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The Cantharidae,
commonly called soldier beetles or leatherwings,
are soft-bodied, often brightly colored insects
known for their aggregating on flowers. They are
distributed worldwide, with some 5,000 species
in 135 genera. They are slow, lumbering fliers
and easily caught; not surprisingly, they have
chemical defenses which make up for these
apparent evolutionary shortcomings.
Unlike many other beetles,
which have a pair of defensive glands at the tip
of their abdomen, the soldier beetles have
paired glands in the prothorax and on each of
their first eight abdominal segments. When
molested, the beetle emits droplets of white
viscous fluid from pores along their sides.
Studies have shown two species of soldiers,
Chauliognathus pennsylvanicus and
Chauliognathus lecontei, are consistently
rejected as prey by birds, mice, other beetles,
ants, and
jumping spiders.
Mantids, assassin bugs, centipedes, and
solpugids also avoid them. Chemical analysis has
shown the secretion in both beetles to contain
(Z)-dihydromatricaria acid, an acetylenic
compound.
[1] |
| A simple bioassay
showed the acid an effective deterrent to
jumping spiders (Salticidae)
in particular. An experiment carried out by
offering to spiders fruit flies tainted with
dilute (less than 2% of the beetles'
full-strength) secretions. As little as 1
microgram proved effective.
[1]
Acetlyenic compounds
are also known from fungi and plants,
including numerous species of aster (Asteraceae),
upon which soldier beetles often congregate.
It is unknown whether the beetles acquire
their defensive chemicals from eating such
plants, or they manufacture them
independently. It has been hypothesized the
insects at some time in the distant past
evolved the capacity to produce the
chemicals, and this adaptation provided them
with immunity to such chemicals in plants.
[1]
These beetles are one of the more abundant large insects in the American Midwest in late summer. They can be seen feeding on nectar and pollen on just about any flowering plant, but goldenrod (Solidago), Queen Anne's Lace (Daucus carota), Water Hemlock, and Ox-eye daisies appear to be among the beetle's favorites. Many contemporary accounts list the adults of this insect as "important predators" of other insects, but I have never seen an adult soldier beetle eating anything other than plant material. |
The orange elytra are retracted when the soft flying wings are in use
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Unfurling the flying wings takes time
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Beetles are known for their elytra,
the hard outer wings used to cover and
protect the flexible, membranous flying
wings underneath. The elytra are held up
at an angle when the flying wings are in
use; it has been posited the elytra act
as airfoils, but I'm not convinced this
is correct. In any event, the process of
uncovering and unfurling the flying
wings takes time, and this interval
sometimes proves fatal for beetles bent
on escape. It is thought the chemical
defenses help ameliorate this
disadvantage.
References
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Thomas Eisner, Maria Eisner, and
Melody Siegler,
Secret Weapons: Defenses of Insects,
Spiders, Scorpions, and Other
Many-Legged Creatures
(Belknap Press, 2005).
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