172 DEVELOPMENT OF TMK WING-VKNATION OF ODONATA, 



the wide, roundish loop of Macromia and Sr/nthemis, while, on 

 the other, we see the extremely elongated and narrow, foot- 

 shaped or Italian''' loop of tlie majority of the Lihellulince and 

 the Eucorduliini. 



There are two theories in the field to account for the develop- 

 ment of this remarkable Italian loop. They may be termed the 

 Theory of Douhle Descent^ and the Theory of Single Descent 

 respectively. They are of special importance, because the whole 

 of one's view of the phylogeny of the Lihellulince depends upon 

 which theory one is willing to accept. 



The Theory of Double Descent postulates a separate origin for 

 the jEschnine and Mac7'omian loops, on the one hand, and the 

 Italian loop on the other. As formulated by Dr. Ris,t and up 

 till now accepted by the majority of students of the Odonata,X it 

 depends mainly on the following interpretation of the venation 

 of the two forms of loop concerned(Fig.4A). The jEschnine and 

 Macromia7i loops are enclosed (as shown above) by A2 as basal 

 side, and A^ as distal side. But the Italian loop is formed with 

 Ao as basal side, and A^ as distal side, while A^ forms its strong 

 midrib. t Hence the two forms of loop are not homologous, and 

 cannot be descended along a single line. According to the 

 exponents of this theory, the Libelluli7ice are descended from 

 narrow-winged forms similar to Tet7'aihemis, and hence the 

 Italian loop arose by secondary broadening from this narrow form 

 of wing. 



The Theory of Single Descent, which I formulated in ]912,|| 

 postulates a single line of descent for all the different known 



* I suggest this as a convenient name for this form of loop, which much 

 resembles the map of Italy in shape. 



t See diagram of tj'pical Libellulive wing (Scapanea frontalis Burm.) in 

 Dr. Ris' LibeUulinen, Fasc. i.-xvi. of de Selys' Monographs, 1910-1914. 



Jin the short account of the anal loop given by Needham (Zoc. cit., 

 p. 722), there is nothing to indicate that he favoured this theory. On the 

 contrar}', his words, so far as they go, seem to oppose it, but no lettering 

 is attached to his figures of loops. It seems unlikely that the question of 

 a double descent ever occurred to him. The theory was, however, a direct 

 outcome of the impetus given to venational studj^ by his paper. 

 § These Proceedings, xxxvii., p. 724, 1912. 



