(2) The development of silkworm includes four stages: fertilized egg, larva, pupa and adult, which is a complete metamorphosis development. After fertilized eggs hatch into larvae, the larvae feed on mulberry leaves and molt once every five or six days. After molting for four times, the silkworm will stop feeding and spinning, and will surround its body and cocoon into a pupa, which will emerge into an adult after a while. So when the baby silkworm begins to spin silk for cocoon, it means silkworm.
(3) The frog's development process is as follows: the male frog calls first to attract the female frog, and when the female frog arrives, the male and female frogs embrace each other to promote the discharge of hermaphroditic germ cells. Androgynous germ cells combine in water to form fertilized eggs, many fertilized eggs form egg blocks together, and fertilized eggs hatch into tadpoles. Tadpoles grow hind legs first, then front legs, and finally their tails gradually disappear and become young frogs, which gradually grow into frogs, that is, fertilized eggs → tadpoles → young frogs → adults.
(4) Reproductive behavior is related to animal reproduction, including male and female animals' identification, occupying reproductive space, courtship, mating, incubating eggs, feeding offspring, etc., which is of great significance to the continuation of the race. The reproductive behavior of some animals can also make offspring take good care of by their parents, greatly improving the survival rate of offspring. "Several early warblers compete for warm trees, and whose new swallow pecks at spring mud" refers to the nesting behavior of swallows.
So the answer is: (1) fertilized eggs.
(2) pupae.
(3) abnormal condition.
(4) reproduction.