And all without dynamite
At High School I had an excellent Biology teacher by the name of Dr Ruppolt, who introduced us pupils to a variety of practical exercises. For one of his favourite experiments he asked us to collect horse dung. He then placed some of it into a dark shoe box, cut a 1 cm window into its lid and sealed the latter with transparent tape. Within a few days approximately 1 cm tall, transparent stalks of the fungus Pilobolus sp. would grow on the dung, all pointing into the direction of the little window in the lid of the box as if they could sense the light falling through it. Soon, however, the little window became covered with Pilobolus’ black spores, which were ejected towards the light from exploding sporangia at the tips of the fungus’ stalks.
We learned that pressures inside the Pilobolus head could reach 4.5 atmospheres and that this fungus was able to shoot its spores more than a metre wide onto fresh grass. A horse (or other grazing animal) would eat the grass, excrete dung plus spores and, thus, allow Pilobolus to spread. What we did not know at that time was that the speeds with which the spores are ejected can reach 25 m per second, a velocity similar to that of bullets in an air rifle.
Interestingly, a little parasitic nematode by the name of Dictyocaulus viviparus, which grows in the lungs of grazing animals and causes a form of bronchitis, allows itself to get propelled with the help of an exploding Pilobolus to fresh grass, from where it can be ingested by its grass-eating host. The worm then invades the lungs of its host, matures, produces eggs that get coughed up and swallowed. The eggs hatch in the gut and the young larvae become expelled with the host’s faeces. The free larvae then search for a Pilobolus and renew the cycle.
Yet, Pilobolus is not the only organism that bursts: The tiny spheres of the freshwater alga Volvox globator break open to release daughter colonies that are trapped inside. Another exploder: seed capsules of a group of plants known as “Touch-Me-Not” (Impatiens spp.) explode when touched, scattering seeds several metres away.
But explosions are even known from some animals (and I am not talking about the sometimes forceful ejection of offspring from viviparous guppy and platy fish or about exploding chicken eggs, toads, whales or cows due to a buildup of gases inside them). I am talking of special defensive castes in certain ants (Camponotus saundersi) and termites (Globitermes sulphureus) that are best to be described as “kamikaze soldiers” or miniature “suicide bombers”. In both cases glands in the body are involved that have no opening to the outside and contain incompressible liquids, but whose composition differs in the ant and the termite.
In termites, for instance, normal soldiers, termed ‘nasuti’, can squirt a sticky substance over a distance of a few centimetres from their heads towards an intruder to immobilize the latter. However, in the self-exploding special kamikaze soldiers the gland mentioned above, containing yellow sticky liquid and occupying a large part of the thorax and abdomen, explodes. Under attack the abdomen of this unique soldier caste contracts so forcefully that the gland ruptures and causes the termite to explode, showering its immediate surroundings with sticky fluid. This suicidal defence system is effective towards invading ants and other small arthropod predators. What is intriguing is that ants like Camponotus saundersi seem to have evolved a similar defence system. But who had it first? Since termites have a much older ancestry than ants, the prize (if we consider such suicidal acts prize-worthy at all) must go to the termites.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2015.
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