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OBSERVATIONS ON THE PUPATION OF
DYTISCUS MARGINALIS L. and HYDROUS
(HYDROPHILUS) PICEUS L.
By HUGH MAIN, F.Z.S., F.R.E.S.
[Read 27th January, 1934.]
(With Two Plates.)
THERE are two large well-known British water beetles,
one of which, Dytiscus marginalis, is very common in
Epping Forest. The other, Hydrous (Hydrophilus) piceus is
rare there. Fowler states that it occurs in Essex, but the only
record I know is of a specimen taken some years ago in the
Walthamstow district.
Although the life histories of these two insects were investi-
gated and described nearly two hundred years ago, the English
text-books to which I have referred give very inadequate and
occasionally erroneous accounts of certain incidents associated
with them.
The fullest account of them appears in Miall's Natural
History of Aquatic Insects (1895) which, besides recording many
observations by himself, his friend and others, gives delightful
descriptions of some of the old zoologists, such as Swammerdam,
Reaumur, Lyonnet and Degeer, and their writings. More
recent books seem to have copied Miall's work to some extent.
DYTISCUS.
Although the adult Dytiscus beetle and its larva are com-
monly found in ponds, it required a long-continued search, with
the assistance of my friend, Mr. J. T. Smith, before the location
of the pupa was discovered by me. All the information we
had about it was practically summed up in Miall's statement,
"At length the larva ceases to feed, creeps into moist earth
"near the edge of the water, makes a roundish cell there and
"changes to a pupa."
The fortunate discovery by Mr. Smith of a pupa in its cell
in the bank near the margin of a pond opened the way for
observation of the method employed by the larva in the pre-
paration of the pupal chamber, and of its further transforma-
tions. Other cells were then found around various ponds, and
one of these was exhibited at our meeting on January 27th,