THE STRUCTURE AND TRANSFORMATION OF THE LABIUM OF AN AX JUNIUS. ARTHUR D. WHEDON, DEPARTMENT OF ZOOLOGY, NORTH DAKOTA AGRICULTURAL COLLEGE. INTRODUCTION. During a recent investigation of the degenerative changes in the muscles of transforming Odonata the author found it con-venient to use the retraction of the labium as an index to approach-ing internal changes. For some hours before the final ecdysis the insect remains quiet, without feeding and partly out of water. It is during this time that the retraction of the labium becomes evident. Some attention was consequently given to its condition and structure. When the literature of the subject was searched it yielded little upon structure, beyond that of the chitinous skele-ton, and nothing regarding changes during transformation, in spite of the fact that the larval labium has long been cited in both popular and technical treatments as one of the most uniquely adap-tive structures in insects. To quote Amans, " Cette piece a de tout temps etc jugee comme tres curieuse et tres interessante ; tons Font decrite avec une minutieuse complaisance, et Dufour en profite pour faire intervenir la Providence et les causes finales." Miss Butler's paper (2) is quite universally cited as the authori-tative treatment of the Odonate labium. It is based upon exten-sive comparative studies of the configuration of the labium in all of the subfamilies, and her bibliography includes the works of many of the greater insect anatomists as far back as 1842. Rather unaccountably, however, she omits Amans' paper on the larva of Aeschna grandis (i), and states that " The powerful muscles and hinges which constitute the mechanism of the labium have never hitherto been investigated " (p. 112). This is followed by a very brief and apparently incorrect description of the mechanism. Her interpretations are in turn followed by Tillyard (4) in 1917. Amans (i) studied the larval labium with considerable care but he, also, overlooked some of the most evident and important 286
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