The mopane worm "masonja" is one of the most important insects in Africa from a cultural point of view. It also has great potential to teach learners about insects and their metamorphosis.

Mopane worms and silkworms have a remarkable life cycle. Mopane worms and other caterpillars start their lives as leaf-eating caterpillars or larvae. Later, the fully grown larva spins itself into a cocoon, inside which it goes into a state of rest while it changes into a chrysalis or pupa and eventually emerges as a moth. This change is an example of complete metamorphoses, or Holometabolous development, and happens in the life cycle of all butterflies, moths, bees, ants, beetles and their close relatives. Complete metamorphosis can be compared and contrasted with incomplete metamorphosis, or hemimetabolous development, which occurs in insects like termites, grasshoppers, crickets and the true bugs. In incomplete metamorphosis the juveniles look like small adults, but are different in that they are sexually immature and lack wings.

As the larvae grow, they moult 4 times (there are five larval stages) before they reach their maximum size (Oberprieler, 1995). It is at this time that they are most sought after as food, and are harvested in great numbers. This is also the stage where the most damage is done to future populations. Possibly the most destructive way of harvesting is to cut down large trees to obtain the caterpillars in the higher branches. However, it is essential to harvest responsibly to ensure a good crop for the following season. One way of limiting the harvest would be to leave all the larvae above a certain height for breeding stock. However, the height of mopane trees is not the same in all areas, so the management strategy would depend on the area concerned. Apart from humans, there are several other predators, such as baboons, birds and insect parasites that reduce the numbers of larvae. When the larvae have finished growing and storing energy reserves, the surviving larvae move down to the ground where they excavate a burrow in which they pupate. The pupa is a very important stage because it is in this stage that the Mopane Worm overwinters and spends 6 to 7 months. At the beginning of summer (November to December) the moths emerge. The moths do not feed and only live for two or three days. Their only function is to mate and produce eggs. The male moth uses his feathery antennae to detect the chemical signals or pheromones of the female so that he can find her. After the moths have mated, the female lays 50 to 200 eggs on leaves or branches of the trees. If there are many moths flying and the food plants are growing well, because there has been sufficient rain, it is a sure sign that there will be a good crop of worms. The eggs hatch about ten days later and the cycle starts again. In good seasons there may be two generations, with a second batch of moths emerging in February to March. Obviously there may be losses to the population in all stages, so to preserve these insects and to optimize the industry, it is necessary to study and manage all four stages of the life cycle..

The origin of metamorphosis.

One of the most intriguing questions in nature is: how, why, when and where did metamorphosis originate? Although these questions have existed at least since the time of Aristotle, there are still no generally accepted answers to them (Toms 1984, 1986). The interrelationships of the major groups arthropods remains one of the most contentious issues in systematics (Giribet et. al., 2001). One of the issues on which there is no consensus is the identity of the ancestor of all holometabolous insects. Although there is no general agreement, some fascinating ideas about insect metamorphosis have been published and more testing of these ideas is needed. Perhaps the first idea, proposed by Aristotle, was that the butterfly or moth represents the true insect and that the caterpillar or larva and chrysalis represent new stages (Toms, 1986). It is suggested that these were added on to the cycle when the embryo began a precocious existence by leaving the egg before it was fully developed. A comprehensive version of the precocious larval hypothesis would have to show how all holometabolous insects radiated from a single hemimetabolous ancestor. However, there is no evidence that this actually happened and no candidate hemimetabolous ancestor has been specified. If the ancestors of holometabolous insects really were terrestrial hemimetabolous insects it would be important to identify them. Presumably they would have to be relatively simple insects, so specialized insects like bugs and preying mantis could not be regarded as potential ancestors. In this hypothesis one might imagine that the most likely ancestor for the holometabolous insects would be a simple insect like a cockroach. However, this idea has not been fully explored and can not be accepted yet!

Another interpretation for the origin of metamorphosis originally proposed by the early French evolutionist Jean-Baptiste Lamanck suggests that insect metamorphosis is directly comparable with sexual maturation in other animals such as humans (Toms, 1986). In this theory the butterfly or moth is simply the adult or sexually mature stage in the life cycle. It has been suggested that the first winged insects were amphibiotic, resembling primitive mayflies and that metamorphosis originated because the different stages of the life cycle were living in different environments as is the case with living mayflies. It has been suggested that the larval stages of the first winged insects were aquatic while the adult stages were terrestrial and that adaptation to these different environments made metamorphosis inevitable. In this hypothesis the most likely ancestors for butterflies and moths are the caddisflies. Caddisflies are small moth like insects with aquatic larvae and already have complete metamorphosis. However, this hypothesis is also not generally accepted, and opponents of this idea have argued that the aquatic larvae of caddisflies are advanced and that in this way the terrestrial larvae of butterflies and moths are more primitive. It is interesting to note that different people have interpreted the available evidence in very different or opposite ways (Toms, 1984), and this matter has yet to be resolved. This means there are lots of opportunities to do interesting research and make new discoveries.

References

GIRIBET, G., EDGECOMBE, G.D. and WHEELER, W.C. (2001). Arthropod phylogeny based on eight molecular loci and morphology. Nature 413, 157-161.

OBERPRIELER, R.G. (1995). The Emperor Moths of Namibia. Sigma Press, Pretoria.

ROBERTS, C. (1998). Long-term costs of the mopane worm harvest. Oryx, 32(1), 6-8.

TOMS, R.B. 1984. Were the first insects terrestrial or aquatic? South African Journal of Science 80, 319_23.

TOMS, R.B. 1986. A synopsis of explanations for metamorphosis in insects and amphibians. South African Journal of Science 82, 372_375.

TOMS, R.B. 2001. The mopane worm and the wonder of metamorphosis. Easy Science 2001(4), 6-8.

Toms, R.B., Thagwana, M.P. and Lithole, K.D. 2003. The Mopane Worm -Indigenous Knowledge in the Classroom.


Toms, R.B. 2003. Worming your way to a sustainable harvest.

Toms, R. and Thagwana, M. 2005. On the trail of missing Mopane Worms. Science in Africa.