t use them unless I have to. The directions say to dose once monthly, but unless you have ‘re-infestation’ issues, I really don’t see the need. A little pricey, (6 tablets for $95) but my last box lasted me 18 months on 4 cats.
I say, ‘Squish away!’ and give Kitty a spa treatment with some diatomaceous earth powder.