“We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions; thus in the larva of the dragonfly… the alimentary canal respires, digests and excretes.”
– Charles Darwin, Origin of the Species, Chapter 6
Dragonflies are wonderful, which is why I have spent the last seven years studying them. I have posted about a range of species before. One of the many reasons that they are fascinating lies in the internal organs of the larvae. Here they are in all their glory (I couldn’t find good anatomical drawings online, so I made my own, and be sure to watch the amazing video at the end):
Dragonfly larvae can grown to 3 or 4 inches in length, making them significant predators in aquatic environments. This is especially true when fish cannot live there, such as in ponds that regularly dry-up. This is an example of a larva that I pulled out of a pond in Ottawa during a biodiversity survey.
Function 1: Digestion – Simply, food is consumed and passes through the intestines where nutrients are absorbed. Waste is then “egested” through the anus. We say that waste is egested because it never really enters the body – the intestines themselves are a tube that runs from mouth to anus. Urine, on the other hand, is “excreted” because it is composed of water and waste products that move into the body across the intestines and then out of the body in the kidneys (in humans, of course).
Function 2: Respiration – Really this is ventilation, where through expanding and contracting the walls of the abdomen, water is drawn over internal gills. The video that I have embedded below clearly shows this “breathing” in the first few seconds.
Function 3: Prey capture – By closing a sphincter at the entrance to the intestines and contracting the walls of the abdomen, the resulting hydraulic pressure thrusts the feeding “labium” towards prey. The intestines and the feeding apparatus are all connected hydraulically, and huge force can be produced to create a rapid predatory strike. Again, watch the video below for a large number of (unsuccessful and successful) strikes.
Function 4: Locomotion – By reversing the process (closing an anterior (or “head end”) sphincter and contracting the abdomen), water can be fired out of the anus at up to 50cm/s, propelling the animal forwards. This can be used to evade predators. However, because the hydraulic system is involved in both prey strikes and jet propulsion, the two functions cannot be used together – one end is closed to provide pressure for the other.
And here’s all of that wonderful adaptation in action. Note the structure of the labium, also known as the “feeding basket”: the structure is hinged to form a grasping structure that can be thrust a considerable distance from the body. You can also clearly see the hairs inside the basket that point inwards to prevent prey from escaping: