Journal Number 102
February 2007

EDITORIAL

Pollination Mechanisms for Nematoceras?
By Ian St George

GM Thomson on Nematoceras macranthum in 1878

In 1878 the Otago schoolteacher GM Thomson read his now famous paper "On the means
of fertilisation among some New Zealand orchids" to the members of the Otago Institute [1].
I will quote in full what he said of Corysanthes macrantha:

"Both this species and C. rivularis (he would be referring to the N. aff. iridescens found around Dunedin) were examined by me, but the flowers are almost identical in structure, the difference not affecting the relations of the parts. They are very striking in appearance, owing to their lurid purple colour, and the long twisted sepals and petals, which give them an extraordinary resemblance to a large spider sitting on a leaf. The upper sepal is large, prominent, and helmet-shaped, and projects forward over the flower.

The labellum is large and involute, almost semi-cylindrical, with its external margin fimbriated and expanded downwards into a longish tip. It is not attached continuously at its base.

On each side of the flower, when in bud, a small slit is seen, which widens by an expansion of the margin (which is thus caused to arch slightly outwards) into a small circular aperture. By the contact of the in-turned edges of the labellum, and the overlapping of the upper sepal, a horizontal aperture is left in the mouth of the flower, which bends at right-angles a little way in, and opens into a tolerably large cavity, Placed quite at the bottom of this is the short, thick column, lying almost horizontally in C. rivularis, and somewhat more erect in C. macrantha.

The stigmatic cavity is deep, and on its posterior margin is the rostellum. This is formed of large cells, covered with a very delicate membrane. If this be touched with a bristle, it is almost instantly ruptured, and a small, very viscid drop of matter exudes. In withdrawing the bristle the pollinia are brought away with it. The anther is terminal (posterior), and has broad lateral projections. The pollinia are four in number, in two pairs, and in the form of plates.

The flowers do not appear to secrete any nectar, but when the surface of the labellum is slightly punctured, a considerable amount of sweetish purple juice exudes, which is probably grateful to insects.

From the shape of the flowers, it is necessary to cut them longitudinally to see the parts. Looking at the position of the anther and stigma, it appears to me almost impossible that self-fertilisation can take place; at the same time it is somewhat difficult to suggest any satisfactory way in which an insect could accomplish either this or cross-fertilisation.

I presume that any insect entering the flower would have to back out again by the same way as it entered, and in doing so it would come in contact with the rostellum, and would remove the pollinia on its head. It is also probable that, in endeavouring to obtain from a second flower any of the sweet juices from the tissue at the base of the labellum, it would slightly advance its head, so as to bring the pollinia attached to it on to the stigma.

Again, it is possible that self-fertilisation might be secured by an insect thus getting the pollinia on its head, and then endeavouring to push its way down through the small lateral apertures. In doing so, it would almost certainly smear the stigma with pollen from the same flower, and I have sometimes been inclined to think that such did take place. At the same time, this would seem like putting an unnecessary difficulty in the way of what is usually a very simple process, and therefore no great value is to be attached to this idea.

"For a time I could not understand why spiders frequented these flowers so much, but I soon found a sufficient cause. The only insects capable of removing pollen which were found about the flowers were small Diptera- probably a species of Culex. In several cases these small flies had penetrated into the tube of the flower, and, in their eagerness after the sweet juices found there, brought their heads in contact with both rostellum and stigma, and partly owing to the viscidity of those parts, and partly to the narrowness and bending of the tube, were unable to withdraw backwards. In some flowers insects were thus found still alive, in others they were dead, while in many others only portions of them, such as legs, wings, etc., were left, the spiders having devoured the rest. In every case in which a captured insect was withdrawn from its trap, the pollinia wore removed also, securely attached to the front of the head."

"I closely examined 148 flowers, and found that in 47 the pollinia were still in the anther cells; from 90 they had been removed, while in 6, dead or living insects were found glued to the stigma. Of the whole number examined, only a small proportion ultimately produced capsules."

