58 



THE BIOLOGY OF INSECTS 



and Grasshoppers), Plecoptera (Stone-flies), Isoptera (Ter- 

 mites), the wings of the two pairs are uncoupled and 

 each wing-bearing segment has its own set of thoracic 

 muscles. But, on the other hand, in the great majority of 

 highly organised flying insects of other orders there is some 

 form of wing-coupHng apparatus, so that the fore and hind 

 wing of each side move in unison, following the rhythmic 

 changes of outline of the thorax due to the alternate con- 

 traction of the sets of flight-muscles. Thus among the bees. 



Fig. 20. — Right wings of Honey Bee {Apis), h, row of booklets on 

 costa of hindwing which hold dorsal edge of forewing during flight. 

 X 8. After Comstock and Needham and Snodgrass. 



wasps, and other insects of the order Hymenoptera a series 

 of curved hook-like bristles along the front edge (costa) of 

 the hindwing catch on the thickened hind-edge (dorsum) 

 of the forewing (Fig. 20) ; it is easy to demonstrate this 

 linkage by manipulating the wings of a large wasp or sawfly. 

 In some of the Lacewing and Scorpion-fly groups 

 (Neuroptera and Mecoptera), as R. J. Tillyard (1920) has 

 pointed out, there is a lobe or process (jugum) at the hind- 

 base of the forewing, and on the costa of the hindwing a 

 corresponding humeral lobe bearing one or two stiff bristles 

 (frenulum). The jugum is present also in many Trichoptera 

 (Caddis-flies) and in a few Lepidoptera (Moths), though in 



