Nick Baker’s Hidden Britain
Reveals a fascinating world of wildlife that we often overlook.
In the depths with dragonfly nymphs
You’re likely to encounter dragonflies and damselflies anywhere there is fresh water – and often some distance away, too. For a couple of heady months, members of this gauzywinged, solar-powered supergroup, the Odonata, are among the most prominent and exciting insects on the wing.
The talents of adult dragonflies and damselflies are often talked about. They boast incredible vital statistics: the most lenses (up to 30,000 per eye) of any insect, and probably the fastest level flight too (some species reach over 48kph). Yet most of their life is spent in the watery realm.
Now is the ideal time to go on a baby dragon hunt – all you need is a pond net and a tray or tank. But while their parents are masters of the air, these aquatic mechanical monsters are the lords below the surface. They lurk at the bottom, unnoticed by all but curious pond-dippers.
Both dragonfly and damselfly larvae, known as nymphs, are as predatory as their parents. Large eyes, elongated bodies and, in the case of damselfly nymphs, three leaf-like ‘tails’ (external gills) make them easy to identify. If you keep one in a tank with a few other pond creatures and keep a close eye on it, you’ll see a special form of skulduggery. To see this you need to pay close attention as it’s over fast.
Within 10–30 milliseconds, the front of the nymph’s head appears to fly off to grab its prey, before retracting to dismember the victim. This ‘mask’ structure is difficult to see in living animals, but look at reeds for empty nymphal cases and you will see it more clearly.
If the exuviae is soft, wet and not too crispy (you may want to soak it in warm water), you can pull out with a pin the weird elbow underneath the head to reveal a swing arm tipped with a couple of hinged, stiletto-like spikes. This device is unique in the insect world.
Another peculiarity of dragonfly nymphs is that you’ll notice they don’t have three tails like damselflies. That’s because theirs are located within their bottom. Not only do they breathe through their anus, but it is also used to good effect in other ways. Thanks to muscular control of a diaphragm and an anal valve, the larvae can draw water in through their rectal cavity – useful if you want to ‘breathe’ actively and not de epend on passive absorption of ox xygen over a gill membrane.
I’ve always been fascinated by y the power of dragonfly n nymphs’ jet bottoms. I used to lo ove watching a nymph squirt its way w around in a tank, and even di irect a jet of water into my eye w when plucked from the surface. B Best of both ends It t may seem that these two sk kills are unrelated, given they in nvolve different ends of the bo ody, but it turns out that one of th he reasons a dragonfly nymph ca an ping its mask forwards at su uch an impressive speed is by si imultaneously clenching its anal va alve, thereby stopping water be eing exhaled, and contracting it ts abdominal diaphragm. This ca auses an increase in fluid pr ressure in the nymph’s body ca avity, to around 0.87 psi.
This pressurised blood, or r haemolymph, has to find so omewhere else to travel. The in ncrease in hydrostatic pressure fo orces blood into the labial mask, th hrusting it forwards. There is al lso a trigger-like mechanism – a ‘bio-spring’, that stores muscle en nergy and holds the mask in pl lace, and this is also released. T The resultant force is enough to o almost guarantee dinner.