212 THE ESSEX NATURALIST
Notes on a Swarm of Dipterous Flies.—On September 11th, 1949, while
exploring a small piece of woodland immediately to the south of the Epping-
Ongar railway line and about 400 yards east of the road from the Lower
Forest to Theydon Garnon a large swarm of small dipterous flies was dis-
covered. The insects were crawling over the stems and leaves of a number of
bracken fronds and on some clumps of the grass, Molinia caerulea Moench.
The whole occupying an area of about two or three square yards and on
both sides of a narrow path. Disturbed by one of my companions passing
along this path before me some of the flies rose into the air but settled again
at once. The entire swarm was in constant motion walking about on the
leaves and moving their wings, but not taking flight unless disturbed. No
copulation was observed. The general position of the swarm did not change
despite the constant movement. The day was warm and sunny with little
or no air movement. It was a simple matter to box a large number of the
flies by closing the lid on to a glass-bottomed box on a bracken leaf.
Upon examination later the insects taken proved to be males and females
in almost equal numbers. A number of them were submitted to Mr. L.
Parmenter for identification and he advised that the species was Sepsis
fulgens Mg. A species that is more usually described in this country as
Sepsis tonsa Duda. He informs me that his identification was based on the
description in Seguy's work (1934 Faune Fr: 28 Muscidae, Acalypterae
et Scatophagidae).
They are small flies about 3.5 mm. long and with a pronounced black
spot near the apex of each wing. The species is generally distributed through-
out the country. Closely related species are reputed to be associated with
dung but there was no suggestion of such an association in this instance.
Mr. Parmenter kindly drew attention to reports of similar swarms described
in various entomological publications.
Dr. Thorpe (1) noted a swarm of the same species on September 21st, 1947,
on a bramble bush at Cambridge and suggested that the phenomenon might
represent some form of mass sexual display and compared it with "court-
ship" swarms of Dolichopodid flies which, like this species of Sepsid, have
spotted wings. He did not observe copulation in the swarm nor did the
insects take wing.
The Rev. Graham (2) observed a swarm of Sepsis fulgens on August 17th,
1948, at Salmonby, Lincolnshire, in a like situation to the swarm Dr.
Thorpe saw. He was fortunate in being able to visit the spot each day and
found that the main body persisted until September 8th, after which date the
swarm dwindled, finally disappearing on September 25th. The Rev. Graham
noted frequent attempts at copulation and when he accidentally disturbed
a part of the swarm "it merely rose into the air and then settled down again
with the rest." The swarm was first noted on hazel, it moved to brambles
and nettles and subsequently spread on to adjacent bracken and grasses.
Two other records of Sepsid swarms came to my notice. R. L. Coe (3)
noted a swarm of Sepsis cynipsea at Bembridge, Isle of Wight, and H. Donis-
thorpe (4) the same species in Windsor Great Park. Both these notes, however,
appear to have been based on "recollections," in one case ten years pre-
viously and the other fourteen years earlier than the publication of the notes.
Both relate to a different species and were not accompanied by details, save
that both were "in summer" and in one case the swarm was noted on
Bramble. An editorial note points out the fact that the name Sepsis cynipsea
has been used for various insects and the consequent doubts cast upon such
names without details of the source upon which such identifications were
based.
At the time my discovery was made I had no knowledge of the reports
of the other swarms so my observation was not so complete as it might have
been. Nevertheless the occurrence of this swarm is of interest, especially