MUSCULAR REORGANIZATION IN THE ODONATA DURING METAMORPHOSIS

¹ DEPARTMENT OF ZOÖLOGY, NORTH DAKOTA AGRICULTURAL COLLEGE

1. There has long existed a misconception as to the time and nature of tissue and organ reorganization in the metamorphosis of the Hemimetabola. It is now determined that most of the changes in the muscles occur after emergence.

2. New thoracic muscles, ultimately to serve the wings, are added from instar to instar during the growth of the larva, and are carried over into the adult stage. The full complement of larval abdominal muscles is present through all the later instars and no new muscles are developed during transformation.

3. The heavier inner layers of larval abdominal muscles (the Primary and Secondary Longitudinal Sternals and Tergals, the Middle Dorso-Ventral Muscles, and the Dorso-Ventral Oblique Intersegmental Muscles) degenerate soon after transformation. The outer and much lighter set remains to function in the adult.

4. For a short time after emergence the muscles which are to degenerate retain their normal appearance and are well supplied with tracheæ and nerves. Later they become yellowish and granular and gradually disappear. The time at which degenerative changes become noticeable varies with the genus, being early in Libellula, Plathemis and Sympetrum and much later in Anax.

5. Degeneration begins in the first segment of the abdomen and proceeds gradually to the last. It is completed in Anax in about three days.

6. The facts recorded make possible a statement of the homologies between the larval and adult abdominal muscles.