The Disappearance of Butterflies. Josef H. Reichholf
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I assumed that all the caterpillars in their various stages that I had collected for my research would continue to develop without any problems, pupate and produce moths. The penny only dropped, as the saying goes, years later, when I had already become involved with a quite different type of species, the small ermine moth. There is a separate chapter devoted to them. Through them, the advantage of life in the water became suddenly apparent: I had had no losses, because the caterpillars and pupae of my aquatic moths had not been attacked by parasites. For practically all the butterflies and moths that live on dry land, parasites are among the main factors that determine their abundance and their development from one stage to the next. With around 96–98 per cent of 694 caterpillars from several breeding groups, the hatching success of my aquatic moths was phenomenally high. I only recorded higher losses for the eggs. I did not discover who or what caused the losses under outdoor conditions, but I considered the egg-eating water mites and the rotting sludge build-up in the heavily silted pools to be the likely causes. With 100 or more eggs per clutch and per female moth, such losses prevent the caterpillars from consuming all the available floating leaves too soon, which can easily happen where brown china-marks exist in large numbers.
This was why the gardeners in the Botanical Gardens in Munich placed their hopes in my research into the aquatic moths when I first encountered the little nymphs. Over the following years and decades, I definitively established that the female brown china-mark will leave the pool from which she emerged if the floating leaves of the water plants have been overconsumed. She will examine the edges of the floating leaves quite thoroughly before laying her eggs, and for good reason. If there is extensive feeding damage, she will leave and search for other waters with better conditions. A tendency to disperse would already be expected, since such small bodies of water are normally only temporary. Under natural conditions, they arise through inundation of the floodplains. New ponds will last a couple of years or a few decades, depending on how large or small they are when they form, and gradually disappear again through sedimentation and plant succession. Species that colonize an environment that is by its nature unstable must seek alternatives in good time.
The dispersal behaviour of the aquatic moths is therefore very particular. As insects, they probably belong to the group of pioneering species that is familiar to us through many land-based plants and that quickly colonizes newly created environments. On the other hand, maybe we are dealing with specialists that need a specific, longer-lasting ‘life zone’: that of floating leaves at the edges of large bodies of still water. Closer to the centre of the body of water there are plants that grow entirely under water, described by specialists as ‘submerged’. The moths seek out shore plants that stand in the water but protrude above it, away from the centre of the pool. These are plants that are ‘emerged’ (in the ecological sense). In order to understand my aquatic moth and to be able to place it properly among its relatives, I would need to deal with the environment of small waterbodies and shores in far greater detail. Was it a pioneer species or was it specially adapted to the specific environment of bodies of water?
A place to live or an ‘ecological niche’
The gravel pits combined all the conditions of the larger waterbodies in a small space. Accordingly, in most of them, I was also able to find the various other species of aquatic moth that exist in central Europe. They form an almost exemplary ‘ecological series’, feeding variously on the shore (or ‘emergent’) plants, through the floating leaves and all the way down to the submerged water plants. This sequence of adaptation is visible in the moths themselves. The caterpillars of the beautiful china-mark, Nymphula stagnata (nitidulata)* feed on bur reed, Sparganium sp. and other species of plant that project out of the water near the bank. Moving out into the water, to the zone where plants with floating leaves grow, is ‘my’ Nymphula nymphaeata. The series continues under water with the ringed china-mark, Paraponyx stratiotata, and the most unusual of all, Acentropus niveus. In side pools, provided they are covered with duckweed, you will also find the small china-mark, Cataclysta lemnata. All these species live next to one another, in the strictest sense of the word, in adjacent ecological niches. All have special adaptations that, in the extreme case of the water veneer, Acentropus niveus (Acentria ephemerella), with its two forms of female, have even led to a permanent life in the water. More about this shortly. First, the bigger picture must be understood. It shows the diversification of moths belonging to the small moth family, the Crambidae, known to be extraordinarily adaptable, from the bank right out into the water. The further they have advanced, the more abundant they are.
The abundance of a species is, in a general sense, an indication of its biological success. The caterpillars of Nymphula stagnata live to a greater or lesser degree on the banks, above the water surface. They are the rarest species in our series. Cataclysta lemnata, whose caterpillars use the small leaves of the duckweed plant to construct their cocoons and for nutrition, usually becomes considerably more common as one moves towards the centre of the body of water, but its occurrence is limited to small waterbodies that are carpeted in duckweed plants, which are also known as ‘water lentil’, or Lemna. The occurrence of my little nymph, Nymphula nymphaeata, is much more widespread and frequent. In order to build their leaf cases, its caterpillars cut out a pair of oval leaf sections, up to 3 centimetres in length. This can be readily seen from land.
In contrast, the ringed china-mark is much harder to detect. Its caterpillars spend their whole lives under water. They do not pass through an air-breathing stage. They do not have to, since they develop thread-like appendages on their bodies, through which the respiratory gas exchange takes place just like in the gills of fish. They are aptly called spiracular gills. Such gills are a very unusual adaptation for moths, although they are normal for the larvae of a very species-rich group of true water insects, the caddis flies. This raises a question that is eminently important for an understanding of the evolution of the Lepidoptera, namely whether spiracular gills are an invention by a genus of aquatic moths, or an ancient legacy linking moths with caddis flies. In other words: are moths and butterflies descended from formerly water-dwelling insects, or early forms of insect that were already established on dry land? There is much to indicate a closer relationship with caddis flies. In any event, they were all ‘born from the water’, just like my little Nymphula.
With a delicate rocking motion, the caterpillars of the ringed china-mark pump water through the loose cocoon, in which they sit under water, eating aquatic plants – in Germany, this is principally water-milfoil, Myriophyllum sp. To do this, they have adapted to survive in warm, oxygen-poor water. In the tropics, these aquatic moths, whose caterpillars develop spiracular gills, have a species-rich network of relationships. However, the non plus ultra of our aquatic moths spends its life as a caterpillar in the depths, among the massive stands of underwater plants that can grow as far as the water surface to flower. It is the tiny water veneer, Acentropus niveus (Acentria ephemerella), which the lepidopterists of the nineteenth century did not even recognize as a moth, taking it instead for an unusual species of caddis fly.
The caterpillars of the water veneer moth are comparatively normal. Their bodies are wettable and they breathe through their skin. Small as they are, they do not require a more efficient gas-exchange mechanism. Skin respiration is quite adequate for them, even in the cool and oxygen-rich shallow lakes in which they are principally found. They pupate under water. Yet what emerges from some of the pupae seems barely credible: females whose wings have been shortened to form pointed paddles. With these, they ‘fly’ around under water. Not fast, but fast enough to avoid simply being pushed to the surface. Their hind legs are fringed with a thick row of bristles and they can use these to steer. Stub wings and rudder legs enable these females to achieve goal-oriented movement under water. The reason for this is evident soon after they have emerged from the pupa. They struggle upwards to the water surface, remaining below it, and thrust only the tip of their abdomen out of the water. Glands on that tip will emit a scent that attracts the male with its normal wings.
The males will swoop around just above the water surface as if entranced until they encounter the abdomen tip of a female that is ready to mate. In the course of the coupling, they are almost pulled into the water by the larger female, but their wings prevent them from