Читать книгу The Disappearance of Butterflies - Josef H. Reichholf - Страница 19

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 being dragged down into the deep. When the sperm has been delivered, the male releases the ‘clamp’ with which it gripped the tip of the female’s abdomen. She, in turn, crawls and paddles down and looks around, ‘flying’ and ‘paddling’ until she finds a water plant that is suitable for egg deposition. I have found the caterpillars of this aquatic moth on curled pondweed, Potamogeton crispus, water-milfoil, Myriophyllum sp., and, above all, on Canadian waterweed, Elodea canadensis, which in the 1960s and 1970s was still relatively common in lakes and larger lagoons among the reservoirs along the Lower River Inn.

Having said this, those females with wings adapted to paddling are rare in southern Germany, in contrast to, for example, Denmark, southern Scandinavia and Britain. In central Europe, the females usually develop with normal wings. They are significantly larger than the males – and this is essential. This is because it is only the females, travelling on the wing, even if they are borne and blown along by the air currents rather than by actually flying, that are in a position to find bodies of water in which there are perfectly suitable stocks of underwater plants. The pools found near rivers and reservoirs have existed for too short a time to be considered permanent. However, the much more constant shallow lakes around the Baltic Sea have also only been there since the last ice age, that is, for around 10,000 years. If this aquatic moth had produced only females with rudimentary wings that were unable to fly, this species would surely not have survived in the long term. The males need their flight capability in any event, in order to search for females.

At this point I would like to mention a genetic peculiarity. With butterflies and moths, the female sex is genetically marked XY and the male XX; that is, the exact opposite from us. For this reason, it is much easier for females to develop two different forms than it is for males. This characteristic manifests itself with particular clarity in forms of ‘mimicry’, that is, through the imitation of poisonous or foultasting examples by non-poisonous imitators that are not protected by unpleasant flavours. In short: in moth and butterfly circles, it is worthwhile for the females to be more highly diversified. We can see this clearly with the female of the brimstone butterfly, which resembles the poisonous cabbage whites. The females, with the precious cargo of eggs in their bodies, have good reason to keep themselves concealed. Generally speaking, we therefore see considerably more male butterflies and moths in nature than females.

But let us return to the species composition of the aquatic moths. With their ecological placement on the banks of waterbodies and their lives on and in small bodies of water, they not only provide a prime example of how the species are distributed across their specific habitats (each in their own ‘niche’), but also illustrate why the evolution of all the related adaptations in physique and lifestyle are so rewarding. The water edges constitute an environment rich in plants and luxuriant with plant stock, less affected by the vicissitudes of the weather. Plentiful nutrition is always attractive; plants that are not protected by special toxic substances all the more so. Vegetation right at the water’s edge and, above all, under water, is particularly attractive, not only due to its utility but also because it is situated in a place that the main enemy, the parasitic insects, have difficulties in reaching. The life and survival of any type of moth or butterfly almost always depends on the success of its caterpillars. If the caterpillars do not find sufficient food plants, then that species will not do well. If there is enough food, but the caterpillars feeding on it are heavily parasitized, then the species will not become (more) abundant. One such case is found among those butterflies with caterpillars that eat nettles. Since nettle plants are in plentiful supply, these particular butterflies should also be extraordinarily numerous. They are indeed numerous, but not exceptionally so, and their populations fluctuate from year to year. More of this in a separate chapter that will provide an insight into the nature of fluctuations. With regard to the aquatic moths, there is yet another question: what do they teach us about this general trend, the disappearance of butterflies?