Insects have an external skeleton consisting of a layer of cells and secreted cuticle. Unlike vertebrates that have muscles attached to internal structures (bones) insects have most skeletal muscles attached to the exoskeleton. In some cases, it would be adaptive for insects to have internal points of attachment for skeletal muscles? What is an insect to do?
Invaginate! In the head of an insect, the outer layer of cells can form pits by growing inward. From the inside, the “pit” is a finger-like projection that contains cuticle and can be hardened into an attachment for the muscles in the head. In the caterpillar head (left), are three pits, one originating from back-center of the head and two originating from the front of the head, one to each side. The three invaginations meet in the center of the head and form a structure called the tentorium. Muscles of the mouthparts and antennae can make attachments to the tentorium. Through invagination, an insect with an external skeleton can create an internal skeletal structure that functions like a bone.