BY R. J. TILL YARD. 63] 



f'U'(ni- 



meshwork present in the newer families, such as the Mijrmfh 

 iid(P. and Asmla^^hidce, is to be traced back to a simpler arrange- 

 ment of numerous parallel sectors supported b}' cross-veins at 

 intervals. It is by the zigzagging of these sectors, and the con- 

 sequent production of innnerous polygonal cellules, that a mesh- 

 work appearance has been produced. 



The essential characters of the original archedictyon, as we 

 find it preserved in the Paljeodictyoptera, are its irregularity, and 

 the delicacy of the cemiles (I use this tei'm as distinct from vein- 

 lets or cross-veins) that form it. Whether these first arose in 

 connection with precedent fine trachete in the wing-rudiment, or 

 entirely independently of them, or perhaps partly in one way 

 and partly in the other, we have now no sure means of proving. 

 We can only say that the irregularity of their courses, and the 

 fact that, in one fossil at least, macrotrichia hav^e been found to 

 exist upon them, strongly suggest a tracheal basis. 



It is now necessary to show the inter-relationship between the 

 archedictyon proper, the veinlets and cross-veins of the wing, and 

 the trichiation. Fortunately all these are preserved together in 

 the fossil Order Protomecoptera, from the Trias of Ipswich, 

 Queensland. 



Plate Ixvii., figs. 9-10, show portions of the wing of Archipanoiya 

 magnijica Till., the only knosvn representative of this Order. 

 Here we may see the archedictyon still complete, but evidently 

 in a stage preparatory to becoming completely merged into the 

 wing-membrane. The venules of the archedictyon are not de- 

 finitely marked out, as in such fossils as HyjJennegethes (Text-fig. 

 19), but appear rather as simple ridges of the cuticle, not showing 

 any definite venular structure. In many places, but chiefiv close 

 to the main veins, and at the angles of the mesh work, there can 

 be seen rounded or slightly oval tubercles, of a diameter con- 

 siderably smaller than those seen upon the veins themselves, and, 

 with few exceptions, less clearly preserved. These are evidently 

 the bases of insertion of macrotrichia; but, partly owing to the 

 weak development of the mesh work that carries them, and parti)' 

 because they apparently lay more fiatly along the wing-surface, 



