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Eating your vegetables under water without drowning - beetle style
By E. Grobbelaar and R.G. Oberprieler*
 We humans like to think that we are extremely innovative, inventing new
technologies and discovering new and better ways of doing things. But how often
does it happen that when looking at Nature, we realize that our inventions are
actually imitations of solutions to physical, chemical and biological problems
which Nature "invented" many millions of years ago?
Take living under water when you can only breath atmospheric oxygen for
instance. We have invented snorkels, aqualungs and diving bells to allow us
brief sojourns into the watery world, but insects have perfected these and other
techniques millions of years ago so as to live an almost totally aquatic way of
life. Water scorpions (a kind of predatory bug of the family Nepidae) breathe
through a long snorkel at their rear end, lurking motionless among vegetation to
seize any unsuspecting prey. Predacious diving beetles (family Dytiscidae) trap
a bubble of air under their wing cases and breath from it, whereas other beetles
have a thin film of air trapped between fine hairs on their underside that
functions like a gill and absorbs oxygen straight from the water. There is,
however, a way of breathing air under water that we humans have not yet been
able to imitate - tapping it from plants.
 Tapping air from plants has been perfected by a beetle,
Donaciasta, from a
group known as longhorn leaf beetles. They belong to the subfamily Donaciinae of
the family Chrysomelidae (leaf beetles), which is one of the largest of beetle
families in the world and contains highly specialised plant feeders such as flea
beetles and tortoise beetles.
The Donaciinae is a relatively small, semi-aquatic group and probably has the
most fascinating biology of all. They live, both as adults and as larvae, on
aquatic and semi-aquatic plants such as sedges and bulrushes (Cyperaceae), reeds
(Poaceae), pondweeds (Potamogetonaceae) and water-lilies (Nymphaeaceae), and
have evolved unique adaptations to feed on the submerged parts of these plants.
Some are even known to live in the brack or salty water of pools along
sea-shores on marine plants such as eel grass (Zosteraceae).
Donaciines occur mainly in the northern hemisphere, with only two genera,
Donacia and Donaciasta, recorded from Africa. Donacia is known from a single
undescribed species from Botswana and Namibia, whereas Donaciasta species are
known to occur on Madagascar, in southern and central Africa, and in China. The
southern African species, Donaciasta goeckei, was only known from a handful of
specimens collected in KwaZulu-Natal, Angola, Uganda and Zimbabwe, until we
discovered a population at Lapalala Nature Reserve (Limpopo Province) and also
found its larvae and pupae during subsequent fieldwork near Ellisras (Limpopo
Province). This discovery is significant as it presents the first account of the
habits and life history of the genus Donaciasta and establishes that the genus
is indeed associated with water-lilies.
A submerged lifestyle requires certain adaptations. Longhorn leaf beetles do
not have gills and therefore cannot extract oxygen directly from the water, as
damselfly and dragonfly larvae do. Neither can they carry a bubble of air from
the surface under their wings or do they have a breathing tube at the tip of
their abdomen. Instead, they obtain oxygen under the water in an ingenious way -
by tapping it from the stems and roots of their food plants. Water-lilies have
large numbers of air pockets in their tissues which keep their leaves afloat on
the water surface, a perfect supply of air for an insect able to
get to it.
 Donaciasta deposits its eggs under water in a gelatinous mass stuck between
overlapping water-lily leaves. This affords the eggs protection from predators
such as fish and other insects from below, and from wasp parasites and insect
predators that could attack from above the water. Creamy white, grub-like larvae
with a small head and three pairs of short, hooked legs on the front (thoracic)
segments hatch from these eggs after a few days. After hatching, they drop to
the bottom of the pond or stream or crawl down on the water-lily stems, until
they reach the dense mass of water-lily roots embedded in the mud at the bottom.
Here they  settle to feed on the plants' rootlets, gnawing them off to leave only
short stumps, or else eating holes into the larger roots. To be able to stay and
feed among the roots for its entire life, the larva has evolved an intriguing
way of breathing. Its last abdominal segment has a pair of long, hook-like,
hollow spines connected to the spiracles (breathing holes) at the end of the
body. The larva pushes these spines through the epidermis of the plant stem or
root, rupturing the cells that contain air. The escaping air is guided by the
spines to the spiracular openings at their immediate base, allowing the larva to
breathe through the plant, as it were, and carry on feeding without ever  having
to break the water surface. When it needs to move off to another root, it simply
withdraws the spiracular hooks and pushes them into the plant somewhere else.
When it is ready to pupate, the larva spins an oval, parchment-like cocoon
that is both water- and airtight and firmly attached to one of the larger roots.
Once spun, the cocoon takes on a silvery appearance, indicating that it is full
of air that seeped out from the slit-like incisions in the root made by the
larval hooks. Oxygen in the cocoon is apparently continually replenished from
the root  through the incisions, ensuring a constant air supply for the pupa and
later the emerging adult beetle. When emerging from the pupa, the young adult
first breaks free from the pupal case in the cocoon and then proceeds to break
open one end of the cocoon to dive into its watery wonderland.
The beetle is now detached from the water-lily and its supply of air, but
suitably equipped to breathe air on its own under water. Its underside is
covered with a dense layer of very fine, silky hairs that trap air contained in
the cocoon to form a thin, silvery cushion, called a  plastron. This film of air
provides the beetle with enough air to reach the surface of the water, where it
now spends most of its time to find a mate and reproduce. Adults of other
species of Donaciinae are reported not to leave the water at all. Their plastron
is more extensive, covering also the antennae and much of the face and body.
When the beetle needs to replenish its air supply, it cuts the stem of a water
plant with its mandibles allowing a bubble of air to escape. This bubble is then
"caught" by the antennae, causing the air it contains to spread
through the plastron to reach the spiracles, or breathing holes, on the beetle's
abdomen. The adults of D. goeckei are coppery brown in colour, elongate, have
slender legs and are about 7mm long. They live on the exposed parts of
water-lilies and may be seen sitting on the upper surface of the lily leaves,
appearing almost fly-like.
Longhorn leaf beetles have thus "invented" ingenious and effective
ways of breathing under water in all stages of their life cycle. Just as
water-lilies have adapted to grow under water, hidden from most
plant;feeding beetles, so longhorn leaf beetles have evolved
counter-measures enabling them to follow these plants into their aquatic habitat
and utilise them as food. In the light of these fascinating adaptations, are we
as wonderfully innovative and imaginative as we think we are?
Are our submarines, submersibles and aqualungs not merely imitations of
devices and mechanisms that Nature "invented" a long, long time
ago?
More Information:
E. Grobbelaar and R.G. Oberprieler*
Biosystematics Division, ARC-Plant Protection Research Institute,
Private Bag X134, Pretoria, 0001
vreheg@plant5.agric.za
* Australian National Insect Collection, CSIRO - Entomology, GPO Box
1700, Canberra, ACT 2601, Australia
Rolf.Oberprieler@csiro.au
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