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"Another such organ was the thymus situated in the chest above the heart. Sometimes in adults this gland is a shrivelled remnant. Its function was unknown, so I was assured that this was yet another leaf not blown off the evolutionary tree. Probably the greatest advance in medicine in the last twenty years has been to understand the function of the thymus." When transplant surgery is carried out, the patient's body, knowing that it has received a foreign organ, mounts a massive defence reaction. But how does the body know that it is 'foreign' tissue? Research has "demonstrated that the thymus imprints this capacity to distinguish between self and non-self on the body's defence mechanism. It does so whilst the baby is in the womb, and in the days immediately after birth."
Prof. Vema Wright - 1988
Symbiosis is said to be the relationship between two different living species where each lives off the other to their mutual benefit. In some cases they are so closely linked that the one cannot survive without the other. The yucca plant is a desert lily which produces numerous creamy white flowers. The flowers are strongly scented and smell most pungent at night time.
The female yucca moth when visiting the flowers has a stereotyped pattern of behaviour. The descends into a flower where she scrapes together pollen from up to four stamens. The wad of pollen can be up to three times larger than the moth's head. She then flies off to another flower where she closely investigates the condition of the ovary, being able to tell if the flower is the right age and whether eggs have already been laid in it. If the flower is suitable she reverses towards the ovary where she drills a hole with her ovipositor. After this she climbs up to the stigmas, which are united to form a tube, and thrusts some of the pollen down into the tube.
The most unusual behaviour of the moth is to lay one egg in each of the three cells of the ovary and to carry out pollination after laying each egg. The remaining ovules, which are numerous, develop into seed, producing a rich supply of nourishment to the moth larvae as they grow. They will eat up to half of the two hundred seeds produced by the plant before emerging and lowering themselves to the ground by spinning a silk thread.
The mother moth never eats. She just lays her eggs, pollinates the yucca to make the seeds ripen, and then dies.
The adult moths always emerge in the flowering season of the yuccas in their area. "The emergence of any one season's brood is spread over a period of three years thus ensuring the continuance of the moth species even if, as occasionally happens, the yuccas fail to flower in a particular year."
The Pollination of Plants - Michael Proctor
"The yucca plant can be fertilized by no other insect and the moth can utilize no other plant." Encyclopaedia Britannica - Vol. 12 - p.869
Three Questions
1. How does the moth know that it must not lay its eggs in the same flower from which it has collected the pollen?
2. The three year brooding period is a marvellous safety mechanism ensuring further generations of both moth and flower. How did this come about?
3. Since the moth cannot exist without the plant and the plant cannot exist without the moth we are caught in a quandary, "Which came first?"
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