314 THE MECHANICAL FUNCTIONS. 



Insects having four thin membranous and trans- 

 parent 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 Hpnenoptera (Fig. 161), in 

 which they are disposed like the ramifications of 

 arteries or veins, diverging at acute angles from 

 the main trunks. The insects belonging to these 

 two orders enjoy extensive powers of flight. Libel' 

 lulcBy and ^slince, which are included 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 great- 

 est 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. 

 'Bees^ which are hymenopterous insects, have often 

 been observed to fly to great distances from their 

 hive in search of food. The humble bee occasion- 

 ally adopts a very peculiar mode of flight, describ- 

 ing, in its aerial course, segments of circles, alter- 

 nately to the right and to the left. The velocity 

 with which these insects move through the air in 

 general much exceeds that of a bird, if estimated 

 with reference to their comparative sizcj" 



* The Agrion, which belongs to the same family, when reposing, 

 closes its wings vertically over the body. 



f I have been favoured by Mr. George Newport with the follow- 

 ing account of the structure of the sting of the Wild Bee, (Antho- 

 phora retusa, Kirby) which he has lately carefully examined, and 

 from whose drawings of the dissected parts the annexed figures 



