Forest on the back

Some beetles (Coleoptera), chiefly weevils (Family Curculionidae), are known to carry literally a forest on their back, that is on the dorsal surfaces of the pronotum and the elytra. Such beetles are known from forests on high mountains of New Guinea. The forest on the insect back is made up of algae including blue-green algae, mosses, hepatica, fungi, lichens, and prothalli of small ferns; the forest is inhabited by minute animals, e.g. psocids, mites, nematodes and rotifers, and the forest denizens also include various Protozoa, diatoms, unicellular algae, and bacteria. Some Psocoptera feed occasionally on algae and fungi. It would be interesting to work out food chains among the components of the forest on the back. This discovery was made by J. L. Gressitt with his assistant J. Sedlacek (Gressitt et al, 1965). They named the phenomenon epizoic symbiosis. Such gardens are found only from 1500 to 3600 m, and are unknown on big weevils with smooth elytra on the lowlands, even if they belong to the same genera. See also Jolivet (1986).

Among the insects in the mountains of New Guinea why such tiny forests are found mainly on weevils? Probably it is because of relatively long life of weevils (5 years or more), rather long for an adult insect. Besides, the elytra and the pronotum of the weevils present on their surface engravings and pits, in which a mucoid epidermal secretion collects. Mites and spores, disseminated by wind, get caught in these tiny collections of mucus, and other organisms follow them.

Some interesting facts about longevity of adult insects. According to Grasse (1949), the queen of termites may live for 80 years or more. Some queens of Nasutitermes may be, he wrote, 100 years old. But among beetles great adult longevity has been noted only in state of diapause; a dessicated diapausing beetle may survive for 10 years or more. Among arthropods only tardigrades may last for a longer period in a state of suspended animation or diapause. Chrysolina and other chrysomelids in the Arctic Circle may live quite long (about 6 years), entering into diapause repeatedly both in the larval as well as in the adult stage.

Another question in this context: Why are cases of forests on insect back known only from forests on mountains of New Guinea? They have not been reported from tropical forests on mountains elsewhere. There are high mountains, covered with forests, in Malaysia, Africa, Borneo and tropical America. These mountainous forests have high humidity and permanent fog or mist. Some of them have permanent glaciers. Thus climatically they are very similar to those in New Guinea. Why then do they not have forest carrying insects? One answer to this question is that such cases may be there in these forests, and they await discovery. The insect fauna of these forests have not yet been well investigated. It is also very difficult to see the lichens on dry insects in collections. The organisms on the body of an insect make a greyish crust almost invisible, when the insect has been dried and stored in museum collections. In New Guinea, this flora can be found commonly on the trunks of surrounding trees. These cryptogamous gardens have been discovered only on big apterous weevils with fused elytra, belonging to the subfamilies Leptopii-nae, Brachyderinae, Cryptorhynchinae, Otiorrhynchinae and Baridinae and also on a species of Drytops (Colydiinae). Probably the gardens exist on the back of other beetles too, but only the Eastern side of New Guinea has been investigated by entomologists, and the higher summits are on the other side, that is on the Indonesian side or Irian Jaya, remaining to be explored. Gerson et al. (1977) mention also some other cases of simple epiphytic symbiosis on beetles and some other insects.

One of us (PJ), when in New Guinea, went to the top of Mount Kaindy, in Eastern New Guinea, near the tropical station of Wau, with J. L. Gressitt. The place harbours the tallest moss known. He collected some big weevils, a Gymnopholus, labelled on its back by Gressitt, as well as other weevils of the mountain, which after dissection showed a parasitic gregarine in its gut; this prorozoa is related to the one from a high central

African mountain. No forests are known, however, on the back of beetles in Africa. PJ there, at 3,000 m in the Ruwenzori, found only galerucines as big beetles. All others, above that altitude, among plants like Lobelia, Erica or Senecio, were tiny specimens. Some middle sized weevils in the Panamean central chains have green algae on their integument, as also fresh water turtles and other aquatic reptiles, but no one shows the mixed presence of a number of organisms, which may be referred to as a forest. Some curculionids in Mt. Humboldt, New-Caledonia, look like covered with a lichen and algal crust. As for some weevils, in the neotropics, this lichen cover is only made of scales and is a case of homochromy with the surroundings.

