354 SAMUEL II. SCUDDEB ON THE 



latter, that we find very varied forms of metamorphosis within the limits of a single order, 

 so that it would require a dismemherment of the orders to uphold the distinction in a 

 logical form. 



In the attempts alluded to above, naturalists have simply selected, as it were, combina- 

 tions of acknowledged ordinal peculiarities in order to form and distinguish their super- 

 ordinal divisions, and have failed to search deeper into the general structure for more 

 fundamental characteristics. Packard, however, has done this, and by employing the 

 terms Metabola of Leach, in a modified sense, and Heterometabola, has brought the 

 Hymenoptera, Lepidoptera and Diptera under the former, and the other orders under the 

 latter. In a paper published six years ago on the Early Types of Insects, 1 gave my adhe- 

 sion to this view, and strengthened it, as I believe, by some additional characteristics drawn 

 from the regional divisions of the trunk. In the Metabola, the thorax, supporting the 

 organs of aerial locomotion — a primary feature of the Hexapoda as a whole — is very highly 

 organized and compact, well differentiated from both head and abdomen, the prothorax 

 very small ; the body is generally cylindrical ; the mouth parts prolonged into a beak of 

 some sort, and the mandibles rarely opposed at tip; the front wings are membranous and 

 larger, generally very much larger, than the hind pair; the larva is cylindrical and. in no 

 way resembles the adult, and the pupa is inactive. In the Heterometabola. on the other 

 hand, the prothorax is large, and the joints of the thorax are less contacted, as a rule, 

 than in the Metabola, or, if compacted, generally massively soldered to the abdomen; the 

 body is usually flattened ; the mouth parts are generally not prolonged into a beak, and 

 the tips of the mandibles are generally opposed ; the front wings are generally more or 

 less coriaceous or with very numerous and thickened veins, and generally smaller than 

 the hind wings ; the larva is usually depressed, often resembles the adult in form (except- 

 ing, of course, in the wings), and the pupa may be active or inactive. 



The exceptions, in special points, to the above general statements, are not few, especially 

 among the less homogeneous Heterometabola, but if any superordinal division of Hexapoda 

 is to be looked for, it would seem to be on the lines here indicated. The points which are 

 especially disturbing are the opposition of the mandibles in the Hymenoptera, and the 

 appearance of many metabolous characteristics among the Neuroptera properly speaking, 

 a group which is, nevertheless, as a whole, admittedly related most nearly to other hetero- 

 metabolous orders. 



That the Metabola should rank, as a whole, higher than the Heterometabola, can scarcely 

 be disputed ; the regional division of the body, the structure of the wings for flight, and 

 especially for strong and directed flight, the complication of the mouth parts, and the 

 universally complete metamorphosis and quiescent, pupal state, — are fundamental features, 

 in which the hexapodal type is carried, as a whole, to its highest development. And yet, 

 as we shall see, there are some features in which its members have held to fundamental 

 characteristics of paleozoic hexapods more firmly than have most of the heterometabolous 

 groups. 



This brings us fairly to the main object of this paper. What were the relations of the 

 ancient to the modern types of winged insects ? In what succession did the two super- 

 ordinal divisions of insects appear, and at what period the different order's as we now 

 recognize them ? What light, in short, can paleontology throw upon the origin and suc- 

 cession of insects ? 



