370 GENERAL BIOLOGY 



even been bred from the open boxes of snuff on a druggist's counter, 

 though tobacco is supposed to be quite injurious to insects. 



After the housefly's eggs are laid it takes about eight hours for them 

 to hatch into maggots. These finish their growth in six to seven days, 

 burrowing into the ground "under the manure pile" (hence the need of 

 concrete floors) and transform into brown puparia, from which they 

 emerge as adult flies in three days. 



Hodge and Dawson have summed up the rapid increase in flies most 

 tellingly in the following words : 



"After coming out as adults they fly about over an area not gen- 

 erally more than one thousand yards in diameter, and feed and drink 

 from two hundred to three hundred times a day for from ten to fourteen 

 days before maturing their first batch of eggs. This actually delivers 

 the enemy into our hands. It means that, with flytraps on every garbage 

 can and swill barrel, and with everything most attractive to flies very 

 carefully kept in these receptacles, not a single fly will succeed in feed- 

 ing for two weeks without getting caught. In this case no more eggs 

 will be laid, and the pests will vanish. 



"Allowing ten days of feeding between emergence and oviposition, 

 figuring that a fly lays one hundred and fifty eggs at a batch and lives 

 to lay six batches, compute the increase of a pair of flies beginning to. lay 

 May 1. Half the progeny are supposed to be females. Test the follow- 

 ing figures : 



May 10 152 flies. 



May 20 302 flies. 



May 30 11,702 flies. 



June 10 34,302 flies. 



June 20 911,952 flies. 



June 30 6,484,700 flies. 



July 10 72,280,800 flies. 



July 20 325,633,300 flies. 



July 30 5,746,670,500 flies. 



"As this last amount makes 143,675 bushels of flies resulting from 

 a single pair of flies in three months, one can estimate what the result 

 will be if allowed to breed unrestrained during August and September 

 beside. 



"The common sense question, then, is, why not let this pair of flies 

 catch themselves in May? This rapid increase also means that anything 

 short of extermination is hardly worth the effort. A fly is possessed of 

 no more cunning than shot rolling down a board, and the last pair will 

 run into a trap as easily as the first. Why not let them all catch them- 

 selves?" 



During the winter, especially in cold climes, most of the flies are 



