IX] THE RESPIRATORY SYSTEM 195 



branches and capillaries run along the edges much like the finer 

 veins in a leaf. Bands of beautiful dark brown or purple pigment 

 often cross these lamellae, and give them a striking appearance. 

 In the Synlestinae (figs. 41, 84 A) the gill is much shorter, and the 

 tip slightly pointed. The branch tracheae come off somewhat 

 obliquely from the main stem. [The median gill of Calopteryx 

 also belongs to this simple type.] 



B. Agrionidae (figs. 42, 84 D-H). There is considerable 

 variety in the type of gill found in this family. It is probable 

 that most of the forms have been descended from a two-jointed 

 or constricted type, though many highly specialized forms shew 

 little sign of this at present. The following types occur: 



(i) Constricted Saccus (fig. 84 c). This is found in Nososticta 

 and Neosticta, of the Protoneurinae. The larvae are rock-dwellers. 

 The shape of the gill suggests a candle surmounted by a flame. 

 There are either two, three or four main tracheae, all of which 

 penetrate through the constriction. 



(ii) Constricted Lamella (fig. 84 D). This very beautiful opaque 

 gill occurs in Isosticta (Protoneurinae). It is clearly derived from 

 (i) by flattening. The larva dwells freely in weeds, roots, or debris. 

 The change from saccus to lamella seems to be correlated with the 

 change from the rock-dwelling to the free-living habit of the larva. 



(iii) Nodate Lamella (fig. 84 E, F). This is a fairly common 

 form of gill, found in many Agrioninae (Caliagrion, Pseudagrion, 

 Austroagrion, Erythromma, Agrion). The node is a reduction from 

 an originally complete constriction. The prenodal border is always 

 armed with spines, which stop short at the node. The largest 

 spine borders the node. The joint may be traced more or less 

 completely across the gill, but is usually only effective close to 

 the node itself. The tracheation is often extremely beautiful, 

 and of a dendritic character. The tip is usually rounded. 



(iv) Subnodate Lamella (fig. 84 G). This common form also 

 occurs in many Agrioninae (Ischnura, Enallagma). The position 

 of the node is now only marked by the termination of the row 

 of prenodal spines, together with a more or less faint trace of the 

 transverse joint. The basal (prenodal) part of the gill is usually 

 slightly thicker than the rest. The tip tends to become pointed. 



(v) Denodate Lamella (fig. 84 H). When all trace of the node 



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