NO. I INSECT THORAX — SNODGRASS lOI 



9. The wings, developed as lateral outgrowths of the dorsum, 

 entailed the development of a second set of modifications in the 

 thoracic structure, superposed on those already acquired in connection 

 with the development of legs as efficient organs for terrestrial or 

 arboreal progression. The wing lobes served at first probably as 

 planing surfaces that enabled their possessors to glide through the 

 air from elevated positions ; there is no good evidence that they ever 

 had any other function. 



10. The wings of all insects except the Odonata are still moved 

 principally by muscles present in the thorax when the wing lobes were 

 first acquired. The evolution of the wings involved the development of 

 a supporting apparatus from the dorsal part of the pleuron, changes 

 in the terga' bearing them, and a general consolidation of the meso- 

 thorax and metathorax. Since the up-and-down motions of the wings 

 are produced by movements of the terga, the development of wing 

 motion demanded from the first a suppression; of the flexible over- 

 lapping of the terga, and this was accomplished, as in the sterna, by 

 a transfer of the precostal margins of the third and fourth terga, 

 together with their muscle-bearing ridges or phragmata, to the seg- 

 ments preceding, where they constitute the postnotal plates. The 

 dorsal longitudinal muscles) could now effect an upward bending of 

 each tergum, giving a down-stroke to the wings, and the tergo-sternal 

 muscles could act antagonistically as elevators of the wings by flat- 

 tening the tergal arches. Since the wings have not been developed 

 alike in the two wing-bearing segments of all the orders, the tergal 

 readjustments vary accordingly. 



11. The ridges on the under surfaces of the wing-bearing terga 

 have been deveU)ped in response to the needs of the terga as parts of 

 the wing mechanism. The tergal areas between the ridges, defined 

 on the exterior by the corresponding sutures, are therefore in no 

 sense primitive component elements of these terga, and are not to be 

 homologized with tergal " divisions " in the other segments. 



12. But few new muscles! have been developed in most insects in 

 connection with the wings. In the Odonata, however, the primitive 

 musculature has been suppressed, and has been replaced by sets of 

 special wing muscles attached directly to the wing bases. The 

 Odonata, therefore, represent a highly specialized line of descent 

 that branched ofT from the main i:)terygote stem at an early period, 

 but apparently since the time of the oldest insects known from the 

 paleontological records. 



