FLIGHT OF INSECTS. 351 



their extremity, and the true wings are folded 

 transversely, so as to cross one another. These 

 hemi-elytra are employed to strike the air in 

 flight, and their movements accompany those of 

 the wings. 



Insects having four thin membranous and 

 transparent wings are arranged under two orders ; 

 namely, the Neuroptera (Fig. 160), in which the 

 lesser nervures form an interlacement of fibres, 

 crossing one another nearly at right angles, like 

 net- work, or lace : and the Hymenoptera (Fig. 

 161), in which they are disposed like the rami- 

 fications of arteries or veins, diverging at acute 

 angles from the main trunks. The insects be- 

 longing to these two orders enjoy extensive powers 

 of flight. Libellultf, and jEschnce, which are in- 

 cluded in the first of these orders, never close their 

 wings, but, when they are not flying, keep them 

 constantly expanded, and ready for instant action. 

 They fly with the greatest ease in all directions, 

 sideways, or backwards, as well as forwards, and 

 can instantly change their course without being 

 obliged to turn their bodies. Hence they possess 

 great advantages both in chasing other insects, 

 and in evading the pursuit of birds. Sees, which 

 are hymenopterous insects, have often been ob- 

 served to fly to great distances from their hive 

 in search of food. The humble bee adopts a 

 very peculiar mode of flight, describing, in its 

 aerial course, segments of circles, alternately to 



