100 



THE PALEONTOLOGICAL DATA 



In the nymph of a Cicada (Fig. 269), the medial trachea is four- branched 

 and about each of these branches a vein of the adult wing is formed. In 

 the monph of a Pentatomid (Fig. 302) the medial trachea is four-branched. 

 In the Odonata media is four-branched (Fig. 228) ; this is also true of the 

 Ephemerida (Fig. 61), and of the more generalized of the Trichoptera (Fig. 

 320). 



When one consults the paleontological evidences regarding the primitive 

 form of media and examines the wings of the Palasodictyoptera one is met 

 by a bewildering variety in the branchings of this vein. And at first sight 

 the evidence does not seem to support the conclusion, drawn from the study 

 of recent insects, that this vein is typically four-branched and that the 

 branching is dichotomous. 



It is natui al that the attention should be focused at first on the wonder- 

 fully well-preserved specimen of Stenodiciya lobata (Fig. 77), the generalized 



Fig. 95. — Didyoneura libelluloides (After Handlirsch). 



features of which have been already discussed. In this insect media is 

 two-branched, and this is the case in all of the six known species of this 

 genus. But of the fifty two genera, representing twenty-one families of the 

 Palasodictyoptera, of which the remains are sufficiently well-preserved to 

 enable us to determine the nature of media, this is the only genus in which 

 this vein is two-branched. Even in the other thirteen genera of the family 

 in which Stenodiciya is placed, the Dictyoneuridas, the media is more than 

 two-branched. It is evident, therefore, that the two-branched condition 

 of this vein in this genus is the result of specialization by reduction, The 

 fact that Stenodiciya is from the Upper Upper Carboniferous and that all of 

 the insects of the Lower and Middle Upper Carboniferous have media more 

 than two-branched confirms this conclusion. 



An even more puzzling matter than the two-branched media of Steno- 

 dictya is the fact that in about one-half of the genera of the Palasodictyop- 

 tera media is of a form that may be termed the isolated front branch type, 

 and in about one-half of these there is what resembles a four-branched 



