G. S. Graham-Smitii 
539 
13. The different species of flies vary in their seasonal distribution 
and in their abundance at different periods of the season. Those species 
which require a high “critical” temperature for emerging from pupae 
appear late and disappear early. The disappearance of flies in the 
autumn is not due to the effects of cold on the adults, but to the non¬ 
emergence of flies from pupae to replace those which are. dying off. 
14. 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 the larvae and pupae by 
braconid and chalcid parasites and other enemies, lack of food for the 
larvae, the larvae of certain species devouring those of other species, 
the attacks of the enemies of adult flies, and the weather and other 
conditions causing great numbers of freshly emerged flies of each 
generation to die. 
15. Traps in the open baited with human excrement and decaying 
animal matter catch approximately equal numbers of flies. Dark 
receptacles in open situations usually catch smaller numbers, while 
traps in shady situations attract few flies. In this country it is 
desirable, therefore, to place all receptacles for garbage, manure and 
refuse in the most shady situations available. 
16. Several of the species, amongst them C. erythrocephala, C. 
vornitoria, F. scalaris, L. caesar, and M. stabulan-s, attracted both to 
human excrement and decaying animal matter seldom enter rooms, and 
are probably of little importance as carriers of pathogenic bacteria in 
this country. These species, however, play an important part in 
spreading disease and causing discomfort under war conditions (see 
footnote p. 493). 
17. Only about 16 % of the flies which visit human excrement 
and decaying animal matter are males. Males very seldom venture 
into dark receptacles. 
18. In the case of the genus Lucilia very young specimens are 
mainly attracted to human excrement as a food, while the older 
specimens visit both excrement and decaying animal matter. Members 
of the genus Calliphora probably behave in the same way. 
19. All the species of flies which visit excrement and decaying 
animal matter also visit ripe fruit, but not in the same proportions either 
as regards species or sexes. 
20. Great contamination of fruit on trees and exposed for sale in 
shops with faecal and putrefactive bacteria is caused by flies and 
wasps. 
Parasitology vm 
35 
