96 THE AUSTRALIAN NATURALIST. 
commonly on the ocean beach, flying briskly for a few yards 
when one is walking near the water’s edge and lighting 
again on the sand. The flies were collected in a slight de- 
pression on a cliff of friable rock beside'the North Wollon- 
gong surf beach.. There must have been many thousands 
of them, forming a coating which quite hid the rock on 
a surface of several yards square, giving the place the ap- 
pearance from a little distance of being painted with tar. — 
he flies furthest in at the back of the cavity were ina 
double layer. They were not pairing, and I am quite at a 
loss to account for their gathering in this way. They rose 
in clouds when approached. I went daily to the spot for 
~ over a week, and on some days found the place deserted, 
while on others perhaps a dozen flies were there, and again 
a multitude, as I have described. The weather was pleasant 
and warm during the whole time. Large numbers of dead 
flies lay on the ground beneath, which were washed away 
at high tide. On the days when the flies were not crowded, 
single lines from a few inches to several feet in length of 
dead flies were sticking to the rock surface, just as if they 
had adhered to something exuding from microscopic cracks, 
but examination with the lens failed to disclose any cracks. 
A few isolated patches of flies occurred on ‘‘crowded’”’ days 
in slight depressions about the face of the rocks, and on one 
oceasion I noticed a trowded patch of flies on an exposed _ 
concrete wall abutting on the beach, at a place stained by 
water running down, but which was quite dry at the time. 
I collected specimens and forwarded them to Dr. Tillyard, 
who has kindly given me the following note regarding 
them :— 
‘«f must thank you for your very interesting letter, and the 
small packet of flies just to hand. ‘ 
“‘] know this fly well, as a frequenter of the sea-shore, but I 
have never seen it so abundantly as you describe. It is almost cer- 
tainly undescribed, and T doubt if there is anybody in Australia. 
competent to deal with it, as it belongs to the great group of the 
Muscidae Acalyptratae, which naw contains more than fifty families, 
all of them distinguished by characters that require the handling of a 
specialist. Even to put it into its correct family, it would be neces 
* sary to have very perfectly preserved specimens, with the antennae, 
_ tarsal joints and: the fine setae of the head and thorax intact. Your 
specimens being sent dry, have all lost the terminal arista and some 
of the setae. Thus I cannot place it definitely in any family for 
you; but I think it probably belongs to the Ochthyphilidae, or ‘‘bank- 
lovers,’’? which are mostly to be found on the banks of rivers and 
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