Wednesday 30 July 2014

Two bugs, predation and evolution

I grew up by the sea, so it is not surprising that I developed a fascination for marine shores and the plants and animals that live there. 1 I loved looking in rock pools at low tide and seeing some creatures dart away, while others clung tightly; and lifting drying seaweed to see what sheltered under the wet fronds beneath. As a teenager, there were plenty of other things to occupy my mind, but I retained my love of Natural History and always enjoyed walking through the Devonshire countryside and around the coast. It was while at University that I discovered the flora and fauna of fresh waters and I then went on to have a research career looking at stream and river ecosystems, while retaining a fascination for all aspects of Aquatic Science. It fuelled my teaching and I hope that my curiosity rubbed off on some of my students, as it is enriching. Underlying my interest has always been a child-like approach - “Look at that!”, “Why is that?” etc. - and I’m pleased that I have failed to grow up in that regard.

While collecting damselfly larvae from a weedy lake for a University vacation project, I first came across two creatures that so interested me that I took them home and kept them in large jars. One was a solitary water stick insect, Ranatra linearis (below left, total overall length 7 cm) and then several specimens of the water scorpion, Nepa cinerea (below right, total overall length 3 cm). Although it is similar in appearance to a stick insect, Ranatra is not related to members of this group, but has evolved the same mechanism to avoid detection by resembling a twig. Nepa looks a little like a scorpion, at least at the front end, but it is not related to the scorpions and is very close, in evolutionary terms, to Ranatra, both being members of the Nepidae group of bugs. Nepa is dark brown in colour and resembles the decomposing leaves amongst which it lives so it, too, is camouflaged.

As they are closely-related, one would expect many similarities in the structure, and behaviour, of these insects. Both are true bugs and thus have incomplete metamorphosis, with larval stages that are similar in form to the adults and that undergo a series of moults, and thus increase in size, until the fully-grown insects emerge. During the larval stages, wing buds are present and the adults have wings, although they do not fly well - or often. All aquatic insects have terrestrial ancestors but these bugs spend almost their entire life in water, retaining a system of tubes within the body for air breathing. Whereas some aquatic insects have developed gills containing branched air tubes, both Ranatra and Nepa have two long extensions at the hind end of the body and these are closely applied and form the equivalent of a snorkel that is held at the surface when the bugs need to replenish air supplies.

The majority of insects have six walking legs, allowing them to move over the substratum by having three points of contact at all times, each tripod providing stability. Ranatra and Nepa have six legs, but the first pair is adapted for catching and holding prey, the other four legs being used to crawl slowly through vegetation. Their camouflaged appearance gives the bugs some protection against attack and, together with their lack of movement, makes them good “wait and see” predators. Any suitable prey that comes within range is grabbed by the fore-limbs and, in Nepa, these have become modified so that the lower part folds into a notch in the upper part, much as a blade folds into a penknife (see the video clip below). Any organism that is caught cannot escape and the bug uses its rostrum of piercing mouthparts to both inject salivary secretions and then suck up the body fluids and digested contents of their prey. It was this predation that fascinated me and the bugs that I kept in jars fed on a range of animals, including water fleas and insect larvae of several types. Especially impressive was the capture by Ranatra and Nepa of tadpoles and I was fascinated by their feeding. There’s something about watching predation that captures our attention, perhaps because it seems a little shocking, as we are witnessing "nature in the raw". I saw both bugs complete several meals and was impressed by how little effort appeared to be required from initial capture to the time the remains of the prey were discarded.


Unless one is a Creationist, Ranatra and Nepa provide excellent examples of evolution. They share the same method of catching prey, using modified fore-limbs, feed using a rostrum, they have invaded fresh waters form a previous terrestrial existence, and both have a breathing tube to enable them to remain submerged. It is likely that these were all features of a common, now extinct, ancestor and the two forms then diverged, with the evolution of different body forms that enable them to live in slightly different parts of habitats. Thinking about their structures, and how they evolved, is as fascinating as watching the bugs “in action” and I’m pleased that I’m not a Creationist. Of course, we can all share a sense of wonder in watching Ranatra and Nepa, but to think of evolution, and all the changes that have occurred through time is amazing, as we have no way of conceptualising a million years, let alone the many millions of years over which changes in body form and behaviour occurred. Natural History is not a religion, but it is certainly rewarding to all of us lucky enough to observe all the extraordinary adaptations that evolution has provided.


[Please play without sound - you know my views on this. 1 The red spots on the legs are water mites.]

1 Roger S. Wotton (2012) Walking with Gosse: Natural History, Creation and Religious Conflicts. Southampton, Clio Publishing.

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