156 THE EARLY STAGES OF TABANID^ 



when fifteen to eighteen days old. The time of the third molt pre- 

 cedes immediately the appearance of the puparium. This period, as 

 has been noted, shows the greatest diversity among individuals of the 

 brood. The third molt in twelve instances was shed between the 

 ninth and twelfth days of life. 



In other individuals the process was not completed within three 

 months, yet the adult fly was an apparently healthy insect. 



Certain unimportant changes in morphology, dependent on the 

 molting process, are noticeable. The loss in size due to contraction 

 of the cuticle preparatory to ecdysis is usually compensated by a 

 substantia extension immediately following the process. The extent 

 of shriveling of the cuticle is represented by 1 mm. in the first molt- 

 ing, 1.5 to 2 mm. in the second stage, and 2 to 3 mm. preparatory to 

 the third stage. There is a notable increase in length resulting from 

 the second ecdysis. A larva, measuring 22 mm. on the day previous 

 to the shedding of the skin, measured fully 25 mm. the following 

 day. In measurements of this sort one must make allowance for the 

 extraordinary amount of telescoping of segments. As much as 5 

 mm. may be involved in this process. 



The structures mainly involved in the ecdysis are the tracheal sys- 

 tem and the appendages of the head. The anal ring of the trachea 

 constituting the syphon is drawn off in each molt in a perfectly 

 cyhndric form. The body trachea is torn from its connections in 

 irregular strands. The entire head capsule, including the chitinous 

 pharyngeal framework, .the great hook, and other mouth structures, 

 are found in perfect form in the various exuvias. These parts upon 

 renewal in the larVa become more heavily reinforced. 



The exuvia (Plate 5, Fig. 81, a, b, c) is usually in a good state of 

 preservation; crumpled, to be sure, but it can be extended in alcohol 

 to three-fourths the length of the larva. Following each ecdysis, 

 the larva is invariably leaden gray with tracheal strands of waxy 

 white. Three anal segments including the syphon become lead- 

 colored and stiffened in structure. They are at this stage more trun- 

 cated, with an anal band of cuticle 1 mm. in depth, making the 

 syphon appear somewhat atrophied. This is no doubt consistent 

 with its restricted function. The color of the viscera has changed 

 from the brilliant red and yellow to an indeterminate white, and the 



