358 SCUDDER ON THE GEOLOGICAL HISTORY OF INSECTS. 



\Vlien we come to the metabolous orders we find a scantier representation, but in the 

 more limited sense necessarily attendant upon this fact nearly the same things are true. 

 Three or four species of Diptera, referred to Chironomidae, Tipulidae, and Asilidae, are 

 found as low down as the Lias, about as many more in the middle Oolite, and some fifteen 

 or twenty in the upper Oolite, of several different families, mostly Nemocera. Of Lepi- 

 doptera, the remains of which are exceedingly scanty even in the tertiaries, we know of 

 two unquestionable Sphingidae in the middle Oolite, and the mines of a tineid moth in the 

 Cretaceous. While of Hymenoptera we have eight or ten mesozoic species, the oldest of 

 which is an undoubted ant from the Lias, next a wood wasp and four or five very obscure 

 remains from the middle Oolite of Solenhofen, two ants again from the upper Oolite 

 (Purbecks), and the eggs of one of the Tenthredinidae from the Cretaceous. 



We find then that the entire change from the generalized hexapod to the ordinally 

 specialized hexapod was made in the interval between the close of the paleozoic period and 

 the middle, we may say, of the mesozoic. These significant changes were ushered in with 

 the dawn of the mesozoic period, and the Triassic rocks become naturally (together with 

 the Silurian) the most important, the expectant, ground of the student of palentomology. 

 Hitherto for fifty years the Carboniferous period has claimed this interest as its birthright. 



The Silurian period has furnished only a single insect, just discovered and already alluded 

 to. The Triassic has four or five representatives in the Old World, while a new locality 

 recently made known in Colorado has yielded a considerable number of specimens of about 

 twenty species, mostly still unpublished. Most of these are cockroaches, and they illustrate 

 and enforce the conclusion we have reached in an interesting way. One of them, the Euro- 

 pean Legnophora of Heer, shows for the first time in the history of cockroaches 1 a thickening 

 of the front wings, rendering the veins nearly obsolete, a characteristic of Blattariae (not 

 always very striking) but never found in Palaeoblattariae. A similar appearance is to be 

 seen in a few of the American cockroaches of the Trias, and in addition to this they are 

 divided between Blattariae and Palaeoblattariae, and the passage from one to the other is 

 traceable. The two exist side by side, but some of the Blattariae have the front wings 

 equally membranous. 



It would then appear that the geological history of winged insects, so far as we know 

 from present indications, may be summed up in a very few words. Appearing in the 

 Silurian period, insects continued throughout paleozoic times as a generalized form of 

 Heterometabola which for convenience we have called Palaeodictyoptera, and which had 

 the front wings as well as the hind wings membranous. On the advent of mesozoic times 

 a great differentiation took place, and before its middle all of the orders, both of Hetero- 

 metabola and of Metabola, were fully developed in all their essential features as they exist 

 to-day, the more highly organized Metabola at first in feeble numbers, but to-day and 

 even in Tertiary times as the prevailing types. The Metabola have from the first 

 retained the membranous character of the front wings, while in most of the Heterometa- 

 bola, which were more closely and directly connected with paleozoic types, the front 

 wings were, even in mesozoic times, more or less completely differentiated from the hind 

 wings, as a sort of protective covering to the latter, and these became the principal 

 organs of flight. 



1 Etoblattina insignis Goklcnb., sp., may, perhaps, be an preservation, as the hind wings share fully the same charac- 

 exception, but the apparent thickening may be due to poor teristic. T»it possibly a " pupal " form? 



