The skeleton is a thickening of the horny cuticle of the annelid on the surface of each segment. The horny cylinder surrounding each segment is composed of several pieces, and on the abdomen these are united by flexible, infolded membranes. This allows the increase in the size of the segment corresponding to the varying size of the digestive and reproductive systems. In this part of the body the skeletal ring of each segment is joined to that of the segments before and behind it in the same manner. But in other parts of the body we shall find the skeletal pieces of each segment and the rings of successive segments fused in one plate of mail. The legs are the parapodia of annelids carried to a vastly higher development. They are slender and jointed, and yet often very powerful. A large portion of the muscular system of the body is attached to these appendages.
But the insect has also jaws. The annelid had teeth or claws attached to the proboscis. But true jaws are something quite different. They always develop by modifying some other organ. In the insect they are modified legs. This is shown first by their embryonic development. But the king- or horseshoe-crab has still no true jaws, but uses the upper joints of its legs for chewing. There are primitively three pairs of jaws of various forms for the different kinds of food of different species or higher groups. But some of them may disappear and the others be greatly modified into awls for piercing, or a tube for sucking honey. Into the wonderful transformations of these modified legs we cannot enter.
The muscles are no longer arranged to form a sack as in annelids. Transverse muscles, running parallel to the unyielding plates of chitin or horn could accomplish nothing. They have largely disappeared. The work of locomotion has been transferred from the trunk to the legs.
The abdomen of the insect is as clearly composed of distinct segments as the body of the annelid. Of these there are perhaps typically eleven. The thorax is composed of three segments, distinct in the lowest forms, fused in the highest. This fusion of segments in the thorax of the highest forms furnishes a very firm framework for the attachment of wings and muscles. These wings are a new development, and how they arose is still a question. But they give the insect the capability of exceedingly rapid locomotion.