Field Notes from the Beiser Field Station: October 10, 2008
Horsehair Worms
| On April 1, 2009, the Biological Imaging Class was at the station and discovered a Horsehair Worm in the pool dug above the main trailhead. This pool, dug in the fall of 2007 by the Field Techniques Class, has hosted green frogs in the past. This year, it held a clump of amphibian eggs, and at least one Horsehair Worm | |
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Horsehair Worms are parasites in the Phylum
Nematomorpha. How they get into their insect hosts is a matter of
some confusion. Some claim that they primarily infest aquatic
insects. Others maintain that they can enter terrestrial insects as
well if the insects come to water to drink (something few insects do).
Still others maintain that the eggs are consumed by insects. Inside
the insect, they hatch out and burrow into the host's hemolymph.
Here they absorb fluids and nutrients from the host and continue to
grow. The specimen to the right is a male. The posterior end
has a pair of claspers; the anterior end has a simple opening
(below). They seem to absorb most of their food through the skin, so
a complex mouth is not needed.
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| Above: Two views of the posterior end of the male; presumably this is used to hold the female during mating. The females are usually thicker-bodied than the males. | |
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The really interesting thing about these worms is the life cycle. The worm completes its development in the host. When it is mature, it changes the body chemistry of the host, causing it to seek out water. When the grasshopper or cricket falls into the water, the worm bursts out into the water, for it lives for some time (the worm pictured here no doubt emerged from its host the preceding fall). If males and females end up in the same pool, they mate, and the female lays eggs. The eggs will be found on vegetation near the former waterline as the pond or puddle dries up, and these eggs will then be eaten by a new generation of crickets or grasshoppers, continuing the cycle.
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