Flea life cycle

Fleas and butterflies have the same life cycle:
adult, egg, caterpillar, cocoon, adult.

flea eggs and egg tooth (inset)

Once on the host the female flea takes two or three blood meals a day and soon begins laying twenty or thirty each day.  Seen with the naked eye, flea eggs look like large grains of salt.  When ready to hatch, baby fleas use their tiny egg tooth to cut their way out of the eggshell. 

emerging flea caterpillar

this is your carpet
mature flea caterpillar, flea feces, eggs

The female flea sucks much more blood than she needs for herself, processing nearly all of it into nutrient-rich fecal pellets. Flea caterpillars crawl around eating whatever they can find, including cookie crumbs, miscellaneous organic matter, and these nourishing little fecal pellets.  (See dark fecal pellets in photo above, on the carpeting and inside the caterpillar.)

flea cocoons and pupa (inset)

When full grown, the flea caterpillar makes a little cocoon, just like butterflies do. Protected in its cocoon, the flea is impervious to insecticides and can wait many weeks for a host to come near. 

adult flea emerges from cocoon
full-sized and ready to go.

When the flea senses vibration from footsteps, it pops out and leaps at any likely source of warmth and vibration. You can spray that rug all you want, but fleas will continue to emerge from their cocoons for about a month.  

Photo source material courtesy of Novartis, manufacturer of Program™ for flea control and Sentinel™ for flea and heartworm control.