At Mincha on Yom Kippur, we read Sefer Yonah as the Haftorah. Chazal selected this sefer to be read on the holiest day of the year because it contains many messages, including a powerful one about Avodat Hashem.
In the Sefer, Hashem uses two diametrically opposite animals to perform His will. First Hashem uses a whale, the largest animal on earth, to save Yonah when he is thrown off of the ship. As we see from the Pasukim, the whale did not hesitate to do whatever Hashem asked of him, and additionally he did not question the logic behind his divine task. Later, Hashem commands a tiny worm to kill the tree that Yonah is resting under, and the worm complies immediately and does as Hashem commands him.
By using both the largest and smallest of creatures to do His will, Hashem is reminding Yonah that he too is among the creations of Hashem, and that the role of all the creations, from the biggest to the smallest, is to serve Hashem. This message does not only apply to Yonah, it applies to all Jews. We must all follow the example set by the whale and the worm, and never forget that we must do always serve Hashem by following his Torah and doing anything that He asks of us. On Yom Kippur, the Day of Judgement, it is especially critical that we come to the realization that Hashem is our Creator, because if we truly believe this, it is simply impossible for us to do anything other than follow the Torah and the way of Hashem, and by doing so we will surely earn a spot in the Sefer Hachayim.