"The flowers of this genus will well repay examination."
 

Pterostylis, Nematoceras and fungus gnats

I wrote [J100 p7, in the context of a discussion of nectaries], "What about Nematoceras? There is a protruberance at the base of the column of Nematoceras species. The pollinators are fungus gnats; some adult fungus gnats drink nectar. The position of dead bodies of fungus gnats found in N. 'Craigielea' and N. iridescens suggest that the protruberance is
what they seek".

In the same issue was a report of Carlos Lehnebach's work [2], including observations of fungus gnats probably pollinating Pterostylis alobula.

He wrote "… these orchids (Pterostylis) may be 'window flowers' with the clear crystalline panels in the hoods concentrating the light on the inside of the flower thus attracting the fungus gnats" (Jones, 1981 [3]).

The presence of colourless translucent areas in the perianth such as those of P. alobula and
P. patens has been reported in other trap flowers (Dafni 1984 [4]; Vogel & Martens 2000 [5]).

These authors explained that since flies are positively phototropic (attracted to light) once inside the floral trap, they will try to escape through the trap's entrance before reaching the reproductive organs. The light that comes through these 'window-panes', usually located at the bottom of the trap and close to the reproductive organs, will lure the insects deep into the trap assuring insect visitation to the reproductive structures."

Pterostylis nutans - dubbed "crystal dome" by George Fuller,
who grew it from an Australian plant.
An insect-pollinated window flower.
 

Only male fungus gnats visited the flowers of P. alobula, and Carlos Lehnebach postulated sexual attraction by fragrance mimicking pheromones, then trapping by the irritable labellum, movement towards the base of the "window-flower", then escape via stigma, rostellum and pollinia, depositing and carrying pollen.

Can we understand insect pollination in Nematoceras similarly? Perhaps so, but we need some important knowledge gaps to be filled.

I suggest this: some Nematoceras flowers exude fragrance attractive to fungus gnats (can we determine the sex of gnats found inside Nematoceras flowers?

George Fuller [6] (see p.10) found female gnats from at least three different species visiting Nematoceras iridescens, and Eric Scanlen photographed gnats' eggs in N. trilobum [J98: 34].but is this more generally true? And do the gnats approach from downwind?

The long filiform tepals act as "pollen guides" as the gnat draws closer.

The light (or ultraviolet) reflected from the galea [7] provides a ring target for the gnat, which enters the narrow flower cavity. Once there it may be rewarded by nectar from the protruberance in front of the stigma, or it may simply be attracted to the light entering via the auricles. It may (if it is very small indeed) exit through those auricles, or it may exit by backing out the way it came in. In doing so it dislodges and carries pollen on its thorax, ready for the next flower it visits. Fungus gnats are attracted to nectar and to light, and Nematoceras appears also to be a "window flower" with its auricles.

New Zealand orchids appear not to have formed the specialised orchid-pollinator partnerships common among Australian orchid species. What seems likely here is that a number of fungus gnats are capable of pollinating a number of orchids - Pterostylis as well as Nematoceras.

Bruce Irwin's longitudinal sections seemed at first a strangely complex, even contrived, way of distinguishing among different members of the N. rivularis group, but they clearly differentiated a range of new taxa, and possibly define the entry route of the gnat, and thus the pollination system of the different species.
 

Is N. papa a self pollinated species?

George Fuller wrote that no pollinators visited N. papa during his observations: N. papa may be self pollinated-or it may spread only vegetatively (has anyone seen fruit on it?). In the Fernery at Pukekura gardens in New Plymouth it grows alongside N. iridescens, they flower together, and they do not hybridise.

What advantages might self pollination confer on N. papa? A guaranteed fertility so that it flourishes in its little ecological niche, and doesn't need to await the arrival of an appropriate gnat. Non-competitiveness with neighbouring Nematoceras. But if its ecological requirements are too narrowly circumscribed - and it does have a very restricted geographical range - it may be in trouble in an extreme season, or with climate change.

