188 THE RESPIRATORY SYSTEM [CH. 



(2) An epithelial syncytium. This is a very flat layer of 

 completely fused cells, underlying the cuticle of the gill. It is a 

 specialized portion of the epithelium of the hind-gut, and may 

 be termed the "respiratory epithelium." The nuclei are oval, 

 numerous, and irregularly placed. Except at the base, the 

 syncytia of the two walls of the gill fuse together, blocking out 

 the haemocoele, and only allowing of the passage of capillaries. 

 Pigment granules are frequently scattered in the epithelium. 



(3) Tmcheal capillaries. These run in the fused syncytial 

 mass. They usually pass along into the gill closer to one wall, 

 and return closer to the other. The lumen is minute, and collapses 

 soon after death. There is no spiral thread. The nuclei of the 

 ecto trachea are few, and jut out strongly at the sides of the lumen. 

 The structure suggests great stretching of an originally shorter 

 and wider tracheal loop. 



(4) Basal pads (bp). At the base of the gill the two walls 

 are not fused. A prolongation of the haemocoele separates them. 

 In this cavity the larger tracheal branches (tr) run. Either on 

 one side, or on both, the epithelium of the gill is seen to be 

 enormously swollen, forming a huge pad, tensely rilled with liquid. 

 These pads are only found on the anterior side of the gill in most 

 forms; in the Aeschninae (A), they occur on both sides. In all 

 forms except the Libellulidae they are diffuse and non-localized, 

 so that they are not noticeable on opening the gill-basket. In the 

 Libellulidae, however, in correlation with the extreme separation 

 of the gill-system into definite lamellae, the basal pads have become 

 restricted and localized into rounded pigmented discs, which are 

 very conspicuous objects in the opened gill-basket. The pads 

 possess huge oval nuclei, from which a fibrillar network radiates 

 out. No separate cell-boundaries can be seen. The protoplasm 

 is restricted to the fibrils and the extreme borders of the pad ; 

 the spaces between the fibrils are filled with liquid, apparently 

 water. The pads appear to form strong basal supports for the 

 gills. 



Just as the thin epithelium of the gill is clearly homologous 

 with the undifferentiated rectal epithelium of other insects, so it 

 seems highly probable that these basal pads are derived from the 

 original swollen portions, known as the rectal pads. 



