Come Eat My Pollen, Breed in My Chamber, Says the Arum to the Fly

Arrow Arum

Peltandra virginica

Araceae

Calendar events prevented John and George’s usual Friday field trip this week, although my son Evan accompanied me to a swampy visit yesterday.  In the Realm of Squishy Shoes abide members of the Aroid Family, the Araceae.    Anyone who has ever attended a wedding reception knows this plant family from the centerpieces with Anthuriums and Calla Lilies.  Gardeners know their Araceae, including Aglaonemas, Alocasias,  Amorphophallus, Caladiums, Colocasias, Dumbcanes,  Monsteras, Philodendrons, Pothos, and more.  Although these plants are mostly toxic, hungry readers may think of Ceriman, Dasheen, Malanga Root, and Taro.  Wildflower buffs favor Golden Clubs and Jack-in-the-Pulpit.  You may be saying by now, oh yes, the plants with a spike inflorescence (the spadix) that looks like a possum tail, associated with a specialized leaf (the spathe).  Or maybe you did not say that.  The spadix is covered with tiny flowers, usually female (fruit-producing) flowers toward the base and male (pollen-producing) flowers toward the top above the female flowers.  Those bumps on the spadix are individual flowers.

Jack is the spadix. The pulpit is the spathe. All photos today by John Bradford.

Arrow-Arum is a wetland resident around here, and is widespread from Hudson Bay to Tijuana.  Despite almost certain poisonous contents, Native Americans probably used parts of the plant as a starchy staple in some places.  The leaves look like arrowheads, and the flower-bearing units in the shade of the leaves are the standard Aroid spathe-spadix combo.  The Arrow Arum spathe wraps tightly around the spadix.  The female flowers on the base of the spadix are wrapped a deep dark chamber formed by the base of the spathe.  The male flowers on the more-exposed upper portion of the spadix open well after the female flowers.  Remember that, it’s important.  The spadix is functionally female and pollen-receptive before it becomes male and pollen-producing.

The spathe, with spadix enclosed, is the pointy object right of center.

And here is why:  the reproductive cycle of the Arum, progressing from the floral bud phase through the female flower phase and on to the male flower phase, sets the behavioral agenda for the Arum’s chief pollinator.  The remarkable dance between Arrow Arum and its pollinator fly Elachiptera formosa came to light in the 1990s thanks to botanist Joseph Patt and collaborators.

(For a photo of a  little Arrow Arum fly see http://bugguide.net/node/view/878136/bgpage.  Note that the fly in that photo is labeled Elachiptera costata.  I do not know if that identification reflects an error for E. formosa, or if  the two fly names are synonyms (unlikely), or if more than one species of Elachiptera visits.)

[Added post-posting.  John caught the fly in the act.  See his Trail to the River.  Nice Bobcat there too.]

To continue the story:  As the spathe and spadix mature, two main fragrances come forth.  Call them Fragrance A and Fragrance B.  Fragrance A smells like a sweet floral perfume, whereas Fragrance B is musky.  During the Arum’s floral bud stage, A and B come forth roughly equally mixed.  As maturation progresses to the floral female phase, the Fragrance A/B ratio shifts in favor of B.  Then later as the floral male phase arrives, fragrance production becomes almost 100% Fragrance B.  In short, as the spathe and spadix mature the fragrance balance shifts from about 50-50 A/B to almost all Fragrance B.

Spathe and spadix. Male region of spadix exposed. Female region at the base of the spadix, the spathe wrapped around.

The fly responds to these perfume ratios as follows:

When Fragrances A and B are roughly equal (floral bud stage), female flies enter the hollow spathe basal chamber and lay eggs among the immature female flower buds.

Subsequently, as the fragrance balance shifts to more B than A (as the female flowers mature), the flies continue to visit but lay fewer eggs.  Fly visitation at this female-flower stage is critical.  The receptive female flowers require pollen brushing off of the fly visitors, even if the insects quit laying eggs.

And later still, as the fragrance balance tips to almost 100% Fragrance B (as the male flowers open to release pollen),  the flies continue coming, but something has changed…they are not laying eggs or dropping off pollen; now they feed.   They eat some of the newly produced pollen, then fly away carrying uneaten pollen on their bodies to a different Arum to drop off pollen, completing their service to our plant.

The beauty of it all is that the Arum orchestrates the fly’s life cycle to serve its own reproductive cycle.  And the fly does okay too: getting a brood chamber as well as a cafeteria.

What becomes of the fly larvae hatching from those eggs we left among the female flowers?  The maggots mature as the spathe and spadix mature.  The larvae presumably hatch in the decaying remains of the spadix.  They probably depart their childhood Arum nursery with a load of pollen stuck to their bodies in service to their botanical masters cradle to grave.