The extended spoon-shaped labium of a libellulid dragonfly nymph.
One of my favorite examples of weird insect anatomy are the strange mouthparts of the aquatic nymphs of dragonflies and damselflies. A close look at the faces of these creatures reveals large hinged structure folded back under the head. In the case of the nymphs of Libellulidae, this structure actually curves up and wraps around the face, very much like a Darth Vader style mask. What are these structures?
Ventral side of libellulid dragonfly nymph, showing the spoon-shaped labial mask.
All Odonata larvae have a prehensile labium, sometimes called a labial mask, which folds under the head and thorax. This lower lip is capable of extending rapidly forward, striking prey before they can react. Hooks on the ends snag the prey and draw it back to the mandibles. All in all, the feature is reminiscent of a frog tongue snatching flies.
Ventral side of aeshnid dragonfly nymph, showing the flat, blunt labial mask.
An aeshnid dragonfly nymph preys on a libellulid dragonfly nymph.
Last week I posted pictures of some dragonfly nymphs, and Jim Johnson of Northwest Dragonflier was kind enough to give me some ID tips in the comments as well as pointing out I had two different types of dragonfly larvae. Taking another look at the nymphs, the differences were not only really obvious but really interesting. So I took a bunch more pictures and made two more posts.
The two larval types I was observing belong to the dragonfly families Libellulidae (skimmers) and Aeshnidae (darners). As can be seen below, libellulid larvae have short, stout bodies with widespread legs that lend them a toad-like appearance. Aeshnid larvae have elongate, torpedo-like bodies and move about much more actively. These two body types are indicators of the two families’ very different lifestyles under the water.
Early instars of libellulid and aeshnid dragonfly nymphs (sprawlers and climbers).
Libellulid larvae are “sprawlers.” These larvae are fairly inactive, instead lurking under mud and debris at the bottom, ready to snatch potential prey that get too close. Special hairs on their body trap debris, further camouflaging them and giving them a fuzzy, almost moldy appearance. In contrast, aeshnid larvae are sleek and aggressive little predators. These “climber” larvae move rapidly through the water, actually jet-propelling their streamlined bodies by taking water in through their mouth and expelling it out their anus. Within half an hour of my moving a few nymphs into a smaller container to observe, the aeshnid pictured here had made a meal of both his libellulid tank-mates. Climber type larvae are more visually dependent than sprawlers, and their eyes tend to be larger and more developed. They also climb vegetation, moving into different water levels rather than resting on the bottom.
A few more categories exist–”clingers,” with adaptations to allow them to remain stationary in flowing water; and “burrowers,” which, as the name suggests, burrow into the mud or dirt. Aquatic insect larvae that can move freely about through the water are called “swimmers.”
An aeshnid dragonfly nymph shown on thumb for scale.
Final instar nymph of an aeshnid dragonfly (College Station, TX).
Two years running our first planned field outing for the spring has coincided with one of Texas’s rare dips into below freezing weather. While fishing around in lakes is not the ideal pasttime for this kind of temperature, it is a decent way to get hold of some insects even in cold weather. One group of insects that’s a relatively common find are the aquatic immatures of dragonflies and damselflies. The dragonfly nymphs in particular are striking little creatures, with the earlier instars resembling squat little brown frogs. Like their adult incarnation, they’re also impressive predators, even tackling small fish.
Early and late instar dragonfly nymphs (Libellulidae, Aeshnidae).
Dragonflies lack a pupal stage, so the developing wings (wing pads or wing buds) can be seen in the late immature instars. I hadn’t caught a final instar dragonfly nymph before, so I was suitably excited to see one up close. The larger fellow above even obliged us by molting into a pretty blue darner during the middle of a lecture on dragonfly metamorphosis to a group of introductory entomology students.
*edit* Thanks to Jim Johnson for family corrections and additional information about nymphal characters.
Close up of the eyes of a libelullid dragonfly (College Station, TX).
This is a libellulid dragonfly, or common skimmer. The family name Libellulidae probably comes from the latin word ‘libella’ meaning ‘booklet’ (interestingly, the spanish word for dragonfly is libélula). This is in reference to the wings: paleopteran insects (dragonflies, damselflies and mayflies) have wings which are controlled strictly by direct flight muscles and held folded up over their backs. Neopteran insects are able to twist their wings and fold them flat along their backs. This development allowed insects to branch into many new habitats that would not have been conducive to managing large delicate wings. Think of beetles which burrow into bark or cockroaches which scurry under doors. Today, 99% of all insect species are neopteran.