A Civic Biology, Presented in Problems. George W. Hunter
the wings are full-sized, having been pumped full of blood and air, and the little insect is ready after her wedding flight to follow her instinct to deposit her eggs on a milkweed plant.
Plants furnish Insects with Food.—Food is the most important factor of any animal's environment. The insects which we have seen on our field trip feed on the green plants among which they live. Each insect has its own particular favorite food plant or plants, and in many cases the eggs of the insect are laid on the food plant so that the young may have food close at hand. Some insects prefer the rotted wood of trees. An American zoölogist, Packard, has estimated that over 450 kinds of insects live upon oak trees alone. Everywhere animals are engaged in taking their nourishment from plants, and millions of dollars of damage is done every year to gardens, fruits, and cereal crops by insects.
Damage done by insects. These trees have been killed by boring insects.
All Animals depend on Green Plants.—But insects in their turn are the food of birds; cats and dogs may kill birds; lions or tigers live on still larger defenseless animals as deer or cattle. And finally comes man, who eats the bodies of both plants and animals. But if we reduce this search after food to its final limit, we see that green plants provide all the food for animals. For the lion or tiger eats the deer which feeds upon grass or green shoots of young trees, or the cat eats the bird that lives on weed seeds. Green plants supply the food of the world. Later by experiment we will prove this.
Homes and Shelter.—After a field trip no one can escape the knowledge that plants often give animals a home. The grass shelters millions of grasshoppers and countless hordes of other small insects which can be obtained by sweeping through the grass with an insect net. Some insects build their homes in the trees or bushes on which they feed, while others tunnel through the wood, making homes there. Spiders build webs on plants, often using the leaves for shelter. Birds nest in trees, and many other wild animals use the forest as their home. Man has come to use all kinds of plant products to aid him in making his home, wood and various fibers being the most important of these.
What do Animals do for Plants?—So far it has seemed that green plants benefit animals and receive nothing in return. We will later see that plants and animals together form a balance of life on the earth and that one is necessary for the other. Certain substances found in the body wastes from animals are necessary to the life of a green plant.
A section of a flower, cut lengthwise. In the center find the pistil with the ovary containing a number of ovules. Around this organ notice a circle of stalked structures, the stamens; the knobs at the end contain pollen. The outer circles of parts are called the petals and sepals, as we go from the inside outward.
Insects and Flowers.—Certain other problems can be worked out in the fall of the year. One of these is the biological interrelations between insects and flowers. It is easy on a field trip to find insects lighting upon flowers. They evidently have a reason for doing this. To find out why they go there and what they do when there, it will be first necessary for us to study flowers with the idea of finding out what the insects get from them, and what the flowers get from the insects.
The Use and Structure of a Flower.—It is a matter of common knowledge that flowers form fruits and that fruits contain seeds. They are, then, very important parts of certain plants. Our field trip shows us that flowers are of various shapes, colors, and sizes. It will now be our problem first to learn to know the parts of a flower, and then find out how they are fitted to attract and receive insect visitors.
The Floral Envelope.—In a flower the expanded portion of the flower stalk, which holds the parts of the flower, is called the receptacle. The green leaflike parts covering the unopened flower are called the sepals. Together they form the calyx.
The more brightly colored structures are the petals. Together they form the corolla. The corolla is of importance, as we shall see later, in making the flower conspicuous. Frequently the petals or corolla have bright marks or dots which lead down to the base of the cup of the flower, where a sweet fluid called nectar is made and secreted. It is principally this food substance, later made into honey by bees, that makes flowers attractive to insects.
The Essential Organs.—A flower, however, could live without sepals or petals and still do the work for which it exists. Certain essential organs of the flower are within the so-called floral envelope. They consist of the stamens and pistil, the latter being in the center of the flower. The structures with the knobbed ends are called stamens. In a single stamen the boxlike part at the end is the anther; the stalk which holds the anther is called the filament. The anther is in reality a hollow box which produces a large number of little grains called pollen. Each pistil is composed of a rather stout base called the ovary, and a more or less lengthened portion rising from the ovary called the style. The upper end of the style, which in some cases is somewhat broadened, is called the stigma. The free end of the stigma usually secretes a sweet fluid in which grains of pollen from flowers of the same kind can grow.
Insects as Pollinating Agents.—Insects often visit flowers to obtain pollen as well as nectar. In so doing they may transfer some of the pollen from one flower to another of the same kind. This transfer of pollen, called pollination, is of the greatest use to the plant, as we will later prove. No one who sees a hive of bees with their wonderful communal life can fail to see that these insects play a great part in the life of the flowers near the hive. A famous observer named Sir John Lubbock tested bees and wasps to see how many trips they made daily from their homes to the flowers, and found that the wasp went out on 116 visits during a working day of 16 hours, while the bee made but a few less visits, and worked only a little less time than the wasp worked. It is evident that in the course of so many trips to the fields a bee must light on hundreds of flowers.
Bumblebees. a, queen; b, worker; c, drone.
Adaptations in a Bee.—If we look closely at the bee, we find the body and legs more or less covered with tiny hairs; especially are these hairs found on the legs. When a plant or animal structure is fitted to do a certain kind of work, we say it is adapted to do that work. The joints in the leg of the bee adapt it for complicated movements; the arrangement of stiff hairs along the edge of a concavity in one of the joints of the leg forms a structure well adapted to hold pollen. In this way pollen is collected by the bee and taken to the hive to be used as food. But while gathering pollen for itself, the dust is caught on the hairs and other projections on the body or legs and is thus carried from flower to flower. The value of this to a flower we will see later.
Field Work.—Is Color or Odor in a Flower an Attraction to an Insect?—Sir John Lubbock tried an experiment which it would pay a number of careful pupils to repeat. He placed a few drops of honey on glass slips and placed them over papers of various colors. In this way he found that the honeybee, for example, could evidently distinguish different colors. Bees seemed to prefer blue to any other color. Flowers of a yellow or flesh color were preferred by flies. It would be of considerable interest for some student to work out this problem with our native bees and with other insects by using paper flowers and honey or sirup. Test the keenness of sight in insects by placing a white object (a white golf ball will do) in the grass and see how many insects will alight on it. Try to work out some method by which you can decide whether a given insect is attracted to a flower by odor alone.
The Sight of the Bumblebee.—The large eyes located on the sides of the head are made up of a large number of little units, each of which is considered to be a very simple eye. The large eyes are therefore called the compound eyes. All insects are provided with compound eyes, with simple eyes, or in most cases with both. The