THE OPEN TYPE OF WING -GROWTH 95 



on the abdominal segments from the second to the eighth 

 inclusive. These are at first slender and elongate (Fig. 52 a) ; 

 after several moults they are seen to be flattened in form and 

 traversed by a set of branching air-tubes (Fig. 52 c) continuous 

 with the tracheal system of the larva. These are the charac- 

 teristic leaf-like abdominal gills of the mayfly grub, each pair 

 attached laterally to the hinder edge of the segment. In 

 some cases projecting into the water from the margin of the 

 abdomen, in others lying over the back, these gills are in 

 frequent motion, and, surrounded by water, are efficient 

 organs of gaseous exchange ; delicate, elongate bristles are 

 seen to fringe them in the typical Ephemerae. 1 As these 

 gills are usually flattened in form, dorsal in position, and 

 traversed by air-tubes they have been compared 2 to the wings 

 borne on the thoracic segments. Recent investigations 3 into 

 their structure, musculature and development 4 show, however, 

 that their true correspondence is rather with the insect's 

 legs, that they are indeed the limbs of the abdominal 

 segments which bear them, specially modified for the function 

 of breathing, and that they indicate thus a further parallel 

 between mayfly grubs and bristle-tails, many of which carry 

 a series of abdominal limbs. In some mayfly larvae the tail- 

 feelers (cerci) also are used as breathing organs, the chamber 

 of the heart in the hindmost segment sending into them 

 delicate arteries through which the blood is propelled, escaping 

 through small holes into the extension of the blood-space 

 which forms the cavity of the appendage, which thus becomes 

 a blood-gill, effecting gaseous exchange through its delicate 

 cuticle with the dissolved air. 5 



The mayfly grub may show no trace of wing-rudiments till it 

 reaches the eighth or ninth stage of its life-history ; then these 



1 A. Vayssiere : " Recherches sur 1' Organization des Larves des 

 Ephemerides ". Ann. Sci. Nat. (Zool.), (6) XIII. 1882. 



2 C. Gegenbaur : " Grundriss der vergleichende Anatomie ". Leipzig. 

 1878. 



3 C. Borner : " Die Tracheenkiemen der Ephemeriden ". Zooloe. Am. 



XXXIII. 1909. 



4 R. Heymons : " Grundziige der Entwicklung und des Korperbaues 

 von Odonaten und Ephemeriden ". Abhandl. K. preuss. Akad. d. Wissensch. 

 Berlin. Anhang. 1896. 



5 O. Zimmermann : " Ueber eine eigentiimliche Bildung des Riickenge- 

 f asses bei einigen Ephemeridenlarven ". Zeitsch. f. wissensch. Zoolos. 



XXXIV. 1880. 



