The first opening in a developing insect embryo always becomes the mouth

The fundamental divide in animal life is determined in the earliest stages of embryonic development. In a group called protostomes—which includes arthropods, mollusks, and annelids—the initial pore that forms in the blastula, known as the blastopore, develops into the organism's mouth. This developmental path diverged from other animals approximately 600 million years ago and is often accompanied by spiral cleavage, a unique pattern of early cell division.