What is the significance of "Vayetar El Hashem" (as opposed to Vayispalel El Hashem)?
Rashi: It means that he (did not just pray, rather, he) prayed intensely. 1
Riva: Hashem gives; he does not take. Moshe was adamant that Hashem take back every animal, lest the Egyptians benefit from the hide, and likewise that all the locusts leave, lest they be able to eat them. Therefore he needed much prayer to remove these two Makos. Pharaoh said Hatiru also for other Makos; he thought that all need much Tefilah.