246 ANIMAL LIFE 



within and without itself, issues in a concerted move- 

 ment, and the butterfly flies away. 



The Diptera or Two-winged flies. — The most im- 

 portant groups of insects are those whose life-histories 

 are more rapidly traversed, or exhibit more profound 

 adaptations for the welfare of the species, than those 

 we have so far reviewed. These are the Diptera or 

 two-winged flies, and the Hymenoptera. 



Flies are one of the scourges of the earth. They 

 have no fear. To them the persons and dwellings of 

 animals and men, and all that comes from them, 

 afford nourishment and shelter. They pierce and 

 suck with the finest and most elaborate of instruments. 

 They transmit typhoid, malaria, cattle-disease, and 

 sleeping sickness. They devastate prairies, and what 

 they do not kill they spoil. Wherever man goes they 

 dog his steps, and where he lives there the house-fly 

 follows. Beelzebub was the appropriate symbol for 

 such a scourge. Each winter of our climate promises 

 to end the plague, and clears the air and earth of flies. 

 But that avid life escapes all checks. In every mild 

 break small gnats emerge, and some fly shakes a loose 

 leg in mockery at the season's persecution, and with 

 the approach of spring detachments of the vast bat- 

 talions issue from cover. Even in the cold of northern 

 Siberia, where spring and summer form but two 

 months of the year, the mosquito then hangs as a veil 

 over the rivers. In the fly's sight earth and its waters 

 form a vast nursery ; its covering of verdure a sus- 

 taining drink ; its inhabitants a still more stimulating 



