56 CLAUDE FULLER. 



upon a fairly level plane. This condition obtains until the 

 insect has progressed well along the last nymphal stadium. 

 It is then succeeded by a phase in which the tracheae are 

 found sinuate and later coiled and overlying one another, 

 fig. 61 (PI. VI). This is due to the growth of the final wing 

 and the lengthening of the tracheee with it. The cuticle 

 encasing it cannot give Avay along its margin, but both the 

 upper and lower surfaces bulge outwards, the growing wing 

 accommodates itself to its narrow confines by a series of 

 longitudinal folds and transverse creases, and so becomes 

 packed up very tightly. 



From quite early stages pale bands are to be seen in the 

 organ. As the largest of these cause faint ridges to form 

 on the dorsal surfHce of a wing-sac but leave no other trace 

 on the moulted cuticle, they must represent a certain differen- 

 tiation of tissue. Their structure has not been studied, but 

 they no doubt consist of modified cells which form about the 

 trachege and cause thickenings between the upper and lower 

 cuticles. The principal trachete have not been noticed dis- 

 sociated from them, but this is not the case with some of the 

 branch trachete. Consequently, certain pale thickenings are 

 found in which thei-e are no tracheae. As development proceeds 

 certain tracheee tend to move out of the thickenings. After 

 such a movement a further thickening may or may not form 

 about the latest course of the trachea and hence one and the 

 same trachea may give rise to two thickenings. Two concrete 

 illustrations may be taken from the wings of Calotermes 

 durbanensis. 



The first is the frequent presence, in the final wing, of a 

 bicornute branch near the base of the media, lying between 

 it and the radius, figs. 112 (PI. VIII), 127 (PL IX). This 

 rib is derived from a similarly shaped, pale thickening 

 that is occasionally to be seen in a wing-sac, and it depends 

 upon the stage in which the tracheation of the wing-sac 

 happens to be whether this is or is not associated with a 

 ti'achea. When it is so, the trachea is furcate, with the tips 

 of its branches turned backwards, fig. 62a (PI. VI) ; when 



