The Oilonatit or Druijoii/lies of South Afrlra. 391 



I 



OETHETRUM (Newman, 1833). 



A genus of many species, many of them common and dominant in 

 large parts of the Old World, more common in tropical and sub- 

 tropical that in temperate regions, especially dominant in continental 

 Africa in number of species and of individuals. Species of OrtJietnuu 

 are very wide-ranging (with but a few exceptions) : none of those 

 found in the present fauna is limited to South Africa ; most of them 

 are known from the entire continent South of the desert belt ; two 

 (chrysostigma, trimtcria) are also from Mediterranean Africa and 

 Europe. 



They are medium-sized or larger Libellulinae ; all the species here 

 discussed similar in colours : male pruinose blue with a pattern of 

 black or reddish brown and yellowish, greenish or whitish, often also 

 covered by dense pruinosity in fully mature specimens, in younger 

 individuals similar to females ; females not pruinose in most species, 

 in some others only so in very old specimens, with pattern similar to 

 male in principle, but mostly with the dark elements reduced. The 

 condition of these species in collections has long been chaotic ; studies 

 by P. P. Calvert and more recently by the writer have established 

 reliable specific characters, and but little uncertainty remains now for 

 some females particularly, or for males in which the geuitalia of 

 second segment are not in sufficiently good condition for examination. 



The following table was established chiefly on characters of colour 

 and pattern. For certain identification the study of genital structures 

 on second abdominal segment of males is always useful, often necessary, 

 and the figures given for those structures are indispensable in addition 

 to the table. 



1. Basal segments of abdomen much widened in lateral and especially in dorso- 

 ventral dimension ; segment 3 constricted, following segments compara- 

 tively narrow, fusiform or almost parallel in males, cylindrical in 

 females ........ 



Basal segments of abdomen not widened in lateral, very little in clorso- 

 ventral dimension ; segment 3 compai-atively broad, not constricted ; 

 following segments very gradually narrowed to end of abdomen and 

 depressed in male, less distinctly narrowed, more cylindrical in female. 

 Ante-nodal cross-veins in sub-costal space of ambiguous colour, blackish 

 in dorsal view, ochreous if seen from the sides. Pterostigma very 

 large, ochreous or light ferruginous between black veins. Membranule 

 whitish or light grey. Base of wings hyaline with no trace of yellow 

 spot. Adult $ with face fuscous, frons above shining black ; thorax 

 and dorsum of abdomen (entirely or to segment 6) with very dense 

 whitish blue pruinosity ; terminal abdominal segments black with a mid- 

 dorsal, basal, cuneiform ochreous spot on segment 6-7 (similar spots on 



