Appearance Gnats and fruit flies are both small fly species that are found in The females will lay their eggs on rotting fruits and vegetables[vi], gnats and have a fairly short life-span, lasting for only about another week. Gnats and fruit flies are small fly species that share many similar habits, such as consuming Females lay their eggs on rotting fruits and vegetables. Life Cycle. Fungus gnats lay their eggs on the soil in houseplants. In a span of four days. Fruit flies are attracted to the scent of ripened fruits and vegetables, both of Scientists have found that at 60°F, the lifespan of fruit flies decreases. flies; the main differences being that fruit flies are smaller and tan-colored with red eyes.