Sprawlers and Climbers – Lifestyles of dragonfly nymphs

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.

Water to Air – Baby Dragonflies

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.

(Extremely) Flat Bug

Adult and nymph flat bugs on bark underside (Aradidae).

One interestingly little bug family that I have occasionally stumbled across while peeling back bark is the flat bugs, family Aradidae.  As their name suggests, these bugs are notable for their extremely flattened body morphology, an adaptation to their lifestyle under bark and in crevices of wood.  These guys are hard to spot, being cryptic both in body and lifestyle, but, as an example of morphology pushed to the limits, they are impressive to observe.

A flat bug shown on a finger for scale (Aradidae).

Because these bugs are of little economic importance (they are rarely agricultural pests) little research has been done on them.  Nonetheless this is a diverse group which is found worldwide.  Most flat bugs feed on fungi in decaying wood, and some are attracted to the pheromones of bark beetles, which may help them to locate food sources.  These bugs tend to be found in gregarious groupings.  Probably the best gathering of information that I have found on aradids is Steve Taylor’s info page, complete with some great pictures.

Side view of the aptly named flat bug (Aradidae).

Be My Valentine: Male and Female Garden Spiders

Sexually dimorphic garden spiders oppose each other on a web (Araneidae: Argiope). (Photo by Jessica Hyde)

For Valentine’s Day we have garden spiders.  These are the big spiders you find with the zig-zag patterns in their webs.  It turns out their mating habits are even more entertaining than I had thought (journal articles are fun!).

The much smaller males build mini-webs in at the edges of the females’ webs, often complete with their own tiny zig-zag.  Then the male commences a careful and prolonged courtship, plucking and vibrating the strands of the female’s web to play her a love song.  He’s trying desperately to convince her to mate with him before she decides to eat him.  In this case, his small size is an advantage.  His lady love may ignore him because he’s of little nutritional value.

Like other spiders, the male uses his pedipalps to transfer sperm.  This is the part where a previously quiescent female may turn vicious–a quarter of males are killed during the first insertion attempt.  The little male is hoping to manage one insertion with each pedipalp, as surviving through two insertions uneaten will increase the number of eggs he fertilizes.

After that, his lover’s appetite will become a moot point:  during the second insertion the male will spontaneously suffer a fatal seizure.   Although this could be interpreted as a romantic sacrificial gesture (box of chocolates, anyone?) it is more likely he is using his body as a plug, to try to block the access of other males.

Be mine, Valentine?

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Thanks to Jessica, who took the picture for me after I ran around the ranch house squeaking about dimorphism and catching grasshoppers to toss to the spiders.  Thanks also to the spiders, who caught me a gorgeous buprestid specimen which I stole and unwrapped.

(Elgar 1991; Foellmer 2004; Hickey & Lee 2004)

Old Wings; New Wings

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.

Blood Suckers – Hog lice and dead pigs

Hog lice displayed on a finger (Haematopinus suis).

I have the oddest experiences as an entomology graduate student.  Last week I found myself tagging along with a friend from the FLIES lab* to examine the decaying hog carcasses they have laid out in a field.  They do this to study the arthropod communities involved in the degradation–maggots, carrion beetles and more. In this instance it wasn’t as bad as it might have been–it’s been relatively cool out for one thing.  On top of that, nearby fire ant colonies had evidently been scouring eggs and larvae from the corpses, something which I learned can virtually halt decay in some circumstances, since the fly communities can’t establish the maggot masses that take a pig apart.

*Forensic Laboratory for Investigative Entomological Sciences (and, yes, I imagine they spent a while on that one)

Close up of two hog lice (Haematopinus suis). (Photo by Collin McMichael)

Donning gloves, I was quickly picking through dead pig fur for lice and other entomological treasures.  It was a novel experience.  So here they are, sucking lice in the louse suborder Anoplura, which I am going to presume are hog lice (Haematopinus suis).  Close up pictures of lice make it evident why an unpleasant human manifestation of this pest is sometimes called ‘crabs.’   Their tarsal claws, specially modified to oppose a tibial spur for the gripping of hairs or feathers, resemble tiny lobster claws.  Their dorsoventrally flattened bodies are secondarily wingless, all of which enables them to cling close and tight to their host’s skin while they take a blood meal.

The bottom picture was taken by my labmate Collin McMichael on a digital scope.  If you want a glimpse into the inside of his head (it’s a scary but entertaining place) you can wander over to Tumble-Bumble where he gathers up bizarre odds and ends from around the internet, pretty much at random.