G. S. Graham-Smitii 
483 
and the depressions the periods intervening between generations when 
many of the flies die. 
Before considering the species in detail certain general features of 
the chart may be discussed. 
The observations already described (p. 442) on the wintering habits 
of flies show that from batches of pupae in certain situations most of 
the adults emerge within a few days of each other, though isolated 
individuals emerge before the majority. When plotted in the form of 
a chart this mode of emergence produces a rapid rise in the curve at 
an early period. This is well illustrated in the curves for C. erythro¬ 
cephala, Lucilia, M. stabulans, H. dentipes, 0. leucostoma and the small 
Anthomyidae. 
It has also been suggested that the period of emergence is dependent 
on the rise of the ground temperature to the “critical” point for each 
species. If this view is correct a species requiring a relatively high 
temperature for emerging would tend to disappear early, since after the 
temperature falls below the required level in the autumn no further 
flies are likely to emerge to replace those which were dying off. Species 
on the other hand with a low “ critical ” temperature would go on emerging 
later. In the chart it will be noticed that the late emerging species 
such as P. groenlandica, Lucilia, M. stabulans, and 0. leucostoma were 
the first to disappear, whereas those which appeared earliest, such as 
C. erythrocephala, S. calcitrans, disappeared late. Certain species with 
peculiar habits seem to be exceptions to this rule. 
The conditions, which limit the numbers of a species, are various, 
and up to the present have been insufficiently studied Among the 
more important are the destruction of larvae and pupae by braconid 
and chalcid parasites (p. 523) and other enemies, lack of food for the 
larvae owing to the eggs being deposited in great numbers on material 
insufficient to nourish all the larvae which develop (p. 504), the larvae 
of certain species devouring those of other species, the attacks of 
enemies of the adult flies, and weather or other conditions causing 
great numbers of the freshly emerged flies of each generation to die. 
All these factors, and probably others at present unknown, operate 
with varying effect in different seasons and at different places. 
Experiments with blow-flies have been described and consequently 
the curves of C. erythrocephala and C. vomitovia need not be discussed. 
Phormia groenlandica seems to be considered a rare species (Austen, 
1909), but in the neighbourhood of Cambridge it is moderately common. 
One hundred and thirteen were caught in traps alone in 1915, and 
