6 ENTOMOLOGICAL NEWS [Jan., '22 



(c) A distal portion, from O' onward, supplied by a true branch 

 of M, viz., Ms, whose basal piece underlies O'. This portion is about 

 as long, in Uropetala, as the other two portions combined. 



Having completed this somewhat complicated analysis of the 

 tracheation of this region, we may now proceed to solve the 

 vexed question of the true homologies of the parts in question. 

 Are we dealing with a single longitudinal vein of complex 

 origin, whose evolution is to be traced out by reference to the 

 courses of the larval tracheae underlying it ; or are we not 

 rather dealing with a single primitive longitudinal vein, whose 

 larval tracheal supply has become more and more specialized 

 during the evolution of the Odonata? If the former, then we 

 cannot hope to find any simple notation which would correctly 

 express the true structure of this vein. If the latter, we can 

 ignore the tracheal specializations, and name the vein accord- 

 ing to its true position as a simple longitudinal vein. 



This problem is really quite easy of solution. For, if the 

 \eia called by Needham R s is really complex, then we may ex- 

 pect to find, in the fossil record, some types at least in which the 

 formation of the bridge-vein is not completed. But a careful 

 study of the fossils shows that, both in the Protodonata and in 

 all the Liassic Odonata, this supposed Rs is a single complete 

 vein, which arises from 'M3 near its origin, and runs parallel 

 to and below M2, without any connection whatever with R. 

 In Typus and some Liassic fossils, such as Hctcrophlcbia, an 

 oblique vein is present in the position of 0' , thus showing a 

 tracheational connection with M2. But, in all the rest of the 

 Protodonata, and in most of the Liassic Odonata, even this is 

 absent, and we find the same simple condition that is still to be 

 seen in all the Zygoptera except the Lestidae, viz., that the 

 supposed Rs of Needham is in reality a true branch of M, 

 with no oblique veins above, and with absolutely no connec- 

 tion with R. This is, then, surely the primitive condition of 

 this vein in the Odonata; and the specializations which have 

 set in during the evolution of the Order are surely trache- 

 ational specializations only, which do not demonstrate to us 

 the course of evolution of the vein, but rather serve to mask 

 its simple origin, by the complexity of the changes that have 

 taken place in the tracheal supply. 