Mind you, a degree of rarity and a nice neat stable structure that humans can easily recognise may, in this new conservation age, confer a special human-mediated advantage to enhance its chances of survival too.

Is there any structural clue to self pollination in Nematoceras? N. papa is a shy, small-flowered, largely green and odourless orchid, its proximity to the big, purple, fragrant, flagrant N. iridescens at Pukekura emphasising the contrast between the two. There are others in both the N. rivulare and the N. trilobum alliances that would appear on a superficial assessment to be self pollinators too.

Do the insect pollinated taxa have a selfing fallback position? To do that the flower would have to tip forward, making the column vertical, allowing pollen to fall onto the stigma; I don't think I have observed that.

Hybrids (see p.12)

We are aware of a number of likely naturally-occurring hybrids between pairs of Nematoceras species [8], and that would be expected if the insect pollinator were not specialised to a single Nematoceras species. The same is probably true of gnat-pollinated Pterostylis species, and indeed some colonies of Pterostylis have all the appearances of hybrid swarms.

Perhaps a single species of fungus gnat may visit several species of Nematoceras, and perhaps different fungus gnats may pollinate the same Nematoceras. In other words perhaps neither the gnats nor the orchids are selective, so hybridization is possible when habitat, flying and flowering times all allow the gnats and the orchids to be there at the same time.

Could that be the situation with (for example) the Nematoceras trilobum complex? Of the Nematoceras species that have chromosome counts, all are diploid (2n = 36), except for the N. trilobum complex that has diploid (2n = 36) and tetraploid (4n = 72) representatives (M.I. Dawson & B.P.J. Molloy, pers. comm.). Or is that 72 an allopolyploid whose number has doubled? Nematoceras hybrids therefore cannot be detected as easily by chromosome studies, as they can in Thelymitra.

Today's Nematoceras taxa may be hybrids that are successful and stable in today's
environmental conditions-but will they be the same ones that dominate in new conditions?
or will other hybrids find those conditions more to their liking?

Hybridisation, especially the wide range of hybrids possible in a hybrid swarm, is an evolutionary means of improving the survival of at least some taxa, and thus of the gene pool.
 

Fungus gnats

The fungus gnats seem to be important insects for New Zealand orchidophiles.
What do we know of them?

These are Mycetophilidae - NZ has about 600 species but there are approximately 3000 described species in 150 genera worldwide, although the true number of species is undoubtedly much higher. Some, like the NZ glow-worm, are bioluminescent.

Adult fungus gnats are about 2.5 mm long, grayish to black, slender, mosquito-like, and delicate with long legs, antennae and one pair of wings. Identification can be made by the vein patterns in the wings.

Eggs are hardly visible, oval, smooth, shiny white and semitransparent (do you recollect Eric Scanlen's photographs of flies' eggs on Nematoceras [J98 p34]? They must be the eggs of fungus gnats). Larvae or maggots are legless, thread-like, white, shiny blackheaded, up to 5.5 mm long and transparent so food in the gut can be seen through the body wall. Pupae occur in silk-like cocoons in the soil.

Fungus gnats reproduce in moist, shaded areas in decaying organic matter such as leaf litter. A great starting point for Nematoceras. The life cycle is about four weeks, with continuous reproduction in homes or greenhouses where warm temperatures are maintained. Broods overlap, with all life stages present during the breeding season. Larvae not only feed on fungi and decaying organic matter, but on living plant tissue, particularly root hairs and small feeder roots. Brown scars may appear on the chewed roots. The underground parts of the stem may be injured and root hairs eaten off. Damage occurs most often in greenhouses or plant beds.

Adults live about 7 to 10 days and deposit eggs on the moist soil surface or in soil cracks. Females lay up to 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage is about 5 to 6 days.

How much more do we know now?

So, where have we got to since George Thomson in 1878? Not much further, truth to tell.
What he deduced from a study of structure alone, we are now beginning to confirm from observations in the field, most notably those of George Fuller (see next page).

But "the flowers of this genus will repay (a lot more) examination (yet)."