G. S. Graham-Smith 
507 
the head only in a very peculiar manner, and apparently all were 
females.” 
Nearly all investigators of this disease have confined their attention 
to house-flies observed in rooms, and do not seem to have taken into 
consideration the possibility of the disease occurring in other common 
flies, which are abundant earlier in the year, and the infection spreading 
from them to house-flies or their larvae, which frequent similar situations. 
In fact the cases just quoted seem to be the only ones recorded of flies, 
other than M. domestica, being attacked. Hewitt (1914) says that it 
is uncommon to find infected house-flies out of doors though he has 
observed some in Canada, at Ottatva. 
Under out-of-door conditions an empusa-infected fly remains in 
a typical and easily recognisable condition for a short time only so that 
much patient labour would be required to ascertain whether the disease 
is common outside either among house-flies or other species. 
The writer has found specimens of Hydrotaea dentipes dead of 
empusa out of doors on four occasions. The interest of this observation 
lies in the fact that this fly is very common and is attracted in large 
numbers to excrement and decc^ying animal matter, where it may come 
into contact with house-flies or their larvae. One was discovered 
on an artichoke leaf on August 4, 1914, and the others in 1915; 
one on August 16 on a Galega leaf (Plate XXVIII, fig. 16), a third on 
September 20 on an elm branch, and a fourth on October 4 on a piece 
of galvanised iron. In these years careful watch was kept for the 
occurrence of empusa disease in house-flies, and in each year an infected 
H. dentipes was found before the first infected house-fly was observed, 
the first case of empusa in the house-fly in 1915 being noticed on Aug. 30 
in the kitchen. 
Blow-flies (0. erythrocephala) become infected occasionally, but the 
growth on the abdomen is often less apparent than in house-flies. 
With careful observation, however, infected specimens may be found. 
In these insects the growth, in the early stages, appears as white, 
regular, wax-like bands between the abdominal segments. In well- 
marked cases the insect later resembles in every respect an infected 
house-fly. Plate XXVIII, figs. 10, 11,14 and 15 reproduce photographs 
of specimens with well-marked rings of growth. The disease killed a 
number of the blow-flies bred from winter pupae and confined in an 
experimental cage (p. 458). The first case was noticed on Sept, 18 and the 
last on Nov. 12; during which period 18 cases of the disease occurred 
at considerable intervals. Approximately 0-33 % only of the blow-flies 
Parasitology vm 
33 
