S. H. SCUDDER ON THE EARLY TYPES OF INSECTS. 15 



(in a more restricted sense than first used by Leach), and to tne former, Heterome- 

 tabola. The Metabola are unquestionably more homogeneous than the other group. 

 One of their primary features is found in the more clearly marked regional divisions of the 

 body ; this is a consideration of great significance, since in the progress of structure, from 

 the worms, through the crustaceans to the insects ; or within the class of insects, from the 

 myriapods, through the arachnids to the hexapods ; or in the developmental history of the 

 Metabola themselves, from the larva, through the pupa to the imago, we discover a con- 

 stantly increasing concentration of the segments of which the body is composed into distinct 

 regions, culminating in the Hymenoptera, where head, thorax and abdomen are most sharply 

 defined. This feature was first insisted upon by Agassiz in his remarkable essay on the 

 classification of insects (I.e., pp. 20-28), but its application to the division of the hexapods 

 has not before been pointed out ; yet a very little consideration will show how much more 

 clearly these regions are marked in the Metabola than in the Heterometabola, especially if 

 the separation of the thorax and abdomen is examined. This is indeed what we might, 

 not unreasonably, look for in the highest members of a group characterized, as are the hex- 

 apods, by the possession of organs of flight : the greater development of these organs would 

 necessitate a more compact and distinctive organization of the region devoted almost ex- 

 clusively to them ; and accordingly in the Metabola we have, on the one hand, a more highly 

 organized thorax, more definitely separated from head and abdomen, than in the Hetero- 

 metabola ; and on the other hand, greater power of continuous flight, of poise, of rapid 

 movement, of sudden and repeated change of direction, and a far greater grace of move- 

 ment in the former than in the latter. 1 This specialization of the thorax led me at one 

 time to think of proposing the term Sternoptena for the Metabola; and, in allusion to the 

 general preponderance of the abdomen in the groups composing it, Gastroptena for the 

 Heterometabola. For the latter series the term Gastroptena would be more distinctive, 

 but the names suggested by Dr. Packard seem to me better adapted to general use, besides 

 having the advantage of prior application, and I accordingly adopt them. 



In addition to the primary features mentioned (which were not stated by Packard), the 

 Metabola are characterized by a usually cylindrical body with a very small prothorax ; mouth 

 parts formed in whole or in part for sucking, the points of the mandibles seldom opposed to 

 each other ; front wings membranous and much larger than the hind wings, which latter are 

 sometimes aborted ; the larva cylindrical and very unlike the adult, and the pupa always in- 

 active. The Heterometabola on the other hand usually have a flattened body, with a very 

 large prothorax ; mouth parts usually adapted for biting, the points of the mandibles then 

 opiwsed to each other ; front wings usually more or less coriaceous or with very numerous 

 and thickened veins, and usually smaller than the hind wings, which latter are only excep- 

 tionally aborted, and never throughout large groups ; the larva is usually flattened, often 

 resembling the adult, and the pupa either active or inactive. 



1 This we affirm only as a general rule, taking each sub- change of flight is very striking ; but these do not affect 



order as a whole. There are, it is true, apterous or sub- the characters of suborders as wholes; and in the exceptions 



apterous Hymenoptera, bungling and inert fliers among the which might be noticed, the specialization of flight is 



Lepidoptera, and Diptera which have a heavy and direct nearly always accompanied to a certain degree by a corres- 



flight ; and on the other hand, groups like the Odonata ponding development and distinctiveness of the thorax, 

 among Neuroptera, whose rapidity and power of sudden 



