NO. I INSECT THORAX — SNODGRASS 4I 



III. SPECIAL STRUCTURE OF A WING-BEARING SEGMENT 



While the perfection of the legs as walking appendages aflfected 

 principally the lower pleural and the sternal parts of the thoracic 

 segments, the development of the wings as efficient organs of flight 

 involved changes mostly in the tergal and the upper jileural regions 

 of the two segments concerned, though the increased importance of 

 the tergo-sternal muscles, and the need of solidarity in the wing- 

 bearing part of the thorax have had a general effect on the entire 

 thoracic structure. 



Flying insects have evolved along two quite different lines, but the 

 similarity in their general structure, and in the structure of the wings 

 themselves shows that both resulting groups are descendents of a 

 common ancestral form, which form had all the features distinguish- 

 ing the Pterygota from the Apterygota. The two lines of dift'erentia- 

 tion in the Pterygota have been established through the adoption of 

 different mechanisms for moving the wings. Among modern insects, 

 one pterygote grou]:» is represented only by the Odonata, the other 

 includes the rest of the winged orders. 



In the non-odonate insects there are few special wing muscles, the 

 muscles that effect the movement of the wings being chiefly the longi- 

 tudinal dorsal muscles of the terga, the tergo-sternal muscles, and a 

 set of muscles that originally were muscles of the coxse. The tergal 

 and the tergo-sternal muscles move the wings indirectly by the pro- 

 duction of alternating changes in the shape of the thorax ; the other 

 muscles are more closely connected with the wings in the adult, but, 

 being leg muscles in origin, they also are not primary wing muscles. 

 It thus appears that when wings were developed and first became 

 mobile appendages, they were moved by muscles already present in 

 the segments bearing them. Only a few special wing muscles have 

 since been acquired by most insects. In the Odonata, however, the 

 wings are moved entirely by muscles inserted directly on the wing 

 bases, and it is impossible to trace any homology between these mus- 

 cles and the wing muscles of other insects. According to Poletaiew 

 (1881) the wing muscles of the dragonflies are formed and de- 

 veloped during the nymphal stages of these insects. The wing mech- 

 anism of the Odonata, therefore, must be one secondarily acquired 

 through the development of special wing muscles, which have sup- 

 planted the ])rimitive musculature of the wing-bearing segments. For 

 this reason, the following descriptions of the structure of a typical 

 wing-bearing segment will be based on that of the mesothorax and 

 metathorax of non-odonate insects. 



