194 THE RESPIRATORY SYSTEM [OH. 



Lamellae may be developed from an original triquetral form 

 in two ways: 



(1) As a horizontal lamella, by a gradual widening of the base 

 of the triangle representing the cross-section of the triquetral 

 gill, and a corresponding flattening of the other two sides below 

 it. Thus the mid-rib, originally placed externally, becomes 

 mid-ventral in position, with the broad horizontal base lying 

 dorsally above it 1 . This is a very rare formation, found only in 

 Argiolestes (fig. 42 A). I do not know whether it is typical for the 

 Megapodagrioninae or not, as we know little of the larvae of this 

 subfamily. It is clearly a specialization to enable the larva to 

 flatten its gills close against a rock, and still expose a large surface 

 to the water. Argiolestes, when at rest, usually raises these gills, 

 like a fan, slightly away from the rock surface. When disturbed, 

 they are at once flattened closely down, so as to escape attention. 



(2) As a vertical lamella, by elongation of the base, together 

 with approximation of the other two sides towards it, in a vertical 

 plane 2 (fig. 86 c). This process is easily understood by comparing 

 cross-sections of the lateral and median gills of Calopteryx. It 

 clearly first arose as a specialization in the case of gills which no 

 longer rested on the surface. Nearly all the forms which possess 

 lamellar gills live freely in masses of water-weed, matted roots, etc. 

 The vertical lamella not only exposes its whole surface to the 

 water, but the three gills together form an efficient parachute, 

 used by the larva when descending through the water. 



We can only briefly mention the various types of vertical 

 lamellae known, together with the types from which they have 

 been derived: 



A. Lestidae (figs. 41, 84 A, B). In all known larvae of this 

 family the lamella is of the simple form, elongated oval in shape, 

 often with a broadly rounded end, and usually of exceptional 

 length. In the Lestinae (figs. 41, 84 B) the secondary or branch 

 tracheae come off from the main trachea at right angles, and run 

 nearly to the border of the lamella before branching. The finer 



1 This applies only to the lateral gills. The median gill has been formed by 

 horizontal flattening of a gill originally diamond-shaped in cross-section. 



2 This is the process for the lateral gill. The median caudal lamella appears 

 to be formed generally by symmetrical narrowing, in a vertical plane, of a gill 

 in which the cross-section was originally diamond-shaped. 



