The action of the acid juices in the bodies of certain specimens--as
many of the Lepidoptera, Coleoptera, and Diptera--will cause the
formation of verdigris about the pin, which in time accumulates and
disfigures and distorts the specimen, and ultimately corrodes the pin,
so that the slightest touch causes it to bend or break. There is no
preventive yet known for this trouble other than the use of pins which
have no brass to be corroded. Japanned pins are made for this purpose,
and are, on the whole, satisfactory, but they bend easily and some
caution is required in handling them. In place of these pins, which are
somewhat more expensive than the steel pins, iron pins may be used.
These are very soft and bend too easily for satisfactory use. The steel
pins may be rendered available for use by an immersion in a silver bath,
which is comparatively inexpensive.
Insects the larvae of which live in wood are particularly subject to
verdigris, as the Cerambycidae and Elateridae in Coleoptera, the Uroceridae
in Hymenoptera and Sesiidae in Lepidoptera. In Hymenoptera the families
Formicidae, Mutillidae, and the endophytous Tenthredinidae verdigris very
rapidly, and most Diptera also. With all these insects japanned or
silvered pins should be used, or when not too large the insects should
be mounted on triangles. This verdigrising is associated with what is
known as greasing, and this, as just indicated, is also associated with
endophytous larval life. The verdigris may be prevented by the methods
indicated, and I would strongly advise, as a good general rule to be
followed, the rejection of the ordinary pins for all species which, in
the larva state, are internal feeders. But there is no way of preventing
greasing or decomposition of the fats of the body, which may affect a
specimen years after it has been in the cabinet. If the specimen is
valuable the grease may be absorbed by immersion in ether or benzine, or
by a longer treatment with powdered pipe-clay or plaster of Paris.
Insects collected on seabeaches, and saturated with salt water, also
corrode the common steel pin very quickly and should be mounted on
japanned pins. It is also advisable to rinse such specimens thoroughly
in fresh water before mounting.
The conviction has been forcing itself on my mind for some time that the
naphthaline cones tend to promote greasing and verdigris, and carbolic
acid in some small vessel secured to the cork, were, perhaps,
preferable.