The Brisker Rav says that every Makkah was done by Moshe and Aharon except for Dever and Makkat Bechorot, which were done by Hashem. Makkat Bechorot was pretty obviously done by Hashem, and Dever was also done by Hashem as it says, ”הנה יד ה הויה,” "The hand of Hashem will strike" (Shemot 9:3). Although it also seems like Arov is from Hashem, both Rashi and Onkelos say that the words “הִנְנִי֩ מַשְׁלִ֨יחַ בְּךָ֜ וּבַעֲבָדֶ֧יךָ וּֽבְעַמְּךָ֛ וּבְבָתֶּ֖יךָ אֶת-הֶעָרֹ֑ב," "I will let loose swarms of insects on you, your servants, your people, and your houses" (Shemot 8:17) implies that Hashem did this through a Shaliach.
What was so special and different about these two Makkot that they were done by Hashem?
To answer this, Reb Dovid Cohen, the Rosh Yeshiva of the Chevron Yeshiva, quotes the GR"A who says that these two Makkot were the only ones which stopped or killed off creation. Dever killed the animals and life in the lower realms, while Makkat Bechorot finished off the firstborn human soul, which is from the upper realm. So because of the odd nature of these two Makkot, they had to be done by Hashem, not a human.
Ramban gives another reason. He says that ten Makkot were divided into two groups: the first five and the second five. The purpose of the first five Makkot was to get Paroh to send out Bnei Yisroel, and the second five were done to harden Paroh’s heart and to inflict him with suffering. This is why it says "ויכבד ה את לב פרעה" regarding the last five Makkot. Accordingly, Dever completed the first five Makkot, and Makkat Bechorot finished off the last five. So we see something special in Dever and Makkat Bechorot: they are the closing of the two different groups.
So being last certainly does not mean you are the least!