Hashem's instructions to Moshe to bring the locusts reads as follows:
וַיֹּאמֶר יְהֹוָה אֶל משֶׁה נְטֵה יָדְךָ עַל אֶרֶץ מִצְרַיִם בָּאַרְבֶּה וכו
And Hashem said to Moses, "stretch out your hand over the land of Egypt for the locusts..."
The vowel under the ב of בארבה is a qamatz, which is in place of the definitive article ה. (See here for more about that.) This indicates that there were some specific locusts that were being referred to by this verse (i.e. send the locusts as opposed to send locusts).
I think Rashi is addressing his point with his comment to this word,
בארבה: בשביל מכת הארבה
For the locusts: for the plague of locusts
thus making the definitive article referring to the plague, as opposed to the locusts.
However, I am wondering if there is any other approach to this in the commentaries. Does anyone explain that there were some specific, otherwise known, locusts that were involved in this plague?