Читать книгу The Disappearance of Butterflies - Josef H. Reichholf - Страница 15

Let us start with a successful search flight in early summer. It is the males who jiggle around in the evening twilight. They are searching for females, newly emerged ones that are still unmated. With a delightful little experiment, I was able to follow what was going on. Taking a female that had emerged in the aquarium at home, I put her into a mini cage of the type that beekeepers use to house queen bees for short periods. The attractant that the female produces escapes through the vented sides and is then dispersed by the air currents. Once the search flights of the males had started, I sent this virgin female off onto the open water of the pond on a polystyrene raft in the twilight. After just a few minutes, the erratic flights of the males became target-oriented trajectories. Dozens of males landed on the cage. With their abdomens stretched through the lattice of the cage, they attempted to reach the female. I had attached the little raft to a piece of string and could thus pull it towards me on the bank. The males followed as if drawn by a magnetic force They were not frightened away when I pulled out the stopper and released the female onto the floating leaves of the water knotweed or amphibious bistort, Polygonum amphibium. Barely was she freed, when one of the males captured her for copulation and would not let her go. The others did not have a chance. They left the pair and continued with their search flights. Here and there I noticed one or other of them landing and pairing with another female sitting on a floating leaf. The slightly more sombre-coloured females were not at all easy to spot in the evening half-light, as they sat on the floating leaves.

The next morning, the newly mated female begins her own search, this time for somewhere to lay her eggs. She flies ever closer over the surface of the water until she finds floating leaves without the notched edges that indicate that there are already caterpillars feeding there. She considers different species of water plants that spread their leaves on the water surface. After landing, the female carefully probes the leaf with her legs. Floating broad-leaved pondweed, Potamogeton natans, water knotweed (amphibious bistort), Polygonum amphibium, yellow floating heart (or fringed water-lily), Nymphoides peltata, and the young, fine leaves of water-lilies, Nymphaea sp. The floating leaves of the yellow floating heart (fringed water-lily), with its striking yellow flowers, are particularly suited for the caterpillars to grow and thrive. But this member of the bogbean family, which has evolved into a water plant, is only very rarely found. The variety of water plants mentioned here illustrates that the brown china-mark is not adapted to specific forage plants. Their caterpillars can actually be successfully fed on lettuce, admittedly with effects that I had not anticipated and that became very instructive.

When the female has found a suitable leaf, she pushes herself backwards towards its edge and curves the tip of her abdomen over it in such a manner as to reach the underside of the floating leaf. This is not at all easy for her, since she must overcome the resistance of surface tension. Close to the edge of the leaf, the female then sticks a cluster of 100–180 eggs on its underside. These are moistened by the water. The small caterpillars develop quickly. Precisely how quickly depends on how warm the water is during the early summer period. As they hatch, the caterpillars bite through the eggshell and gnaw their way into the tissue of the leaf. If the leaf is thick, as with the water-lily, then they will simply carry on eating; they ‘mine’, to be precise. With thinner leaves, they soon cut out a little piece and cover themselves with it. The finest insoluble silk threads hold the leaf segment, initially only 2–3 millimetres long, in place.

In this state, and in the next stage following the first moult, the caterpillar is moistened by the water in its tiny leaf tube. It breathes through its skin. The breathing holes, the spiracles and the trachea already exist, but remain closed in the first two stages of the caterpillar’s life. Later on, air will be drawn through them into the body and the excreted carbon dioxide expelled. After the first moult, the caterpillar fashions for itself a proper leaf tube with base and lid. This also contains water and the caterpillar continues to breathe through its skin. However, this changes when it reaches the third larval stage. The caterpillar, having moulted the skin that had become too small, now shimmers as if made from silk. Water droplets run down its skin. It stretches its head above the surface of the water and is immediately enveloped in a gleaming silver layer of air. It fashions the new case of such a size that it can completely withdraw into it. It is filled with air. The body of the caterpillar will now remain hydrophobic until pupation. The spiracles are open. Now the exchange of respiratory gases takes place in the normal manner, but with a significant peculiarity: if the carbon dioxide level rises in the air bubble that surrounds the caterpillar, then a portion of it will automatically pass over into the water. This is because carbon dioxide ‘readily’ dissolves in water, as chemists will casually declare. The resulting lowered pressure is offset by the oxygen, which in turn penetrates the air bubble surrounding the caterpillar. The caterpillar thus breathes in part with something resembling a physical lung. This is not yet essential, since, from the third larval stage, the caterpillar of the brown china-mark eats the floating leaves from the top down. In doing so, it absorbs their waxes, which are responsible for ensuring that the top surface of the leaf does not become wet but instead floats on the water. Without this waxy surface, an ordinary rain shower would be enough to submerge the leaves.