Nectar is an exudate from plant tissues. It is produced in floral structures called nectaries. The
nectary is simply an epidermal area composed of many permanently open stomata, which "bleed"
a sugary liquid. The nectar may accumulate in the base of the flower, or perhaps even in a long pouch called a nectar spur.

Nectar is produced by the flowers in order to attract pollinating animals. Nectaries are generally at the base of the perianth, so that pollinators are made to brush the flower's reproductive structures while accessing the nectar.

Nectar that is produced outside the flower is generally produced to attract predatory insects.
They will eat both the nectar and any plant eating insects around, thus functioning as "bodyguards". Some carnivorous plants will use nectar to lure prey insects into the trap organs
of the plant. Nectar is economically important as it is the sugar source for honey.

The main ingredient is sugar - glucose, fructose, saccharose - but there are also amino acids, lipids, organic acids, minerals, vitamins, enzymes, antioxidants, ions and metabolites. The European orchid Epipactis helleborine appears to inebriate its insect visitors, and its nectar contains narcotic/hallucinogenic compounds, as well as ethyl alcohol, thought to be from yeast fermentation of the sugars [1].  

Orchid Nectar

A comprehensive survey from 117 orchid species in the literature and from field studies of fruit set, showed that nectariferous orchids are more successful in setting fruit than are nectarless species. Overall fruit set figures for nectarless and nectariferous orchids were 19.5 and 49.3% for North America, 27.7 and 63.1% for Europe, 41.4 and 74.4% for the temperate southern hemisphere, and 11.5 and 24.9% for the tropics, demonstrating that the dichotomy is consistent across all geographical areas. On average, the provision of nectar doubled the probability of fruit set in both temperate and tropical areas [2].

M.Stpiczyska, K.L.Davies and A.Gregg reviewed nectar secretion in Orchidaceae [3].

The morphology of orchid nectaries has been widely studied; van der Pijl and Dodson [4], and Dressler [5] believe the "lily-like ancestors of the orchids probably had shallow nectar glands between the perianth and the ovary".

In extant orchids, however, nectar is not produced in sepal glands but in a relatively shallow nectary on the lip or tepals or between the column and the lip (e.g. Bulbophyllum, Cirrhopetalum, Epipactis, Listera, Pleurothallis, Stelis), in glandular ringlike nectaries at the top of the receptacle or in spurs (e.g. Angraecum) or in tubular nectaries embedded in the ovaries (e.g. Brassavola, Rhyncholaelia).

Other orchids (e.g. Cymbidium, Grammatophyllum, Vanda) produce nectar at the base of the outer surface of the tepals, and the mentum may also function as a nectar spur (e.g. Scaphyglottis, Dendrobium). However, only rarely has the column ever been observed to secrete nectar (e.g. Stelis).

The orchid Maxillaria coccinea, whose flowers produce copious amounts of nectar, has (for instance) a "faucet and sink" arrangement: "the nectary is represented by a small protuberance on the ventral surface of the column and nectar collects in a semisaccate reservoir formed by the fusion of the labellum and the base of the column foot. The nectary comprises a single layered epidermis and three or four layers of small subepidermal cells. Beneath these occur several layers of larger parenchyma cells". The colour and shape of M. coccinea suggest it is ornithophilous (bird pollinated); a protuberance similar to its nectary is found in the entomophilous (insect pollinated) species M. parviflora.

The authors point out that some orchids (e.g. Oncidiinae) have a protuberance on the ventral surface of the column called a tabula infrastigmatica. It is not a nectary, but serves as an anchor for insects, who grasp the tabula with their mandibles, leaving their legs free to collect reward.

Some species conserve energy by resorbing their nectar when it is no longer needed. For instance pollination triggers nectar reabsorption in flowers of the epiphytic orchid Mystacidium venosum. The amount of sugar in the nectar of M. venosum decreased by more than 50 percent within 72 hours of pollination. Hand pollinated flowers from which nectar was previously removed set significantly smaller fruit with a lower percentage of viable seeds than hand pollinated flowers containing nectar, suggesting that resources reclaimed by nectar resorption are allocated to fruit production [6].  

Australasian Orchid Nectar

The major syndromes in Australian orchids are wasp pollination by pseudocopulation (15 species) and the bee pollination syndrome of food mimicry (5 species - here the orchid suggests that food, pollen or nectar, is available by appearance or fruity fragrance, but conserves its energy by failing to provide it).

Nectar and pollen reward systems do operate in a small number of species [7]. Australia's Dendrobium smilliae is a nectar producing, bird pollinated epiphyte, and New Zealand's Winika and Earina species produce nectar and are insect pollinated [8].

Kores [9] reported an analysis of Diurideae, and in it he wrote, "Prasophyllinae … appear to have … secretory tissue at the base of the labellum.

Australian species of Acianthus and Cyrtostylis are reported to secrete nectar from paired glands at the base of the labellum (Jones [10]). Secretory tissue associated with the base of the labellum has also been reported within New Caledonian Acianthus by Kores [11] and for species of Genoplesium and Prasophyllum by Jones et al. [12].

Nectar production in Microtis has been suggested by Peakall and Beattie [13]. In their study of pollination in Microtis parviflora, they noted that ants forage persistently, visiting individual flowers and inflorescences repeatedly for nectar. They also observed that ants visited only newly opened flowers and that pollinia attachment and pollen transfer to the stigma occurred while they probed the base of the labellum.

There are no reports of nectar production in Townsonia or Corybas, but the former is reported to be autogamic while the latter relies on pollination by deceit [12]. As a consequence, the secretory function of the labellum may have been secondarily lost in these genera".

The critically endangered Australian Prasophyllum robustum is fragrant, and its labellum "produces quantities of nectar on which a wide range of insects feed. Some of these, particularly native bees wasps and beetles, are effective pollinators" [14]. The strong fragrance, swarms of insects and often chewed appearance of P. hectori suggests some similarities (see photograph below).

The Australian Caladenia arenaria may secrete a floral scent suggesting it offers nectar; visiting insects pollinate the flower in their search for the nonexistent reward.

Weston and colleagues listed Sydney orchids and their pollination mechanisms: those that produce nectar are Acianthus caudatus, Calanthe triplicata, Genoplesium spp., Microtis spp., Prasophyllum and Spiranthes.  

New Zealand Orchid Nectar

Carlos Lehnebach's masters study [8] at Massey is the only one I know of that specifically mentions nectaries in NZ orchids (see p8).

From the Australian work mentioned above, the following NZ terrestrial orchids seem likely to produce nectar: Microtis spp., Prasophyllum spp., Genoplesium spp., Spiranthes, and Cyrtostylis spp.

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 (below) suggest that the protruberance (arrow) is what they seek: is it a tabula infrastigmatica or is it a nectary?

And what about those Corybas cheesemanii spurs? are they really spurs? do they contain nectar?
It seems unlikely in a self pollinating species.