Thursday 10 May 2012

Ruby, Ruby, Ruby, Ruby

Eh? What's this? His Tenseness is a Kaiser Chiefs fan?

Not as such, though in our family, their 2007 hit is very much pertinent at this time of year.

Large Red Damselflies have started to emerge from the Tense Towers pond, each leaving behind a tell-tale empty larval cases, or exuvia, prompting many happy shouts of "Zuvi, zuvi, zuvi, zuvi!" at their discovery.

The fact that the Large Red Damselfly is... er, red in colour, does make the choice of lyric all the more apt. And, as I have just discovered, 'Zuvi' is a girl's name derived from the Kashmiri word which means 'life'. The emergence of the adult damselfly from the exuvia constitutes a life change for the creature. It has recently undergone a metamorphosis, triggered by an increase in day length and Spring temperature. One of the most profound changes that occurs at this time is the switch from gill-breathing to air-breathing. The larva's gills are in fact the caudal lamellae, the three leaf-like appendages at the tip of the abdomen. During the last few days of metamorphosis, the larva has to breathe directly from the air, so will migrate to the water margin near a place where emergence can take place. Here, the spiracles on its thorax, which have been sealed during its underwater life so far, are now opened enabling it to breathe air directly (my thanks to the Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland by Brooks and Lewington for these details). Once this has occurred, the insect has effectively crossed the Rubicon (or Rubycon!), there is no return to underwater life and it must now carry on with emergence into a winged insect.

Fully grown larva in the final stages of metamorphosis on a Marsh Marigold leaf
Exuvia of a Large Red Damselfly on a dead Water Mint stem
The white thread poking out from the larval case is the remains of one of the tubes linking the spiracles of the larva to the outside world. These are left behind as the adult body emerges.

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