A flea is a type of jumping insect that feeds on the blood of mammals. A common symptom of someone who has fleas is itchiness, and scratching at itchiness is typical behavior. To scratch is to run one's fingernails over a skin surface to relieve itching. To "create something from scratch" is to make something with rudimentary ingredients, or from fundamentals. The flea was created from scratch. The joke is playing on the word "scratch" and its different meanings.
Scientists have created a flea from scratch .