340 



EVOLUTION AND ANIMAL LIFE 



FIG. 200. Egg case of the 

 cockroach. 



adaptations tending toward the preservation of the young. / 

 In the Marsupials, which stand next to the Monotremes the 

 kangaroo, opossum, etc. the young are born in a very im- 

 mature state and are at once seized by the mother and 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. 199). This is a 

 singular adaptation, but less special- 

 ized and less perfect than the condi- 

 tion found in ordinary mammals. 



Among the insects, the special pro- / 

 visions for the protection and care of 



the eggs and the young are widespread and various. The eggs 

 of the common cockroach are laid in small packets inclosed 

 in a firm wall (Fig. 200). The eggs of the great water bugs 

 are carried on the back of the male (Fig. 201): and the spiders 

 lay their eggs in a silken sac or cocoon, and some of the ground 

 or running spiders (Lycosidce) , 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, feed- 

 ing on each other. Many insects have 

 long, sharp, piercing ovipositors, by 

 means of which the eggs are thrust into 

 the ground or into the leaves or stems 

 of green plants, or even into the hard 

 wood of tree trunks. Some of 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. 

 The various gall insects lay their eggs 

 in the soft tissue of plants, and on 

 the hatching of the larva 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 in- 

 sect 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 (Figs. 2J2 and 203). 



The movements of migratory fishes are mainly controlled 



FIG. 201. Giant waterbug, 

 Serphus, male carrying 

 eggs on its back. 



