Wednesday, June 11, 2014

A Dragonfly is "Born"

The word "born" is being used here with poetic licence, as dragonflies are not born in any literal sense of the word. In the course of their life cycle they go through incomplete metamorphosis, and don't be mislead by that word "incomplete". The metamorphosis is incomplete only insofar as odonates don't pass through a pupal stage, but otherwise the change is very complete indeed, and when one compares the naiad to the adult dragonfly it's hard to believe they are formed from the same DNA.

This is a Dragonhunter, Hagenius brevistylus, and with a length of 75 mm to 90 mm it's one of our larger dragonflies. As its name suggests in addition to large butterflies and moths Dragonhunters prey on other dragonflies. A picture is worth a thousand words, here's the story of a dragonfly's journey to adulthood, with the first signs of emergence occuring shortly before 11:40 AM ...

11:40 AM

11:42 AM

11:43 AM

11:45 AM

Time out for a brief interlude here – I've always wondered what the white "strings" (visible near the thorax) on the exuviae might be. I did read on a blog or website somewhere or other that they are involved with the changeover of the insect's breathing system, but couldn't understand what or how switching from internal gills to pairs of spiracles on the abdomen had to do with these string-like structures. So I posed the question in an Internet group, and here's the response ...

"The internal tracheal system is 'closed', which means that the spiracles (openings to the outside) are sealed in the nymph, but must obviously open at the time of the adult moult. The tracheal system extends throughout the body and in the adult opens to to outside through spiracles in the thorax and the abdomen. In the adult the thoracic trachea are very well developed because of the very high oxygen requirements of the flight musculature and hence the evidence of them in the exuviae is particularly obvious. How or if the naiads shed the lining of these tubes in their aquatic moults is a mystery to me. Perhaps the spiracles open at that time to allow moulting.

The internal tracheae serve as the transportation system throughout the body for the oxygen picked up from the water through the thin cuticle of the gill. (Yes, insect rectums are also lined with cuticle.) I don't know how much of a role in oxygen transport the blood plays, but in most larger insects other than bloodworms it is fairly minor. In damselfly naiads you can see the branches of the tracheal system throughout the external gills, hence the term 'tracheal gills'."

And there we have it. The naiad not only morphs from a wingless underwater crawler to one of the most superlative fliers in the natural world, and somehow that amazing prementum which shoots the jaw out to grab passing prey turns into the much different (but just as deadly, from an insect's point of view) jaw structure of an adult, but the insect's entire breathing system changes over as well. In a case like this maybe a picture isn't worth a thousand words after all, because it can only convey a pale shadow of what's really going on here ... and so much for the "brief interlude", now back to the show ... 11:47 AM ...

11:52 AM

11:58 AM ... the teneral dragonfly is free of the exuviae.

12:01 PM

12:11 PM

12:22 PM

12:48 PM

12:54 PM ... the wings are fully deployed. We left at this point and returned at 1:16 PM, by which time the teneral dragonfly had already taken its maiden flight into the shoreline shrubs. The time from when the naiad's exoskeleton first split to flight – about an hour and fifteen minutes.

The exuviae was collected for future reference, in fact it was the discovery of a Dragonhunter exuviae about two or three days old that suggested it might be a good idea to keep an eye out for molting naiads. Whilst this one was about half way through its growing pains another naiad crawled ashore to make its own transformation.

The flattened, dead-leaf-like Dragonhunter naiads and exuviae are unmistakeable, they are a good healthy 40 mm long and have the enlarged third anntennal segments characteristic of clubtail naiads (the two rounded nubs at the front of the head).

So folks – tune in and watch for the next exciting installment of this adventure, it's been ongoing for nearly 300 million years, and still happening at a wetland or river near you ...