27G A BOOK OF INSECTS 



flight above the waters of a pond, while male gnats are 

 sometimes so numerous as to form a flickering haze in the 

 evening air. 



Vast swarms of butterflies have been encountered by 

 ships far from land. A few species are known to migrate 

 annually along definite courses, and to have extended 

 their range in historic times. The American monarch 

 butterfly (Anosia pleccippus) has already been mentioned. 

 It is evidently tropical in its origin, yet in the summer 

 season it ranges far northward into Canada in search of 

 food-plants for its young. At the approach of winter the 

 butterflies of the new generation congregate in flocks, and 

 commence their southward journey, resting at night 

 among herbage and the branches of trees. Those in- 

 dividuals that fail to join in the flight to warmer climes 

 appear to perish, for no hibernating monarchs have been 

 found in the northern States or in Canada. 



The gregarious habits and migratory instincts of locusts 

 have been closely studied. These insects usually breed 

 on dry and rather elevated plains, whence they sally in 

 immense and devastating swarms. A writer in Nature 

 states that a flight of locusts passed over the Red Sea in 

 November 1889. It appeared to be 2000 miles in extent, 

 and he estimated its weight at 42,850 millions of tons, 

 reckoning each locust as one-sixteenth of an ounce. 

 Independent testimony and official statistics tend to 

 support the accuracy of these figures. The fact that 

 locusts usually invade districts at irregular intervals, and 

 not in succeeding years, seems to indicate that the migra- 

 tory instinct is called forth by over- population of their 

 breeding haunts. Favourable climatic conditions, and 

 relative freedom from the attacks of parasites, apparently 

 combine in some seasons to foster excessive increase, and 

 the insects must either migrate or starve. Instincts which 



