[Illustration: FIG. 78.--Ox Bot-fly (_Hypoderma bovis_) enlarged. (After
Brauer.)]
[Illustration: FIG. 79.--The Collecting Shears. (After Kiesenwetter.)]
[Illustration: FIG. 80.--A Bee-fly (_Anthrax hypomelas_). _a_, larva
from side; _b_, pupal skin protruding from cutworm chrysalis; _c_, pupa;
_d_, imago--all enlarged.]
Most Diptera frequent flowers and may be collected with a sweeping net
without much difficulty. The best season is from April to June, and the
bloom of the Willow, Alder, Plum, Cherry, Dogwood, Blackberry, etc.,
will ordinarily yield a bountiful supply of specimens and species.
Parasitic and saprophytic forms may also readily be obtained by
breeding, the former as in the case of the parasitic Hymenoptera, and
the latter from decaying vegetable matter and fungi. The Diptera
require the most delicate treatment, and the greatest care must be
exercised both in collecting and handling. A light sweep net is the best
implement for collecting and the contents of the net should frequently
be emptied into bottles provided with blotting paper to absorb the
excess of moisture. Very small Diptera should not be killed when they
can not be immediately pinned, and hairy flies should never be taken
from the net with the hand, but should be handled with fine forceps. A
pair of special collecting shears has been used by Lord Walsingham very
successfully. It is represented in the accompanying figure, and consists
of a pair of screen-covered disks, between which the fly is caught. The
insect is at once pinned through the screen and may be removed and
transferred to a box containing a sponge soaked in chloroform. The use
of this implement is especially advisable in the case of the Bee-flies
(_Bombiliidae_) and other hairy forms which are liable to be rubbed when
collected in the ordinary net. The Gall-making Diptera (_Cecidomyidae_)
are of little value unless accompanied with their galls, and the aim
should always be to collect the galls and rear the insects rather than
the keeping of specimens taken in the course of general collecting with
a sweep net. The rearing of Cecidomyidae is, however, a delicate task,
and requires considerable experience. Some knowledge of the habits of
the species is very essential to success. From immature galls no
rearings need be expected. A good plan is to examine the galls from time
to time and collect them when it is found that the larvae are beginning
to abandon them. In the case of species like the common Cone Gall-gnat
of the Willow, the larvae of which do not leave the gall to undergo
transformation in the earth, it is advisable not to gather the galls
until the transformation to the pupa state takes place, which, in this
species, occurs in early spring. The various leaf-mining and seed
inhabiting species can be treated as in the case of the
Microlepidoptera.
[Illustration: FIG. 81.--A Syrphus-fly.]