section_path: Reproduction and development/Life-cycles
section_name: Life-cycles
paragraph_text: Life-cycles
  
  The majority of insects hatch from eggs. The fertilization and development takes place inside the egg, enclosed by a shell (chorion) that consists of maternal tissue. In contrast to eggs of other arthropods, most insect eggs are drought resistant. This is because inside the chorion two additional membranes develop from embryonic tissue, the amnion and the serosa. This serosa secretes a cuticle rich in chitin that protects the embryo against desiccation. Some species of insects, like aphids and tsetse flies, are ovoviviparous: their eggs develop entirely inside the female, and then hatch immediately upon being laid. Some other species, such as in the cockroach genus Diploptera, are viviparous, gestating inside the mother and born alive. Some insects, like parasitoid wasps, are polyembryonic, meaning that a single fertilized egg divides into many separate embryos. Insects may be univoltine, bivoltine or multivoltine, having one, two or many broods in a year.
  
  Other developmental and reproductive variations include haplodiploidy, polymorphism, paedomorphosis or peramorphosis, sexual dimorphism, parthenogenesis, and more rarely hermaphroditism. In haplodiploidy, which is a type of sex-determination system, the offspring's sex is determined by the number of sets of chromosomes an individual receives. This system is typical in bees and wasps.
  
  Some insects are parthenogenetic, meaning that the female can reproduce and give birth without having the eggs fertilized by a male. Many aphids undergo a cyclical form of parthenogenesis in which they alternate between one or many generations of asexual and sexual reproduction. In summer, aphids are generally female and parthenogenetic; in the autumn, males may be produced for sexual reproduction. Other insects produced by parthenogenesis are bees, wasps and ants; in their haplodiploid system, diploid females spawn many females and a few haploid males.