A very remarkable instance of insect help is found in the pollination of the yucca, a semitropical lily which lives in deserts (to be seen in most botanic gardens). In this flower the stigmatic surface is above the anther, and the pollen is sticky and cannot be transferred except by insect aid. This is accomplished in a remarkable manner. A little moth, called the _pronuba_, after gathering pollen from an anther, deposits an egg in the ovary of the pistil, and then rubs its load of pollen over the stigma of the flower. The young hatch out and feed on the young seeds which have grown because of the pollen placed on the stigma by the mother. The baby caterpillars eat some of the developing seeds and later bore out of the seed pod and escape to the ground, leaving the plant to develop the remaining seeds without further molestation. [Illustration: The pronuba pollinating the pistil of the yucca.] The fig insect (_Blastophaga grossorum_) is another member of the insect tribe that is of considerable economic importance. It is only in recent years that the fruit growers of California have discovered that the fertilization of the female flowers is brought about by a gallfly which bores into the young fruit. By importing the gallflies it has been possible to grow figs where for many years it was believed that the climate prevented figs from ripening. [Illustration: Pod of yucca showing where the young pronubas escaped.] Other Flower Visitors.--Other insects besides those already mentioned are pollen carriers for flowers. Among the most useful are moths and butterflies. Projecting from each side of the head of a butterfly is a fluffy structure, the palp. This collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. The scales and hairs on the wings, legs, and body also carry pollen. [Illustration: A humming bird about to cross-pollinate a lily.] Flies and some other insects are agents in cross-pollination. Humming birds are also active agents in some flowers. Snails are said in rare instances to carry pollen. Man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields. [Illustration: A cornfield showing staminate and pistillate flowers, the latter having become grains of corn. An ear of corn is a bunch of ripened fruits.] Pollination by the Wind.--Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. Such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. Most of our common trees, oak, poplar, maple, and others, are cross-pollinated almost entirely by the wind.