Number. There are approximately 900,000 known species of insects on Earth, but the total number, known and unknown, can only be estimated by scientists. It may be anywhere from 2 million to 30 million living insect species. As to the number of individual insects, there are about 200 million insects for every man, woman, and child on the planet today. They live in every habitat from fresh water ponds to arctic streams; from the tropical rain forests to the arid deserts. Insects such as the honeybee or the silkworm (larva of silk moth) are helpful to man; others are toxic (blister beetles) or carry diseases (mosquito and malaria). Size and Body Parts. Insects range in length from 1/4 inch flies to 2 1/2 inch beetles. All insects have three main body parts: the head, the thorax and the abdomen. The exoskeleton is “worn” on the outside of the insect body. In most species, two pairs of wings sprout from the thorax, but some, like the common house fly have just one pair. All insects have a pair of antennae and two compound (mosaic) eyes on the head. Six legs grow out of the abdomen. Reproduction. Insects mate by copulation. The male deposits sperm in the female’s vagina and she stores the sperm, usually for life, and lays fertilized eggs periodically. Life cycle. Insect young are sometimes miniatures of adults; as they grow they shed their hard outer layer, the exoskeleton, then grow a new and larger one. Alternatively insects start as wormy looking feeders (larvae). A caterpillar is the larva of a butterfly; a maggot is the larva of a fruit fly. After a period of feeding, both types of larva form a cocoon-like case (pupa), develop further, then burst out as fully formed adults. This last process is called metamorphosis. Not long after their debut as adults, they start looking for a mate, and the cycle begins anew.