It may be pointed out here that molluscs and turtles are known to carry a complex of organisms on their shell. Marine Turbellarian, Convoluta roscoffensis, is associated with algae. The giant molluscs of the genus Tridacna have algae on their gills. The gastropod Elysia atroviridis uses free chloroplasts from marine algae. Whales and marine iguanas may be covered with barnacles and vegetation. Fishes have algae infested gills. Fresh water Hydra has zoochlorellae in its body. Marine anemones and some sponges are with algal association. But they are all aquatic animals. Instances of terrestrial animals, carrying and transporting various organisms, with the exception of parasites, cyanobacteria, algae or fungi, on their body, are very rare. One rare instance is that of the South American sloths living on trees. Their fur lodges blue-green algae and some fungi, and also some pyralid moths. The algae give these dark animals a greyish-greenish colour, and thus the advantage of camouflage through homo-chromy with their surroundings. The adult moths seem to feed on the algae. They lay eggs on the sloth excreta, from which their caterpillars seem to derive their nourishment. Among terrestrial animals with vegetation, we may mention the giant Galapagos tortoises which carry lichens on their shell. These lichens on the female tortoise eventually get worn because of the friction during copulation. In the same way old New Guinean weevils loose their scales when getting old and also some part of their flora, mostly lichens, by rubbing against each other or against the leaves. Another case of vegetation on a vertebrate is the growing of bryophytes on the head of a Mexican lizard (Gradstein and Equihua, 1995). A Chinese geometrid moth is also known to lodge algae on its body, also some grasshoppers and an Australian spider. In southern

Brazil, in the Cardoso island, in the rain forest, there were found cyanobacteria and two liverwort species (Hepaticae) growing epizoically on the dorsal scute of the harvestman, Neosadocus, near variabilis (Machado and Vital, 2001).

It is likely that many insects host microscopic flora/fauna on their body, but we have failed to detect them. Generally, insect specimens are studied in a dry preserved condition, and in this condition it is very difficult to detect and make out microscopic organisms, if they were there on their body.

Do the New Guinean weevils get any advantage from this forest on its back? Most probably the advantage is of camouflage through homo-chromy with the surrounding foliage and mosses. It has been noted by naturalists that the forest carrying weevils are nontoxic and quite edible to predators. The only defensive device with the insect seems to be the little forest carried on its back. The weevils show only small mobility. The relative sedentary habit also appears to be a part of their defensive strategy. The predators in those altitudes are birds of paradise, and several marsupials, like Antechinus, Petaurus and Eudromicia. Really, the weevils are

— Fig. 28.1. A-B: Two giant New-Guinean weevils carrying lichens, algae, and mosses over their elytra (after Jolivet, 1986). Wau, New Guinea.

difficult to see. PJ's students detected some during mountain climbing, which PJ himself did not even see.

In Ohio, in 1976, a neuropteran larva (Nodina pavida), which is a chrysopid, was discovered carrying an accumulation of cryptogamic vegetation somewhat comparable to that on the back of the New Guinean weevils (Skorepa and Sharp, 1971). It was composed of lichen soredia and thalli, pieces of bark, parts of moss gametophytes, pollen grains, fungus spores and other debris from plants and insects. This too is a case of symbiosis, because the lichens are dispersed by the insect. There is also a larva of a Colombian lacewing, a Chrysopa, covered by soredia of several lichens. Thus the forest carrying phenomenon does not seem confined to the New Guinean weevils. An analogous situation is in nymphs of some reduviid bugs and some beetle larvae carrying a trash on their body for purpose of camouflage. The trash consists of dead insects, cast skins, feces and other debris.

It appears that chill, constant high humidity in the environment, longevity of the insect and its relative immobility and big size are prerequisites for the insect to have a flora on its body. All the terrestrial cases of animals carrying flora are known to occur in moist situations.

A recent study on two algal symbionts, found in the sea anemone Anthopkura elegantissima (Lewis and Muller-Parker, 2004), suggests the directions in which the New Guinean weevil tiny forests may be investigated with likelihood of getting interesting and significant results.

References

Gerson, U. and Seaward, M. R. D. 1977. Lichen Ecology. Academic Press, London & New York: 71-119.

Gradstein, S. R. and Equiha, C. 1995. An epizoic bryophyte and algae growing on the lizard Corythophanes cristatus in Mexican rain forest. Biotropica 27: 265-268.

Grassé, P. P. 1949. Traité de Zoologie. Tome IX. Insectes. Masson & Cie. pubis.: 1118 pp.

Gressitt, J. L., Sedlacek, J. and Szent-Ivany, J. J. H. 1965. Flora and fauna on backs of large Papuan moss-forest weevils. Science 150 ( 3705): 1833-1835.

Jolivet, P. 1986. Insects and Plants. Parallel evolution and adptations. Flora and Fauna Handbook. Brill, New York: 197 pp.

Lewis, L.A. and Muller-Parker, G. 2004. Phylogenetic placement of

"Zoochlorellae" (Chlorophyta), algal symbiont of the temperate sea anemone Anthopleura elegantissima. Biol. Bull. 207: 87-92.

Machado, G. and Moreira Vital, D. 2001. On the occurrence of epizoic cyanobacteria and liverworts on a neotropical harvestman (Arachnida: Opiliones). Biotropica 33 (3): 535-538.

Skorepa, A. C. and Sharp, A. J. 1971. Lichens in "packets" of lacewing larvae (Chrysopidae). Bryologist 74: 363-364.

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