THE ENTOMOLOGIST. 15 
As a contribution to this subject I beg to put on record an 
incident which my brother and I witnessed during an after- 
noon’s ramble at the latter end of August last. We were 
“prospecting” in a favourite nook of ours in Epping Forest, 
near the village of Woodford, when we chanced upon an 
astonishing sight: a patch of fern and broom, about four 
yards square, was literally blackened by a swarm of a little 
fly, Sepsis cynipsea, Z.; every frond and twig seemed alive 
with the myriads of insects, slowly moving about and gently 
fanning their beautiful, spotted, iridescent wings with a 
steady and simultaneous motion. Some idea of their pro- 
digious numbers may be formed when I mention that two or 
three sweeps of a butterfly-net secured a mass of flies which 
weighed more than half a pound! We noticed that the mass 
exhaled a rather strong, and by no means an unpleasant, 
odour of “lemon-thyme.” The swarm consisted of males 
and females; but a long examination of the spot failed to 
throw any light on the cause of this assembly. The larve, 
Mr. Walker informs me (I am indebted to him for the name 
of the insect), feed on decaying matter, but we could find no 
difference in this respect in the small patch of herbage 
covered with the insects, or the ground beneath them, com- 
pared with the surrounding open forest glade. Mr. Walker 
once found a large cluster on a statue in Highgate Cemetery. 
I shall be glad if this notice leads to the publication of 
similar facts, for a rational explanation of this class of 
phenomena, based on observation, would certainly be wel- 
comed by all lovers of Nature-—Win. Cole; The Common, 
Stoke Newington, N. 
Certain Insects emerge from the Pupa by Hydraulic 
Pressure.—Being only a beginner and having seen nothing 
in any work I have read on the emergence of insects from the 
pupa, but that they “wriggle out,” 1 was surprised and 
delighted when I saw the wonderful power at their command 
to effect their deliverance. On the 14th of July last, as I sat 
‘watching some Bembeciformis dry themselves after their birth 
on the stem of an old willow, I took in my fingers a pupa that 
had just come to the mouth of its tunnel, and holding it 
between my eye and the light, being in a gloomy part of a 
wood at the time, I saw that the anal segment of the case was 
empty, and the enclosed insect emitting several drops of fluid 
