96 PSYCHE [October 



As a preliminary^, certain of the old larval structures are gradually broken 

 down. The fat in the anterior end of the body disintegrates and is made ready to 

 be used in building up the new structures of the imago. 



As the old muscles of the head disintegrate the whole anterior end shrinks 

 away from the chitin and the new head begins to take shape. If the chitin be 

 removed at this stage we find the condition shown in Plate VI, figs. 3, 4 and 5. 

 The antenna and mouth parts have begun to form but are still very crudely 

 shaped ; the rostrum is already apparent, though broad and short and with walls 

 transversely corrugated. 



The antennae also are forced into many folds by reason of their rapid increase 

 in length. The folds suggest the segmentation of the adult antenna. Real 

 internal segmentation, however, does not take place until the pupal stage. 



More folds occur on that part of the rostrum immediately back of the labrum 

 than posterior to the base of the antenna. This in the adult is the longer part of 

 the beak. 



The mouth parts of the old larva are thick, broad and shapeless, yet resemble 

 in general outline homologous parts in the adult. The labrum is short and slightly 

 bilobed. In the very old larva the large mandibles, always very prominent, are 

 somewhat, though not deeply, serrated on their inner edges and otherwise quite 

 similar in form to the stout biting jaws of the adult. Ventral views of the develop- 

 ing rostrum (figs. 4 and 5) show the bilobed maxillae and trilobed labium. The 

 labial glossa and paraglossae at this stage are large and distinct. In Mononychus 

 (fig. 4) the hypopharynx protrudes beyond the labrum. 



A faint differentiation of tissue upon each side of the epicranium shows the 

 outline of the large compound eye. Near the center of this is a small opaque spot 

 where the optic lobe of the brain is developing. 



The Development of the Beak. 



In its general characteristics the hypodermis is the same for all the parts of 

 the head. From the beginning of metamorphosis it is very thick. The cells are 

 large, multiply rapidly and become closely crowded together and elongate. The 

 nuclei are oblong and large, occupying fully one third of the cell. They are filled 

 with large chromatin granules. 



With the multiplication of cells and increase in size of the parts the hypoder- 

 mis is forced into folds. In this way the entire beak and antennae are crowded 

 under the old larva skin. When the pressure is removed by the shedding of this 

 skin these parts stretch out (PI. VIII, figs. 10 and 11), assuming suddenly more 

 than half their final length, and the pupal stage is entered upon. 



