10 PROCEEDINGS OF THE ACADEMY OF [Jan., 



The theory of Walton, '00, who considers the "meron" as a vestigial 

 leg, seems likewise highly improbable. From an examination of a 

 large number of insects, it would appear that the meron is merely a 

 portion of the coxa. In such generalized forms as the Blattidse, it is 

 not at all, or only partially, distinguishable from the remainder of the 

 coxa; but in less generalized forms, as for example the Lepidoptera, 

 it becomes more separated from the coxa, and in the highly specialized, 

 swiftly-flying Diptera it is drawn quite into the pleural region, doubt- 

 less as the result of muscular tension. 



Walton's other theory, that the pterygoda represent a pair of epi- 

 meral wings, is fully as improbable as his meron hypothesis. The 

 pterygoda bear absolutely no resemblance to wings in structure or in 

 development, and, furthermore, no fossil remains show any traces of 

 more than one pair of wings to each thoracic segment. Walton has 

 tried to evade these facts by suggesting that the tegulse, etc., are wing 

 "fundaments." With regard to this supposition, all that can be said 

 is, that, so far as our present knowledge extends, the tegulse have funda- 

 mentally nothing in common with wings, and any attempt to discuss 

 what they might develop into belongs wholly to the realm of specula- 

 tion. 



If, as Patten, '90, states, two pairs of tracheal invaginations occur 

 in each segment of Acilius, this would indeed be a strong proof of seg- 

 mental fusion. In the adult Acilius, however, this is certainly not the 

 case. Embryos of this insect were not accessible, but in the embryos 

 of Chrysopa, and the far more primitive Forficula, there are no traces of 

 more than one tracheal invagination to the segment. Furthermore, 

 in all illustrations of other insect embryos that I could find, only one 

 tracheal invagination is indicated in each segment, and there are no 

 evidences of a double nature in the ganglia or any other important 

 segmental structures. 



This lack of embryological evidence is the chief argument against 

 the hypothesis of segmental fusion, and until proof more convincing 

 than that brought forward in support of the above cited theories can 

 be offered, it would seem preferable to adopt a mechanical explanation 

 — as, for example, muscular tension, etc. — to account for the origin of 

 the separate sclerites. 



In attempting to apply this thery it must be borne in mind that the 

 sclerites are not produced in a more or less haphazard fashion, as such 

 extremists as Graber seem to think, but one can trace the systematic 

 following out of a ground plan common to all three of the thoracic seg- 

 ments. 



