Journal Number 98
February 2006

THE COLUMN

Flies' Eggs in Nematoceras triloba agg
By Eric Scanlen

Tricia Aspin rang on 20 July 05 saying that they were out in
numbers, at Craig's place, so the Column got over to Pollok smartly.
"They", were Nematoceras "tricraig", the fourth form of N. triloba spotted by Tricia on the Awhitu Peninsula.

You may have heard about the other three, N. "pygmy" form 1,
N "trijuly" and N "tridodd" [J89:23]. But this extensive N "tricraig" colony is in scrubby bush where cattle have access, about 20m horizontally and 30m up from Tricia's original N. "Pollok" colony.

Last year the one remaining withered N. "tricraig" on 19 July 04, was disappointing in the pix so they had their portraits made this year, inside and out (photo right).

Weeks later on 15 Aug 05, there were more and slightly bigger flowers on intermingled colonies. Why so late in 2005? or perhaps early in 2004?

This taxon is the same as the Aug/Sep. flowering N. "tribrive" [J89:23] from the Bridal Veil Falls,
Te Mata, only different. It makes one wonder just how many forms of N. triloba there are. N "tricraig"
is possibly an hybrid N. "tribrive" x N "tridodd" going by the slightly different forms flowering nearly
a month apart in adjacent colonies but its earlier start to flowering than either putative parent, means more study is needed to sort it out.

Anyway, the Column sectioned one flower along its midribs
on 20 July 05, for internal photography. What wasn't seen at
the time but what showed up in the slides, were 3 minuscule
flies eggs tucked neatly into that typical labellum pocket, of
N triloba and N macrantha, at the base of the cleft. Why would any fly do such a thing? There is nothing there for emerging larvae to eat.

Slides of another N. triloba agg. from Moki Road, Uruti from
18 Sep 93, also showed flies eggs, 3 in the side of the labellum,
two in the cleft, and how many in the cleft pocket out of sight underneath?

The late Tony Bishop wrote in his 1996 Field guide to orchids of NSW, p166: "Corybas flowers may mimic small toad-stools."

Aha! He said that fungus gnat pollinators may be attracted by the orchid's fungus scent which
could explain a lot.

You may have seen how fungus gnat larvae ravage field mushrooms so it seemed possible that
the gnats treat these mimics as fungi due not only to fungus scent but also the typically dome
shaped dorsal sepal on our N. triloba agg. The handy central cleft in N triloba agg. could mimic
the space between mushroom gills (albeit upside down), placed so that pollination occurs at the
same time as egg laying?

It is typical of many sneaky orchids to dupe their pollinators without providing any reward to the
long suffering bugs so this could well be another example. If emerging larvae perished because
of the fungus absence, it would be no concern of the orchid, provided it were pollinated but the
crawlers might drop out and feed on the orchid's mycorrhizal fungus.

George Fuller's fungus gnat (Mycetophila deusa Tonnoir) J52:20-21 shows pollinia stuck to its
thorax after visiting Nematoceras iridescens at Pukekura Park. This orchid has a cleft labellum
but no pocket therein and bears no resemblance to a toadstool.

All the gnats that George observed were female; intent on egg laying? To get pollinia on its thorax,
as photographed, she would have had to bend double in the base of the flower; just to lay her eggs?

The mind boggles. But it could also explain why stigma and pollinia in Nematoceras are so close
together. The fly wouldn't need to double up much more to pollinate the next flower she visited
with the pollinia load on her thorax.

Ian St George's gnat, in J79:4, seems to have squeezed herself bodily into the cleft, head down by
the pollinia, got stuck in tiny N. "craigielea" [J82:16 Fig. 4] before laying any eggs and perished in
the attempt. The rounded, toadstool-like tops to these orchids in manuka debris, [J82:9] could
fool more intelligent beings than this pin-brained fly.

In July 1961 Gloria Scanlen photographed a fly that the
Column thought to be a fungus gnat. It had claimed a
Corybas cheesemanii [J59:12] on our kitchen table in
the Hunua Ranges.

Neither the 90 joule flash nor the people leering at it could shoo it away. What was the attraction of this flower with no cleft or pocket? Was the fly biding her time for some peace and quiet to lay eggs here? It had no ovipositor like Ian St George's gnat so the Column sent an enlargement to Ian Townsend for identification.

Ian consulted fellow entomologist Ian Andrew and they agreed that our 1961 fly was not a fungus
gnat but an "out-house-fly" (Sylvicola neozelandica) which feeds on dung, would you believe?
Could it be that our sole NZ Corybas smells like dung to this fly? Perhaps Tony Bishop's Aussie
Corybas species smell like fungus and ours has adapted to another smell to attract another fly?
The Aussies would appreciate that "Ours smell like mushrooms, yours smell like dung."

Who has ever detected that smell in Corybas cheesemanii or incidentally,
in Cyrtostylis oblonga?

S. neozelandica was also spotted in August 1998 by Geoff Stacey, [J71:24] wrecking and efficiently pollinating C. oblonga at Wharekawa. John Early at the Auckland Museum identified it from Geoff's specimen, but possibly didn't like to disclose its dung propensities. How many other NZ orchids are pollinated by these "out-house flies"?

Gnats have been caught several other times in or about NZ orchids. [i.e. J59:14; J91:12] The eggs in N. "tricraig" are about the right size for a tiny gnat, (be they fungus or dung species) at 0.9mm long and 0.3mm diameter, measured from the slides. Ian Townsend and Ian Andrews noticed that Fig. B & C eggs were different shapes so would be from different species of fly.

Can anyone throw further light on flies' eggs in Nematoceras triloba? One can easily check
N. triloba clefts by stretching oneself out in the wet moss, compost mud or stream-bed with
a x20 lens. It's not necessary to zap open the flowers, is it? If you do spot eggs, the thing to do
is to take them home and, when they hatch, give them a choice of dung or mushrooms to feast
upon and see what species of fly emerges, according to Ian Townsend.

The Editor would be ever so pleased to hear more reports of flies' eggs or larvae etc. in Corybas
alliance or any other orchids for that matter and especially if flies are seen stuck, coming or going
from N. triloba or etc. with pollinia stuck on wherever part of their anatomy.