20 S. H. SCUDDER ON THE EARLY TYPES OF INSECTS. 



teristics of the wing structure of living insects (which show, indeed, a variety of type truly 

 marvellous, and ranging from exceeding simplicity to a complexity which nearly baffles all 

 attempts at homology), we should not need to modify our statement in the least particular 

 to include the wing-structure of the insects of earliest times. The plan of neuration upon 

 which the wings of insects were then constructed is the plan we find in all existing types. 

 At the same time, as stated above in a note, there was an unusual degree of homogeneity 

 in the wings of palaeozoic insects. 



This review clearly indicates that the laws of succession of the insect tribes are quite 

 similar to those which have long been known to hold in other groups of the animal king- 

 dom ; and that the facts are, in the main, such as the theory of descent demands. The ex- 

 ceptions to theory, however, and indeed the general facts, are such as to indicate that pro- 

 found voids exist in our knowledge of the earliest history of insects. The appearance of 

 hexapods in the middle Devonian long previous to any traces either of myriapods or of 

 arachnids ; the apparent advent of generalized groups of a comparatively narrow range, 

 before those which are wider in scope and embrace the former ; the apparition of Cole- 

 optera, which present no indication of any divergence from the subordinal type, in Carbon- 

 iferous beds first yielding an abundance of insect remains, — that is, as early as any insects 

 whatever, excepting the homogeneous-winged Heterometabola of the Devonian ; and the 

 occasional discovery of highly specialized types at very early periods : — all point to the 

 far earlier existence of widely comprehensive types, from which all these comparatively 

 specialized but still more or less synthetic forms must have originated. The additions to 

 our knowledge of palaeozoic insects within the past twenty years, and the increasing indi- 

 cations of dry land at earlier and earlier epochs, 1 must leave little doubt in the reflecting 

 mind, not only that insects existed in no scanty numbers in Devonian and even in Silurian 

 times, but that persistent research over wider fields will probably enable us, at no distant 

 day, to replace hypotheses with facts. 



In conclusion, we may recapitulate, as follows : — 



1. With the exception of the few wings of hexapods known from the Devonian, the 

 three orders of insects — hexapods, arachnids and myriapods — appeared simultaneously 

 in Carboniferous strata. 



2. Hexapod insects may be divided into a higher group (Metabola), including Hymen- 

 optera, Lepidoptera and Diptera; and a lower group (Heterometabola), including Coleo- 

 ptera, Hemiptera, Orthoptera and Neuroptera. 



3. All Devonian and Carboniferous insects are Heterometabola, the Metabola making 

 their first appearance in the Jurassic period. 



4. Many synthetic or comprehensive types existed in jmlaeozoic times, combining the 

 characters either of all the Heterometabola ; of Orthoptera and Neuroptera ; or of Neur- 

 optera proper and Pseudoneuroptera. 



5. The Devonian insects either belong to comprehensive types related to the two lower 

 suborders only, or are low Pseudoneuroptera ; and were undoubtedly aquatic in early life. 



0. The lower suborders of Heterometabola, — Orthoptera and Neuroptera, were much 

 more abundant in palaeozoic times than the higher, — Coleoptera and Hemiptera. 



1 Cf. Lesquereux. Land plants, recently discovered in the first page of this paper will be found a resume of our 



the silurian rocks of the Uniird States. < Proc. Amer. knowledge of this subject. 



Philos. Soc, xvil : 163-73, pi. 4. 8°. Philadelphia, 1877. On 



