Summer World_ A Season of Bounty - Bernd Heinrich [27]
Each larva hatches into a white grub in exactly the spot where the egg was deposited. Bald-faced hornets chew their prey into semiliquid gobs and carry these home to the nest to feed to their white grubs, who are neatly stacked in a nursery where the temperature is controlled to maintain their rapid growth. The nests trap heat, and the adults stay warm inside and remain ready for immediate, fast takeoff, even when the outside temperatures may be close to the freezing point. Throughout this time they indiscriminately defend themselves against potential predators, generally those who would make the contents of their nests—the fat juicy grubs and the pupae that they turn into—a feeding bonanza. After achieving full growth in its respective crib, each larva spins a fine white cocoon around itself with silk from its salivary glands. This silk then seals the larva off in a coffinlike cocoon at the same spot where it started as an egg and then molted into a pupa before molting once more into an adult.
The queen’s first successive batches of larvae need to grow fast and become a crowd of workers and nest defenders. She needs these sterile daughters, who are single-minded in their tasks and undistracted by sex, to help raise males and new queens in the fall at the end of the colony cycle. Keeping her brood warm accomplishes fast growth, and she encloses her brood comb with more and larger envelopes of insulating paper. She builds the paper layers from the top down, by attaching them to the petiole at the tops of the cells and extending it at the bottom edge until only a small entrance hole is left at the bottom. After she has enclosed her brood in her upside-down “bell” of paper, she also adds a long tubular extension, at the entrance hole, like that on a weaverbird nest. This construction reduces convective air movement through the entrance. Now, whenever she is inside the nest with her eggs, grubs, and pupae, she shivers and produces heat. The warm air is trapped in the bell chamber. She thus creates and maintains her own very local tropical “summer” climate despite what the weather may bring.
To the queen hornet, time and temperature are inextricably linked. She is programmed to behave in ways that shorten the development time of her offspring, so that she can raise as many as several hundred of them in the one summer allotted for her life. Whenever the nest cools, the eggs stop developing and the young stop growing. She needs to keep her baby factory going, to raise lots of workers during the summer. Her first batches of workers are slaves that promote her objective of raising many reproducing offspring later. These sterile workers cooperate because their evolutionary “objective” is to help to raise brothers and sisters who will insert the same genes they have into the next generation. Meanwhile, the colony as a whole is in competition with other colonies for resources to fuel its economy.
By middle to late October the shortening photoperiod has activated the enzymes that have transformed the leaves’ physiology and caused them to be shed. The first heavy frosts are expected. On mornings when there is no wind, the remaining leaves on the tree come rustling down as the sun comes up, illuminates them, and melts the last thing that then holds them on the tree—a bit of ice. A week later all the leaves are down, and now many bird nests and wasp nests that were hidden earlier are suddenly revealed. There is little or no more insect prey and the wasps’ colony cycle is completed. It is now safe to examine hornet nests to learn much about what they have accomplished during the summer.
The hornet nests are now the size and shape of a basketball, and they are sturdy structures that have withstood summer rains and produced generations of workers, then a brood of drones, and finally a brood of queens. I pulled one down (on 26 October 2003)