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utilized. In general, those animals are highest in development, with best means of holding their own in the struggle

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FIG. 173. The altricial nestlings of the Blue jay (Cyanocitta cristata).

for life, that take best care of their young. The homes of animals are specially discussed in the volume on Ani

mal Life, but those instincts which lead to home-building may all be regarded as useful adaptations in preserving the young. Among the lower or more coarsely organized

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FIG. 174.-Kangaroo (Macropus rufus) with young in pouch.

birds, such as the chicken, the duck, and the auk, as with the reptiles, the young animal is hatched with well-devel

FIG. 175.--Egg-case of California barn-door skate (Raja binocu

oped muscular system and sense

organs, and is capable of running about, and, to some extent, of feeding itself. Birds of this type are known as præcocial (Fig. 172), while the name altricial (Fig. 173) is applied to the more highly organized forms, such as the thrushes, doves, and song-birds generally. With these the young are hatched in a wholly helpless condition, with ineffective muscles, deficient senses, and dependent wholly upon the parent. The altricial condition demands the building of a nest, the establishment of a home, and the continued care of one or both of

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lata) cut open to show young the parents. inside. (Young issues natu

rally at one end of the case.)

the

The very lowest mammals known, duck- bills (Monotremes) of Australia, lay large eggs in a strong shell like those of a turtle, and guard them with great jealousy. But with almost all mammals the egg is very small and without much food-yolk. The egg begins its development within the body. It is nourished by the blood of the mother, and after birth the young is cherished by her, and fed by milk secreted by specialized glands of the skin. All these features are adaptations tending toward the preservation of the young. In the division of mammals next lowest to the Monotremes-the kangaroo, opossum, etc.-the young are born in a very immature state and are at once seized by the mother and

FIG. 176.-Egg-case of the cockroach.

thrust into a pouch or fold of skin along the abdomen, where they are kept until they are able to take care of themselves (Fig. 174). This is an interesting and ingenious

adaptation, but less specialized and less perfect an adaptation than the conditions found in ordinary mammals.

Among the insects, the special provisions for the protection and care of the eggs and the young are wide-spread and various. Some of those adaptations which take the special form of nests or "homes" are described in the volume on Animal Life. The eggs of the common cockroach are laid in small packets inclosed in a firm wall (Fig. The eggs of the great water-bugs are carried on the back of the male (Fig. 177); and the spiders lay their eggs in a silken sac or cocoon, and some of the ground or

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FIG. 177. Giant water-bug (Serphus). Male carrying eggs on its back.

176).

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FIG. 178.-Cocoon inclosing the pupa of the great Ceanothus moth. Spun of silk by the larva before pupation.

running spiders (Lycoside) drag this egg-sac, attached to the tip of the abdomen, about with them. The young spiders when hatched live for some days inside this sac, feeding on each other! Many insects have long, sharp,

piercing ovipositors, by means of which the eggs are deposited in the ground or in the leaves or stems of green plants, or even in the hard wood of tree-trunks. Some of

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the scale insects secrete wax from their bodies and form a

large, often beautiful

egg-case, attached to

and nearly covering the body in which eggs are deposited (Fig.

179). The various gall insects e S. lay their eggs in the soft tissue of plants, and on the hatching of the larvæ an abnormal growth of the plant occurs about the young insect, forming an inclosing gall that serves not only to protect the insect within, but to furnish it with an abundance of plant-sap, its food. The young insect remains in the gall until it completes its development and growth, when it gnaws its way out. Such insect galls are especially abundant on oak trees (Fig. 180). The care of the eggs and the young of the social insects, as the bees and ants, are described in Chapter XXII.

FIG. 179.-The cottony cushion scale insect (Icerya purchasi), from California. The male is winged, the female wingless and with a large waxen egg-sac (e.s.) attached

to her body. (The lines at the left of each figure indicate the size of

the insects.)

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