364 
Parasites of Common Flies 
and that numbers of females are often seen emerging one after the other from 
a puparium through a single opening. 
If several $s are confined in a corked tube they often endeavour to escape 
by excavating a tunnel through the cork. In one instance a tunnel 3 mm. 
long and about 0-5 mm. in diameter (PI. XIX, fig. 8) was excavated in 
the part of the cork in contact with the glass, and all the $s, 40 in number, 
escaped through it. In this case, as in many others, the process was 
carefully watched. Only one $ worked at a time, and the others remained 
quietly cleaning themselves in various parts of the tube. When the worker 
retired another took her place, and no attempt was made to start fresh 
channels. 
Sometimes though the parasites become adult they fail to emerge from 
the puparium and die. The cause of this phenomenon has not been ascer¬ 
tained. 
Habits of M. acasta. 
Males. As mentioned previously the males usually reach the adult con¬ 
dition several days or even weeks before the females in the same puparium. 
The males vary greatly in size and colour. The largest reaching a length of 
2-25 mm. and the smaller specimens only 1-25 mm. They are larger and more 
stoutly built than the females, but they appear to be blind and owing to their 
ill-developed wings are incapable of flight. Their colour varies from light 
yellow to brown. 
The males which emerge in opened puparia (see p. 363) remain there 
though free to get away, if they desire to do so. They sometimes move slowly 
about amongst the female nymphs, frequently touching them with their an¬ 
tennae and apparently examining them closely, but spend most of their time 
in cleaning their antennae and legs. If two males emerge in the same puparium 
they soon meet and fight. 
The habits of the males are more easily studied in glass tubes, especially 
when confined with females. A single male was frequently seen to mate with 
many females. It may be mentioned that when a male has been present all 
the females which leave a puparium seem to be fertilised. Though all the 
movements of the males are slow, and they are seldom in motion for more 
than a few seconds, their attitude is alert. Especially when in the neighbour¬ 
hood of other males they walk with the antennae lifted, widely separated, 
and waving in a peculiar menacing manner. When two males approach one 
another a fight almost invariably occurs, and a living male has been seen to 
attack the shrivelled body of a dead male. 
Several of these fights were carefully observed. One between two large 
specimens of almost equal size lasted nearly an hour. Much of the time was 
occupied in manoeuvring to obtain favourable positions. Eventually one 
specimen was able to get on to the back of the other, and, securing a firm hold 
with his legs, buried his mandibles in the dorsal part of his adversary’s head, 
and continued to bite for five minutes. During this time the other ceased to 
