1913] Biology of May -flies. 383 



can be moved a little. The breathing movement is, however, 

 slow and feeble. The nymphs are thus provided with large 

 breathing organs, but also burdened with an unweildy load. 



In Siphlurus the gills are double and are stiffened by strong 

 tracheae and moved by muscles at the base, so that they can 

 be held upright and can also be vibrated with great rapidity. 

 In addition there are narrow spinous bands upon the inner 

 sides of the upper lamellae. Those of Callibaetis (PL XLIII, 

 Fig. 5), are held in upright position, and can be rapidly vibrated 

 like those of Siphlurus. They are much smaller and lie 

 farther dorsad when pulled down close to the body. They 

 are better protected because less conspicuous, and better breath- 

 ing organs because their rapid vibration enables them to absorb 

 as much oxygen as if they were broad and bulky. 



In the gills of Baetis the marginal bands are hardly in- 

 dicated, but those of Ameletus are broadly bordered by thick 

 spinous bands of chitin. In them the single lamella is fairly 

 supplied by tracheae. Its base is inserted into a shallow 

 notch in the posterior margin of the tergite. Its attach- 

 ment is thus slightly protected and at the same time it is 

 allowed to swing freely. Adjustment to the conditions 

 in water falls is always marked by an increase in the tracheal 

 supply. - In Chirotonetes (PI. XLVI, Fig. 13), this has been 

 made by a great increase in the number of fine tracheoles 

 which supply the lamella and by the development of a fimbriate 

 gill at the base main trachea of which is a branch of the main 

 trachea of the lamella. The margins of the lamella are bor- 

 dered with thickened spinous bands similar to those of Amele- 

 tus, but it has also a stiff rib extending from base to tip. 



A second group of Baetinas in which the gills are much 

 specialized includes those which have been adjusted to an 

 environment of mud and sand. Nearly all of these nymphs 

 have the number of gills reduced. In Ephemerella excrucians 

 there are but five pairs of gills and these cover but two ab- 

 dominal segments. The attachments are in every case pro- 

 tected by lateral spinous extentions of the abdomen. In 

 Ephemerella rotunda and E. excrucians a wide hollow shelf 

 is -formed from these spines, upon which the gills rest. Each 

 gill consists typically of a thickened lamella, which completely 

 overlies the delicate fimbriate-lamelliform division beneath. 



