The four species resemble some of the most important organs in the human body. They hint that one must use all these body parts to serve the Creator.

These four species possess an additional attribute, which is that they resemble human organs of great significance. The citron resembles the heart, which is the locus of understanding, symbolizing that one must serve his Creator with his understanding. The palm branch resembles the spine, which is a person’s main support, symbolizing that one should set his entire body upright to serve God. The myrtle [leaf] resembles the eye, symbolizing that one must not stray after his eyes “on the day of his joy.” The willow leaves resemble the lips, with which one completes all his acts of speech, hinting that one should exercise restraint over his mouth and control his speech, and should fear God, even at a time of rejoicing. (Sefer HaĤinnukh 324)

Further reading: For more on the symbolism of the four species, see A Concise Guide to the Sages, p. 291.