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The Giant Ichneumon Wasp – Stump Stabber Extraordinaire

Friday 3 June 2016

What is the fastest, tallest, heaviest, lightest? We love to compare members of the animal kingdom in these terms.  One word you may hear too is longest but when it appears in a question it is normally asked in terms of total length.  In that case, the Giant Ichnuemon Wasp (Megarhyssa macrurus), found in the USA, is nothing much to write home about being just two inches long.  However, if the question was “which insect has the longest ovipositor known to science?” then the female of this species would be the answer. And holy egg laying organs, it’s some length.

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So, yes.  That long thing extending from the wasp, twice the length of its body, is not a stinger or a rear antenna (which might be unusual and interesting).  It’s the wasp’s ovipositor, and is used to lay its eggs: but that’s not all.  It’s also a drill.

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The female wasp must first find a perfect drilling location and she does this using her antennae. Yet the Giant Ichneumon is not simply looking for a snug location for her egg.  She is searching for a host (and does so between the months of June and September).  This is a parasitoid species.  The object of her exploration is the larvae of the pigeon horntail (Tremex columba, Symphyta).  Once she has detected one the process of egg laying begins – it is not the easiest of jobs.

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She will stand about five millimeters away from where she wants to drill which involves pulling herself upwards to an almost vertical stance and then placing her ovipositor directly on the drilling spot so it can go in to the wood straight down.  This can often go wrong and she often has to start from scratch.  However, she persists as the next generation is depending on the success of this mission.

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She is built for this, however.  Her legs and abdomen are stretched to the limit and work as stabilizers for the ovipositor.  This is itself made up of three filaments, two of which are valvulae and come together to create a cylindrical sheath which supports the final filament which is the tube through which the eggs will journey to their ligneous destination.

As she drills in to the wood it is thought that a fluid is secreted from a pouch in to her ovipositor.  The fluid contains enzymes that digest wood and so makes it soft enough for her to drill downwards.  This impressive feat has given rise to the wasp's most popular nick-name: the stump stabber.

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All this takes time.  Depositing a single egg can take over half an hour and during this time the female is exposed not only to potential predators but to other females covetous of her spot on the log.  This particular distraction can lead to the female defending herself with her antennae and, in doing so, damaging her ovipositor.

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When the process is successful, however, the egg finds its home. That would be right on top of a larva of the pigeon horntail.  The egg hatches and the Giant Ichneumon larva begins to ever so slowly devour its host.  It is a feast which lasts the entire winter.  When spring comes the larva will pupate and emerge as an adult from the drill hole from June up to the beginning of September.

Although the males do not have an ovipositor, naturally, they do have one cunning trick up their sleeves. They gather around the logs or trees where the females are about to emerge, searching with their antennae. Once they find a hole they will wait until a female comes out and will then mate.  Often they will go one step further.  They will insert their abdomen in to the hole, and will not remove it until the female has mated with them.

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The whole process takes a year.  Yet once out of the log which has been its home for so long the female has less than a month before she must oviposit. She will die soon afterwards, worn out from her exertions.

The Giant Ichneumon Wasp has attracted a great deal of attention and not just for its gruesome tactics to ensure continuity of the species or indeed the length of its ovipositor.  As many say, it’s not the length that’s so important it’s what you do with it.  Scientists at Imperial College in London, taking the way that the ovipositor can twist and turn as their inspiration are currently researching the concept of steerable needles by biomimicking the wasp’s ovipositor.

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It’s strange to think that this organ which has such devastating consequences for the host it seeks may indirectly help to save many human lives.

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