71-4 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



rates from vein J^+s far to the proximal side of the nodus, and about 

 in the more usual position of the proximal end of the bridge/' 

 In this group l)ridge and oblique vein are lost; but similar parts are 

 found, as will be noted later, extending the attachment of the radial 

 sector to the media still nearer the arculus. 



THE REGION OF THE ARCULUS. 



Across the base of the wing a strong transverse union of all thd 

 principal veins is effected by means of arculus in the middle, triangle 

 at the rear, and antenodal cross veins at the front. Distinct tra-: 

 cheal twigs precede the antenodals of the first (costal) series, while; 

 those of the second (subcostal) series are of independent cuticulai 

 origin.'' Specialization is to be traced among these cross veins ini 

 their reduction in number and matching in position in the two series;.{ 

 and in the hypertrophy of some of them to form stout triangular 

 trusses, which entirely fill, in section, the furrow between the costal 

 and the radius. Two antenodals, some distance apart, are thus hyper- 

 trophied in most aEschnidfe, one at either side of the arculus; in the. 

 Thorina% but one, and that one meeting the arculus; in SyritJiemit: 

 alternate antenodals are thickened, ])ut to a less degree. Theii 

 reduction in numbers will be discussed under the general subject ol 

 "cross veins." 



We have already seen that arculus and triangle are formed where 

 bends in the media and the cu])itus, respectively, are met l)y strong cros,' 

 veins. We have seen that the medial and the cubital trachete are at 

 first straight or slightl}^ curved as in other insects, and that the bend- 

 ing takes place late in nymphal life, at the time when the veins are 

 forming. The genus Anax (Plate XL, tig. 3), while highly specialized 

 in man}' ways, has preserved a rather primitive condition of the 

 arculus. It is composed here largely of cross vein, which the media 

 bends but slightly to meet, and the veins J/1-3 and J/^ depart from it 

 in straight lines. Media tends to descend the arculus, and the veinsii 

 Mi-^ and J/^, departing from it, ])ecome arched strongly toward the 

 radius. In the LiVjellulida? the branches of the media become fused 

 at the base whih; arching upward. This may be traced in the stagesj 

 attained by such genera as Neocordulia (Plate XLII, fig. 1), Raphismid 

 (Plate XLIV, fig. 3), and Pachydiplax (Plate XL VII, fig. 1). The 



« Indicatiug that in this group at least a recurrent tracheole, such as precedes th€ ' 

 bridge in the .'Eschnidte, may have developed into the basal attachment of the radial 

 sector to the media. 



*In other orders of insects cross veins are generally wanting from this space, and 

 where present are very few in number. 



It will be observed that the tracheal twigs which precede the antenodals of the 

 first series are derived from the subcosta; those of the postnodals of the first series, 

 from the radius. These tracheal branches are clearly the homoiogues of the longei 

 anterior branches of the subcosta and the radius in the Neuroptera and the 

 Orthoptera. 



