BY K. J. TILI.YARD. 



99 



Tiills exceedinglj- narrow, quite unpigmented, the pointed tip not 



formed by sudden nairowing-in of the gill-edges 



Argiocnemi.'< Helys; (PI. ii., tig. 17). 



C4ills moderatelj' narrow, lightly pigmented near tip; the tip verj' 

 sharplj' pointed, and formed by sudden narrowing-in of the gill- 

 edges from the broadest part of the gill 



Xavthocnemh Tillyard; (Text-fig.27). 



The gills of the following species are known :— 



1. AnstroGnpmis sj^lendida Martin. Length of gills, wlien full- 

 grown, median 7 -6 mm, laterals 8 mm. Median gill strongly bent 

 like a bow, basal third narrowed to a stalk, the rest broadly 

 falcate, crossed by three very irregular transverse bands of choco- 

 late-brown pigment. Lateral gills much narrower, not bent; 

 basal third narrowed to a stalk, the rest lanceolate, with three 

 pigment-bands as in median gill. Tracheal system not easily fol- 

 lo>ved out, owing to the semi-opacity of the gill. The broad por- 

 tions of the gills edged with some long, delicate hairs (Plate ii., 

 figs. 18-1 9). 



Of the three specimens of this rare larva secured, one was bred 

 out, the other two preserved. One of these had a regenerated 

 lateral gill, which I have figured in Plate ii., fig. 20. This gill is 

 only 3--1 mm. long, very narrow, and has three main longitudinal 

 trachea^. The great point of interest about it, however, is that 

 it shows, not far from the tip, the very distinct transverse divi- 

 sion into two parts seen in the gills of Agrion mercttriale and 

 Pyrrhosoma nyinphnla. This appears to me to afford definite 

 proof of the descent of this remarkable gill-form from the true 

 nodate type. 



2. Agriocnemis rubescens Selys. Length of gills, when full- 

 grown, median 5'6mm., laterals 5'6mm., the median slightly 

 wider and less pointed than the laterals. Base of gill narrowed 

 to a stalk, rest narrowly lanceolate. Colour pale yellowish or 

 creamy, rarely slightly greenish; no pigment-bands. Tracheal 

 system closely resembling that of Ischnura, but the branch 

 tracheae inclined at a somewhat less angle to the main stem. 

 Spines of the outer edge reduced to short bristles; from base up 

 to about two-fifths of the gill-length, these bristles are set in 



