On the criteria of species diversity, distribution, and sheer numbers, Insects must be regarded as the most successful class of animals ever to live. There are over 700,000 species of insects known, outnumbering all other forms of life combined. The appendages are variously modified for walking, feeding, sensory perception, copulation, and defense. Reproduction In insects, reproduction is usually sexual, with separate male and female animals.
Fertilization is usually internal.
In most species, sperm are deposited directly into the female's vagina at the time of copulation, though in some species the male deposits a sperm packet outside the female, who then picks it up. The female reproductive system may also include a spermatheca, a blind sac in which sperm may be stored for a year or more. Though most insect species produce a multitude of eggs, some flies produce live offspring, usually a single young at a time. Many insects mate only once in a lifetime. The cuticle can be modified into a thick, hard armor over some parts of the body or be paper-thin and flexible in other locations, such as the joints of the appendages.