This section is from the book "The Standard Cyclopedia Of Horticulture Vol2", by L. H. Bailey. See also: Western Garden Book: More than 8,000 Plants - The Right Plants for Your Climate - Tips from Western Garden Experts.
Experts guess that there are from 2,000,000 to 10,000,000 different kinds of insects in the world. Only about 400,000 of these have yet been described and named by man. Between 30,000 and 40,000 are now known in North America. Four-fifths of all the kinds of animals are insects; some single families of insects are said to contain more species than one can see stars in a clear sky at night; and there are as many butterflies as birds in North America. The larger part of the land animals are insects, and it is asserted that the larger proportion of the animal matter existing on the lands of the globe is probably locked up in the forms of insects. Insects vary in size from little beetles, of which it would take 100, placed end to end, to measure an inch, up to tropical species 6 or 8 inches in length, or of equal bulk to a mouse. Insects have a very long, but, as yet, very imperfect pedigree extending through the geological ages to Silurian times. Fossil remains of many different kinds of insects have been found in the rocks (Fig. 1295); even such delicate insects as plant-lice left their impress on the rocks ages ago.
In the carboniferous or coal age, the insect world was evidently quite different from that of today, for fossils of veritable insect mammoths have been found; dragon-flies with a wing-expanse of 2 to 3 feet then existed. Insect fossils found in the tertiary rocks indicate that there were even more kinds of insects then than now.
Fig. 1296. The four stages in an insect's life - egg, larva, pupa, imago. - The codlin-moth. (Egg much enlarged; others Xl 1/2)
Fig. 1299. Tent-caterpillar.
Insects begin life as an egg; in some cases the egg stage is passed within the body of the mother, which then gives birth to living young. The eggs of insects exhibit a wonderful variety of forms, sizes, colors and characteristic markings. A single scale insect may lay thousands of eggs, while some plant-lice produce only one. Remarkable instinct is often shown by the mother insect in placing her eggs where her young will find proper food. From their birth the young of some of the lowest or most generalized insects closely resemble their parents, and they undergo no striking change during their life; hence are said to have no metamorphosis. In the case of grasshoppers, stink-bugs, dragon-flies, and many other insects, the young at birth resemble their parents, but have no wings. As they grow, wings gradually develop and often changes in markings occur, until the adult stage is. reached. The growth, however, is gradual, and no striking or complete change occurs, and these insects are said to undergo an incomplete metamorphosis. The young insects in all stages are called nymphs (Fig. 1297); thus insects with an incomplete metamorphosis pass through three different forms during their life: an egg, the young or nymph stage, and the adult.
From the eggs of butterflies, moths, flies, beetles, bees and some other insects, there hatches a worm-like creature, much unlike the parent insect. It is called a larva (Fig. 1298); the larvae of butterflies and moths are often called caterpillars (Fig. 1299); maggots are the larvae of flies (Fig. 1300); and the term grub is applied to the larvae of beetles and bees (Fig. 1301). When these larvae get their full growth, some of them go into the ground where they form an earthen cell, while others proceed to spin around themselves a silken home or cocoon (Figs. 1302-1304). In these retreats the larvae change to a quiescent or lifeless-appearing creature which has little resemblance to either the larva or the parent insect. It is call a pupa (Fig. 1305). The pupae of butterflies are often called chrysalids. Flies change to pupae in the hardened skin of the maggot. Some pupae, like those of mosquitos, are very active. Wonderful changes take place within the skin of the pupa. Nearly all the larval tissues break down and the insect is practically made over, from a crawling larva to a beautiful, flying adult insect. When the adult is fully formed, it breaks its pupal shroud and emerges to spend a comparatively brief existence as a winged creature.
Such insects are said to undergo a complete metamorphosis, and pass through four strikingly different stages during their life: the egg, the wormlike larva, the quiescent pupa, and the adult insect. Such remarkable changes or transformations make the story of an insect's life one of intense interest to one who reads it from nature's book. Various kinds of adult insects, or imagoes, are shown in Figs. 1306-1311. No two kinds of insects have the same life-story to tell. Some pass their whole life on a single host; some partake of only a certain kind of food, while others thrive on many kinds of plants; some are cannibals at times, and others, like the parasites, are boarders within their host, while many prey openly on their brethren in the insect world. Usually the life of the adult insect is brief, but ants have been kept for thirteen years, and the periodical cicada has to spend seventeen years as a nymph underground before it is fitted to become a denizen of the air. The winter months may be passed in any of the different stages of the insect's life. Two very closely allied insects may have very different fife habits. How they grow. - Many persons think that the small house-flies grow to be the large ones.
While most insects feed after they become adults, they get little or none of their growth during their adult fife. Insects grow mostly while they are larvae, or nymphs. The maggots from which the little house-flies develop doubtless do not have as luxuriant or favorable feeding-grounds as do those of the larger flies. In thirty days some leaf-feeding caterpillars will increase in size 10,000 times; and a certain flesh-feeding maggot will in twenty-four hours consume two hundred times its own weight, which would be paralleled in the human race if a one-day-old baby ate 1,500 pounds the first day of its existence! The skin of insects is so hard and inelastic that it cannot stretch to accommodate such rapid growth. But nature obviates this difficulty by teaching these creatures how to grow a new suit of clothes or a new skin underneath the old one, and then to shed or molt the latter. The old skin is shed in its entirety, even from all the appendages, and sometimes remains in such a natural position where the insect left it as to easily deceive one into thinking that he is looking at the insect rather than at its cast-off clothes. Some insects are so neat and economical that they devour their old suits or skins soon after molting them.
Larvae, or nymphs, may molt from two or three to ten or more times; the larvae do not often change strikingly in appearance, but the nymphs gradually acquire the characters and structures of the adult.
Fig. 1300. A maggot. Larva of a dipterous insect.
Fig. 1301. A grub. Larva of a beetle.
Fig. 1302. Cocoon of Pro-methea moth.
Made in the roll of a leaf. The insect weaves a web about the leaf-stalk and ties it to the parent stem, so that the leaf cannot fall
Fig. 1303. Lengthwise section of the Promethea cocoon.
Showing at apex the val ve-like op e n i n g through which the moth escaped.
Fig. 1304. End of cocoon of Cecropia moth. Inside view, showing where the moth gets out.
Fig. 1305. Pupa of tomato worm.
Fig. 1306. The cabbage butterfly.
To the horticulturist, the mouth-parts of an insect are its most important organs or appendages. The mouth-parts are built on two very different plans. Grasshoppers, beetles, caterpillars and grubs have two pairs of horny jaws, working from side to side, with which they bite or chew off pieces of their food, that then pass into the food-canal for digestion (Fig. 1312). The scale insects (Fig. 1313), plant-lice, true bugs (Fig. 1314), mosquitos and others have these jaws drawn out into thread-like organs, which are worked along a groove in a stiff beak or extended under-lip. Such insects can eat only liquid food, which they suck with their beak-like mouth-parts. The insect places its beak on the surface of the plant, forces the thread-like jaws into the tissues, and then begins a sucking operation, which draws the juices of the plant up along the jaws, and the groove in the beak into the food-canal of the insect. Thus a sucking insect could not partake of particles of poison sprayed on the surface of a plant. Its mouth-parts are not built for such feeding, and as it is impracticable to poison the juice of the plant, one is forced to fight such insects with a deadly gas, or each individual insect must be actually hit with some insecticide.
A knowledge of these fundamental facts about the eating habits of insects would have saved much time and money that have been wasted in trying to check the ravages of sucking insects with paris green and similar poisons. Some insects, like the fruit flies, have mouth-parts fitted for lapping up liquids.