538 THE PANORPOID COMPLEX, iii.. 



The variations in the tracheation of the wings of Lepidoptera 

 during pupal life appear to nie to be of such importance that I 

 have paid special attention to them in dealing with the venation 

 of this Order in Section xiv. 



In the case of wings of merotracheate type, the pupa should 

 always be examined, when obtainable; for some evidence of value 

 may be gained in this way. Moreover, such wings may also 

 show a progressive change in their venational scheme, as the 

 pupa ages, and these changes must be noted. 



(B) Study of the Wing-Trichiation. The distribution of the 

 macrotrichia upon the wing-veins in the Panorpoid Complex is 

 of the greatest value in determining homologies in difficult cases. 

 The rule, which I have already proved much more fully in Part 

 2 of this work (25) may be briefly stated as follows: — In the 

 more archaic types of every Order within the Complex, the main 

 veins and their branches carry macrotrichia, while the cross-veins 

 do not. The specialised groups, in which the cross- veins, as well 

 as the main veins, carry macrotrichia, are the Raphidioidea, and 

 the higher families of the Planipennia. These will scarcely 

 be needed at all in our search for the Archetypic Venational 

 Schemes of the separate Orders. 



latter case, there are many examples of the retention of the 

 base of the wing, in a number of types, where the chitinisation of 

 the basal parts of certain veins, particularly M and Cu, may be 

 so weakened as not to carry macrotrichia; or (b) where the 

 basal portion of the weaker (more concave) of two veins taking 

 part in a fusion becomes more or less transverse in direction, and 

 begins to take on the character of a cross-vein. But, even in this 

 latter case, there are many examples of the retention of the 

 macrotrichia . 



It should also be borne in mind that true cross-veins are not 

 preceded by tracheae. A main vein, or part, of it, may occa- 

 sionally lose its trachea; but true cross-veins never possess 

 tracheae in the freshly-turned pupal wing, though they may 

 sometimes gain them as special outgrowths from the main 

 tracheae, either later in pupal life, or at metamorphosis. 



(C) The Palaeontological Evidence. This is of the utmost im- 

 portance, but needs to be used with great care. The tracheation 

 test cannot be used in the case of fossils; but it frequently 

 happens that the sockets of the macrotrichia are preserved, and 

 their presence or absence may decide a point of great value. 



