Rashi on Exodus 8:14/רש"י על שמות ח׳:י״ד

1להוציא את הכנים TO BRING FORTH GNATS — to create them and to bring them forth from some other place (not from dust but to turn some other material into gnats).

2ולא יכלו BUT THEY COULD NOT — for the demon (through whose agency this was to be done) is powerless in the case of a creature smaller than a barley corn) (Sanhedrin 67b). Therefore they had to admit,