Friday, 19 July 2013

Looking at the skins of dragons

One of my tasks over the coming months is to go through the many hundreds of damselfly cases that have been collected as part of our reed warbler study. The warblers feed their young on a range of insects, damselflies included, and we need to secure a measure of how the availability of these changes throughout the reed warbler breeding season. The cases are collected from a standardised area of reed bed on a weekly basis, a task that can take several hours of careful searching.

As you probably already know, damselflies spend a greater part of their lives in water as larvae, emerging only when they are ready to transform into an adult. At our site, with its extensive fringing reedbeds, much of this emergence takes place on the reed stems themselves, the larval damselfly hauling itself out of the water ahead of the transformation.

The delicate case left behind after emergence is known as an exuvia (plural exuviae) and this forms a perfect replica of the larva, still showing a series of features that can aid identification. Thanks to the incredible efforts of Steve Cham, we now have two guides dedicated to the identification of dragonfly and damselfly larvae and exuviae, Simple to use and full of illustrations the guides are going to be well-thumbed this year as I work my way through the hundreds of sample tubes packed with specimens.

These are not be the only samples to be tackled this season, as we also have measures of the availability of other insects, notably small flies, collected via a series of pan trans. These traps, which take the form of yellow bowls placed on posts and which are filled with water, sample flying insects using the reed beds. Not only do we plan to examine the seasonal availability of potential reed warbler prey but we also want to look at how it varies across the site. This should enable us to relate characteristics of individual nesting attempts to prey availability and to establish whether some parts of the site are better for breeding reed warblers than others. The answer to that question is some way off, however, not least because we have all these specimens to go through first.

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