BY R. J. TILLYARD. 653 



present upon the wings of archaic types of all the Orders of the 

 Panorpoid Complex. The line of evolution is towards reduction 

 in size in all the Orders, with the single exception of tlie Myio- 

 daria in the Diptera, where there is an increase in size of the 

 microtrichia upon the ujyper surface of the wing only, resulting 

 in the interesting condition found in the Blowfly and allies (Plate 

 Ixix., fig. 14). 



The Macrotrichia : — A study of archaic types, includ- 

 ing the fossil Archijyamnya, shows us that these were originally 

 present upon both archedictyon and main veins, but not upon 

 cross-veins. Moreover, probably because of the comparative 

 weakness of the venules of the archedictyon, there was, from the 

 very first, a considerable difference in size between the macro- 

 trichia of the veins and those of the mesh work. The macrotrichia 

 of the veins also tended to be more erect than those of the mem- 

 brane; so that, in the fossil ArchijKinorpa, their bases of in- 

 sertion show the two concentric circles of the raised disc very 

 clearly, while those of the meshwork rarely do so. 



We are thus led to expect the following lines of evolution, 

 which do, in fact, actually take place in the various Orders of 

 the Complex : — 



(1) If the direction of the evolutionary effort be towards re- 

 duction, then the macrotrichia on the meshwork should be 

 affected first, since they are smaller in size from tlie first, and 

 inserted in less strongly built bases. 



As a matter of fact, it will be seen that the tendency towards 

 reduction in the smaller macrotrichia becomes accelerated in the 

 course of evolution; so that, in many cases, they become quite 

 eliminated, while the macrotrichia upon the veins may even pro- 

 liferate, though usually undergoing some reduction in size as 

 well, as in the higher Diptera. 



(2) If the direction of the evolutionary effort be towards the 

 formation of scales, then the macrotrichia of the meshwork should 

 be affected first, since they are in a more favourable condition, 

 both as regards their delicacy of structure and the condition of 

 their bases of insertion, to undergo this change. 



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