Bilateral Symmetry

This is a photo of a fly. The fly demonstrates bilateral symmetry. Bilateral symmetry is an organism with body shapes mirror along the mid-line called the sagittal plane. This is an external property and does not apply to the internal organs that may be distributed unsymmetrically. The fly in the picture shows bilateral symmetry because if an imaginary line were drawn down the center of its body, one side would reflect the other. This trait applies to 99% of animals. Bilaterally symmetrical animals have an advantage over non bilaterally symmetrical animals because their sensory organs are clustered at the anterior (head) part of their bodies. This is called cephalization and occurs because bilaterally symmetrical animals have a forward motion and this means that the anterior is the first to encounter environmental stimuli and allows the organism to react. This trait is also advantageous because it allows organisms to be more resistant to air flow or water and allows them to be more efficient and smooth-running. Organisms with bilateral symmetry demonstrate better visual perception, better control of movement, and even allows them to indicate their health condition.       

Editors. “Bilateral Symmetry - Definition and Examples.” Biology Dictionary, Biology Dictionary, 28 Apr. 2017, 
        biologydictionary.net/bilateral-symmetry/.