458 Observations on Habits and Parasites of Common Flies 
flies—upon plenty of places for the insect to develop and for the larvae 
to feed, upon an average of ten days to a generation in midsummer 
(this period increasing in the autumn and being greater also in the 
spring time), and also upon a period of ten days after emerging of the 
adult flies before sexual maturity is gained (this point of the duration 
of the existence of the adult fly before the attainment of sexual maturity 
has been the weak element in other calculations that have been made 
of house-fly abundance)—let us start then on April 15 with a single 
overwintering fly which on that day lays 120 eggs, and we will have 
the following table: 
April 15th, the overwintering female lays 120 eggs. 
May 1st, 120 adults issue of which 60 are females. 
May 10th, 60 females lay 120 eggs each. 
May 28th, 7200 adults issue, of which 3600 are females.” 
Nine further generations lay eggs on June 8, 20, July 10, 19, 29, 
August 8, 18, 28 and finally on 
“September 10th, 5,598,720,000,000 adults issue, of which one- 
half are females.” 
Other workers have made somewhat similar calculations and the 
writer decided during the seasons of 1914 and 1915 to investigate 
experimentally what actually occurs, since it is quite evident that 
multiplication under natural conditions is much more limited than such 
figures suggest. 
For these experiments blow-flies were allowed in the autumn of 
1914 (see p. 444) to deposit eggs on the carcases of rabbits in a large 
open-air net cage. This cage was 6 ft, high, 15 ft. long and 6 ft. broad, 
with a floor of bare earth, surrounded by boards sunk into the ground 
(Plate XXIV). All the adults were dead by Nov. 18, 1914. Adult 
blow-flies commenced to emerge in this cage on April 24, 1915, the 
first female appearing on May 4. Altogether 206 flies emerged between 
April 24 and May 16. Judging by the results of a control experiment 
in which the flies were killed as they emerged, the sexes were in approxi¬ 
mately equal numbers (p. 444). These flies and their descendants bred 
in this cage throughout the season and the flies were counted daily 
after they settled down for the evening with the aid of a counting 
machine, recording consecutive numbers after each pressure of the 
finger. 
They were kept well supplied with food and water and dead animals 
to lay their eggs upon. These animals were placed in large earthenware 
