462 Observations on Habits and Parasites of Common Flies 
Chart 4, which shows the numbers of G. erytJirocephala caught on 
days selected as being the most favourable, as well as on those days 
which did not appear so favourable, but on which large numbers of 
flies were captured, resembles in many ways Chart 2 illustrating the 
conditions in the experimental cage. 
The numbers present in both are almost identical in the early part 
of May; both drop to a low level at the beginning of June, both show 
high peaks with intervening depressions in August, September and 
October. The increase in numbers amongst the “wild” blow-flies, as 
indicated by comparing the highest peaks of these two charts, was four 
times smaller than amongst those in captivity. This, however, might 
be expected since both the “wild” flies and their larvae were exposed 
to many enemies such as birds, spiders, wasps, competing and carnivorous 
larvae of other insects, parasites etc., and to adverse conditions such as 
shortage of food, from which those kept in captivity were to a great 
extent protected. 
The deep and frequent depressions in Chart 2 are not found in 
Chart 4, for though many deep depressions occurred in the daily 
record they were omitted as being possibly due to unfavourable weather 
conditions (see Chart 7). 
These two charts exhibiting the seasonal increase amongst blow-flies, 
as ascertained by different methods, resemble each other sufficiently 
closely to warrant the belief that they afford an index of the conditions 
which usually prevail, and show that under natural conditions the 
descendants during the season of each female blow-fly, which emerges 
from a winter pupa, number about 130 under favourable conditions. 
No doubt in very favourable seasons these figures might be increased, 
but under no conditions could the alarming figures of Howard be 
remotely approached. 
The descendants of the flies bred in this cage which passed the 
winter of 1915-16 as pupae emerged between April 1 (see Table I) and 
May 21, 1916. Altogether 2462 flies, 1207 and 1255 $, appeared. 
In 1916 twelve times as many flies emerged in this cage as in 1915. 
The experimental conditions have resulted therefore in a great increase 
in numbers, an increase much greater than is ever likely to occur in 
nature. 
The causes of mortality amongst flies. 
Hitherto no attempts have been made to ascertain with any degree 
of precision the causes of mortality amongst flies. Vague statements. 
