!94 



ORTHOPTERA 



CHAP. 



they migrate do so in 



the direction taken by predecessors. 

 Their movements are to a large ex- 

 tent dependent on the wind, and it 

 is said tliat they make trial flights 

 to ascertain its direction. When on 

 the wing probably very little mus- 

 cular effort is necessary. Their 

 bodies contain elastic air sacs in 

 communication with the tracheae, 

 and at the time of flight it may be 

 presumed that the body is compara- 

 tively empty, food being wanting, 

 and the internal organs of repro- 

 duction, which occupy a large space 

 when in activity, yet undeveloped, 

 hence the sacs have full room for 

 expansion, as explained on p. 283. 

 Thus the Insects exert but little 

 Portions of body of elfort in their aerial movements, and 



Caloptenus sprelus to show some ..,,. , t ■ a ^ i 



of the air-sfcs. (Modified from are, it IS believed, chiefly borne by 

 Packard.) A, Dorsal aspect of ^he wind. Should this become uu- 



anterior parts ; B, lateral aspect .. ■ ^ ,^ , ,^ vii. 



of posterior parts of body ; a, en- favourable it IS Said that they alight 



largements of tracheae in head ; r^^^ ^a,it for a change. 



b, pair of large sacs in thorax ; ... 



c, sacs on the tracheal trunks of The mOSt obSCUre pOlut in the 



abdomen ; 5, spiracles. natural history of the migratory 



locusts appears to be their disappearance from a spot they 

 have invaded. A swarm will alight on a locality, deposit there 

 a number of eggs, and then move on. But after a lapse of a 

 season or two there will be few or none of the species present 

 in the spot invaded. This appears to be partly due to the young 

 locusts dying for want of food after hatching ; but in other cases 

 they again migrate after growth to the land of their ancestors. The 

 latter fact is most remarkable, but it has been ascertained by the 

 U.S. Entomological Commission that these return swarms do occur. 

 In South Africa it would appear that the movements of 

 the migratory locusts are frequently made before the Insects have 

 acquired their wings. Mrs. Barber, in an account of " Locusts and 

 Locust-Birds in South Africa," ''■ has illustrated many points in the 



1 Tr. S. Afr. Phil. Soc. i. 1880, p. 

 sulcicollis Stal. 



193. The species is thought to be Pachytylus 



