350 
Parasites of Common Flies 
On several occasions the flies caught in a trap baited with excrement were 
counted hourly, and the maximum temperature recorded by a thermometer 
exposed in the sun during each hour noted. The results obtained on two 
consecutive days, July 26 and 27, 1916, are shown in Chart 2. On July 26 
the sun first appeared at 1.30 p.m. and the afternoon was very fine. The fly 
curve corresponds closely with the curve for the maximum “sun” tempera¬ 
ture. The following day was very fine and hot throughout, and the flies were 
most numerous about midday. This seems to be the normal condition on very 
fine, sunny quiet days. 
The following flies were common in 1914, uncommon in 1915 and again 
common in 1916: M. hortorum, G. maculata, P. albolineata, P. eriophthalma. 
P. greenlandica was very common in 1916. 
26 July 1916 27 July 1916 
Chart 2. Number of flies caught in a trap baited with excrement (broken line) and maximum 
temperature recorded by a thermometer in the sun (line) during each hour on July 26 and 
27, 1916. 
HABITS OF FLIES. CLEANSING. 
The cleansing habits of flies, especially of S. carnaria, were carefully studied 
in a large cage, 12 x 8 x 7 ft, out of doors—under conditions resembling as 
closely as possible those in which these flies live. 
Throughout the day flies spend a very large proportion of their time, 
probably several hours daily, in cleaning themselves; not infrequently a leg 
or wing is being cleaned even in the act of feeding. Cleansing is almost in¬ 
variably undertaken after feeding, and the procedures usually follow each 
other in a definite order. The wings are first treated on both surfaces, and 
they receive more attention than any other structure; next the posterior legs; 
