258 Mr. S. H. Scudder on the Relation of 



8. The Devonian insects were of great size^ had membranous 

 wings, and were prohahly aquatic in early life. The last state- 

 ment is simply inferred from the fact that all the modern types 

 most nearly allied to them are now aquatic. As to the first, 

 some statements have already been made ; their expanse of 

 wing probably varied from 40 to 175 millims., and averaged 

 107 millims. Xenoneura was much smaller than any of the 

 others, its expanse not exceeding 4 centims., while the pro- 

 bable expanse of all the rest was generally more than a deci- 

 metre, only Homothetus falling below this figure. Indeed, if 

 Xenoneura be omitted, the average expanse of wing was 

 121 millims., an expanse which might well be compared to 

 that of the iEschnidse, the largest, as a group, of living 

 Odonata. There is no trace of coriaceous structure in any of 

 the wings ; nor in any are there thickened and approximate 

 nervules — one stage of the approach to a coriaceous texture. 



9. Some of the Devonian insects are jylainly precursors of 

 existing forms, while others seem to have left no trace. The 

 best examples of the former are Platejihemera, an aberrant 

 form of an existing family, and Homothetus, which, while 

 totally difierent in the combination of its characters from any 

 thing known among living or fossil insects, is the only Palseo- 

 zoic insect possessing that peculiar arrangement of veins found 

 at the base of the wings of the Odonata, typified by the arcu- 

 lus, a structure previously known only as early as the Jurassic. 

 Examples of the latter are Gerephemera, which has a multi- 

 plicity of simple parallel veins next the costal margin of the 

 wing, such as no other insect, ancient or modern, is known to 

 possess, and Xenoneura, v7\\q.xq. the relationship of the interno- 

 median branches to each other and to the rest of the wing is 

 altogether abnormal. If, too, the concentric ridges, formerly 

 interpreted by me as possibly representing a stridulating organ, 

 should eventually be proved an actual part of the wing, we 

 should have here a structure which has never since been 

 repeated even in any modified form. 



10. They show a remarUahle variety of structure, indicating 

 an abundance of insect life at that epoch. This is the more 

 noticeable from their belonging to a single type of forms, as 

 stated under the seventh head, where we have seen that their 

 neuration does not accord with the commoner type of wing- 

 structure found in Palseozoic insects^. These six wings 

 exhibit a diversity of neuration quite as great as is found 

 among the hundred or more species of the Carboniferous 

 epoch : in some, such as Platephemera^ the structure is very 

 simple ; in others, like Homothetus and Xenoneura, it is some- 



* Cf. Mem. Bost. Soc. Nat. Hist. iii. 19, note 1. 



