CH. IX] THE RESPIRATORY SYSTEM 183 



lies with its axis of symmetry in a position corresponding with 

 one of the odd numbers on the clock-face diagram. This is because 

 the holobranch is formed, not from the right and left sets of cross- 

 folds originally supporting one main-fold, but from the right set 

 of one together with the left set of the next. Now we have already 

 shewn that two such sets have their trachea connected with a 

 single series of primary efferents (fig. 78 A). Hence the Duplex 

 System combines a definite increase in the complexity of the 

 gill-system, with a very useful simplification of the efferent 

 tracheal system. As the tracheae to the main-folds are aborted, 

 each separate gill now has its own secondary efferent, and each 

 holobranch its own single series of primary efferents. 

 Three main types are easily recognized : 



(1) The Implicate Type (fig. 79 B ; fig. 80, 4). This occurs only 

 in the Brachytronini. The gills form a series of obliquely-placed 

 projecting concave tiles, slightly overlapping one another from 

 before backwards. The true axis of symmetry of a holobranch 

 lies above the letter B in fig. 79. Owing to the main-folds being 

 still present, but non-functional, the cross-fold character is not 

 yet lost, and the overlapping ends of the gills from two consecutive 

 holobranchs (to the left of B) are connected with the underlying 

 main-fold. Thus the gills can be spread out on a slide so as to 

 resemble very closely the Undulate Simplex Type, from which 

 they have been clearly derived (cf. fig. 80, 4, i). Pigmentation is 

 very rarely present. 



(2) The Foliate Type (fig. 79 c, D; fig. 80, 5-7). This is 

 found only in the Aeschnini. Here the gills are constricted off 

 so as to form a series of separate leaves or foliae, the secondary 

 efferent representing the stalk. The free ends of the gills are much 

 enlarged, with capillary loops spreading all over them in immense 

 numbers. There are two sub-types, (a) the normal foliate sub-type, 

 found in Aeschna (fig. 79 c ; fig. 80, 5), with the foliae resembling 

 crinkled rhubarb or cabbage-leaves, and (6) the papillo-foliate 

 sub-type, found in Anax (fig. 79 D ; fig. 80, 6, 7), with the foliae 

 swollen up into oval humps on short stalks. All over the free 

 surface of these humps a number of minute papillae arise. These 

 differ from the larger papillae of the Papillate Simplex Type in 

 not containing any axial trachea, but simply five or six drawn-out 



