After the orchids, no flowers show greater executive ability, none have adopted more ingenious methods of compelling insects to work for them than the milkweeds.Wonderfully have they perfected their mechanism in every part until no member of the family even attempts to fertilize itself; hence their triumphal, vigorous march around the earth, the tribe numbering over nineteen hundred species located chiefly in those tropical and warm, temperate regions that teem with insect life.
Commonest of all with us is this rank weed, which possesses the dignity of a rubber plant.Much more attractive to human eyes, at least, than the dull, pale, brownish-pink umbels of flowers are its exquisite silky seed-tufts.But not so with insects.Knowing that the slightly fragrant blossoms are rich in nectar, bees, wasps, flies, beetles, and butterflies come to feast.Now, the visitor finding his alighting place slippery, his feet claw about in all directions to secure a hold, just as it was planned they should for in his struggles some of his feet must get caught in the fine little clefts at the base of the flower.His efforts to extricate his foot only draw it into a slot at the end of which lies a little dark-brown body.In a newly opened flower five of these little bodies may be seen between the horns of the crown, at equal distances around it.This tiny brown excrescence is hard and horny, with a notch in its face.It is continuous with and forms the end of the slot in which the visitor's foot is caught.
Into this he must draw his foot or claw, and finding it rather tightly held, must give a vigorous jerk to get it free.Attached to either side of the little horny piece is a flattened yellow pollen-mass, and so away he flies with a pair of these pollinia, that look like tiny saddle-bags, dangling from his feet.One might think that such rough handling as many insects must submit to from flowers would discourage them from making any more visits; but the desire for food is a mighty passion.While the insect is flying off to another blossom, the stalk to which the saddlebags are attached twists until it brings them together, that, when his feet get caught in other slots, they may be in the position to get broken off in his struggles for freedom precisely where they will fertilize the stigmatic chambers.Now the visitor flies away with the stalks alone sticking to his claws.
Bumblebees and hive-bees have been caught with a dozen pollen-masses dangling from a single foot.Outrageous imposition!
Does this wonderful mechanism always work to perfection? Alas!
no.It is a common thing to find dead hive-bees and flies hanging from the flowers.While still struggling to escape, the unhappy victims will be attacked by ants, beetles, and spiders, or killed by heavy showers.Larger and stronger insects than honeybees are required to regularly effect pollination and free themselves, especially when they are so unfortunate as to catch several feet in the grooves.Doubtless it is the bumblebee that can transfer pollen with impunity; but very many other insects, not perfectly adapted to the flowers, occasionally benefit them.Among the large butterflies the Papilios, which suck with their wings in motion, are the most useful, because in using their legs to offset the motion of their wings they rapidly repeat those movements which are necessary to draw the pollinia from the anther cells and insert them in the stigmatic chambers of other flowers."Large butterflies like Danais," says Professor Robertson, "hold their wings still in sucking, spending more time on an umbel, but generally carrying pollinia.Small butterflies are worse than useless.They remain long on the umbels sucking, but resting their feet superficially on the flowers.
Since several moths were found entrapped, pollination must often be brought about by night-flying Lepidoptera.As a rule, Diptera (flies) either do not transfer pollinia at all, or become hopelessly entangled when they do."Occasionally pollen-masses are found on the tongues of insects, especially on those of bees and wasps, which move about with their unruly member sticking out.Probably no one has ever made the exhaustive and absorbingly interesting study of the milkweeds that Professor Robertson has.
Better than any written description of the milkweed blossom's mechanism is a simple experiment.If you have neither time nor patience to sit in the hot sun, magnifying glass in hand, and watch for an unwary insect to get caught, take an ordinary housefly, and hold it by the wings so that it may claw at one of the newly opened flowers from which no pollinia have been removed.It tries frantically to hold on, and with a little direction it may be led to catch its claws in the slots of the flower.Now pull it gently away, and you will find a pair of saddlebags slung over his foot by a slender curved stalk.If you are rarely skilful, you may induce your fly to withdraw the pollinia from all five slots on as many of his feet.And they are not to be thrown or scraped off, let the fly try as hard as he pleases.You may now invite the fly to take a walk on another flower in which he will probably leave one or more pollinia in its stigmatic cavities.