N0.1331. DRAfiON-FLY WING VENATION— NEEDHAM. 711 



rigidity, would seem to allow more eflfcctive flexion of the distal parts 

 of the wing/' 



We have already .soen the trachea Bs descending- at the nodus and 

 crossing tracheie M^ and Jf.,. The veins formed about these tracheje 

 bind them all solidly together. For convenience of reference we now 

 designate that portion of the radial sector which unites the lower end 

 of the nodus with the median vein as the .^nhiKxhis {sn.) and tli(> short 

 oblicjue portion of the radial sector appearing as a cross vein Ix'hind 

 vein J/, as the ohJique vein (<>), and the trunk secondarily developed 

 to connect the radial sector proximally with vein i/j+^, as the 

 hridge (br). 



In the sul)order Anisoptera we may then note that the radial sector 

 fuses with vein J/, for a little way, carrying the oblique vein a varia- 

 ble distance beyond the subnodus. The bridge is outlined in tracheae 

 of two very different types, which are almost characteristic of the two 

 families of the suborder. (1) In all the ^Eschnina^ of which 1 have 

 had nymphs for examination the antecedent trachea is simple, and 

 springs from the radial sector near what will be the distal end of the 

 bridge (tig. 5) and extends in a direct line proximally toward vein 

 J/i+2. (2) In most Libellulida? the trachea which precedes the ])ridge 

 springs from the radial sector near the origin of the latter (tig. 6), 

 descends to the level of the bridge that is to be, forks, and sends its 

 branches in opposite directions to meet veins 31^+, and ^.v respec- 

 tively. In the Macromiime, however, it is formed by a moditication 

 of the latter type, as shown for Dldyrnojps transversa in tig. 7 A. Fig. 

 7 B shows how, b}' a further division of the radial sector near its l>ase, 

 two oblique A'eins are formed in the CordulegasterintB and Petalurinffi, 

 Compare also with fig. 28. 



In the suborder Zygoptera, so far as known to me, trachea Rs 

 appears as a branch of the media, the subnodus, while formed in the 

 usual place, being destitute of a tracheal trunk. The Zygopterous 

 nymphal wing figured herewith (Plate XXXI, fig. 2) shows that the 

 trachea Rs has, in this (well grown) stage, at least, no direct commu- 

 nication with the radius at all. But if we compare the adult wings 

 of the two suborders there can be no question as to the identity of 

 the vein Rn^ or of its homology in the two groups. 



The explanation of this at first rather surprising state of the tia- 

 chea> I believe to l)e that the trachea 7?.s' has l)een detached from the 

 radius and attached to the media. There are everywliere bi^twecMi the 

 principal tracheae open channels of con)munication. formed by the 



« This suture seems to have cut off the subcostal trachea, or else to have dwarfed 

 and diverted its tips. In the cicada the subcostal trachea crosses the nodal suture; 

 and doubtless it once did so in Odonata, for the adult vein extends a little waj' 

 beyond in three living jrenera of .Eschnina' (t-oinpare Stdiiroiilihljia, Plate XXXIX, 

 fig. 2), and was well developed l)eyond in the fossil genus ^Et<<'}niidinm. The nodus 

 when present in other orders is nearer the wing apex than in the CKlonata. 



