42 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



GROUND-PLAN OF A WING-BEARING SEGMENT 



Though the general structure of the thorax was determined 

 through the speciaHzation of this region of the body as the locomotor 

 center of the insect, and through the transformation of the subcoxse 

 into chitinous pleura, probably long before the wings appeared, the 

 special features of the thoracic segments in winged insects have un- 

 doubtedly been evolved as characters correlated with the development 

 of the wings. 



The paleontological history of insects shows that lateral tergal lobes 

 were present on the prothorax of many of the earliest known winged 



Ti Jz 



Fig. 19. — Carboniferous insects with tergal lobes on the prothorax (Palseodic- 

 tyoptera). 



A, Stenodictya lobata Brong., with large lobes (a) on protergum. (Brongni- 

 art, 1890.) B, Enbleptiis danielsi Handlr. (drawn from specimen No. 35576, 

 labeled holotype, in U. S. Nat. Mus.), showing small protergal lobe (a), well- 

 developed postnotal plates (PM2, PN?.) in mesothorax and metathorax, and ten 

 abdominal segments. 



species (fig. 19) ; but there is no fossil insect known from the geologi- 

 cal period, probably the Silurian, when the wings were in the course 

 of development. There can be little doubt, however, that the wings 

 were evolved from lateral lobes of the dorsum in the mesothorax and 

 metathorax. We can only speculate as to what service these lobes 

 were to insects during their early stages of evolution. The most 

 popular explanation is that they were gills, and that the tracheos, 

 which mark later the courses of the wing veins, penetrated the lobes 

 first for respiratory purposes. This theory implies that the ancestors 

 of winged insects passed through an aquatic period in their evolution 

 after having acquired a tracheal system during a previous period 

 when they dwelt on land. Osborn (1905), in proposing this explana- 

 tion of the origin of insect wings, suggests that insects lived in the 



