parshat Vaera #3: why couldn't the magicians create lice?

Daat Zekenim Mibaalei haTosafot offers an amusing explanation of why the magicians were not able to reproduce the lice - Shemot 8:14:

יד וַיַּעֲשׂוּ-כֵן הַחַרְטֻמִּים בְּלָטֵיהֶם לְהוֹצִיא אֶת-הַכִּנִּים, וְלֹא יָכֹלוּ; וַתְּהִי, הַכִּנָּם, בָּאָדָם, וּבַבְּהֵמָה.

14 And the magicians did so with their secret arts to bring forth gnats, but they could not; and there were gnats upon man, and upon beast.

Immediately preceding, we see that even the dust of the earth had turned to lice:

יג וַיַּעֲשׂוּ-כֵן, וַיֵּט אַהֲרֹן אֶת-יָדוֹ בְמַטֵּהוּ וַיַּךְ אֶת-עֲפַר הָאָרֶץ, וַתְּהִי הַכִּנָּם, בָּאָדָם וּבַבְּהֵמָה: כָּל-עֲפַר הָאָרֶץ הָיָה כִנִּים, בְּכָל-אֶרֶץ מִצְרָיִם.

13 And they did so; and Aaron stretched out his hand with his rod, and smote the dust of the earth, and there were gnats upon man, and upon beast; all the dust of the earth became gnats throughout all the land of Egypt.

The suggestion is that the magicians could not create the gnats because they were standing on lice or gnats rather than dirt. Magicians draw their power from the earth, as we see in the story of Shimon ben Shetach and the witches, in which he had his students lift up each witch off the ground, for these witches drew their power from the ground: כל חד וחד מינכון יגוף חדא ויטלטליניה מן
ארעא דעיסקיה דהדין חרשא טלטלתניה מן ארעא לא יכול עבד כלום