Insect reproduction

With the exception of parthenogenetic insects, insects are most often sexually dimorphic, with the males and females of most insects differing in appearance.

Insects have several strategies for locating mates, including the use of sex pheromones, mass swarming (lekking, as in honey bee nuptial swarming), and nuptial gifts.

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. Many insects lay their eggs in large masses, while others favour greater dispersion and lay single eggs in different locations. Herbivorous insects may lay their eggs in discreet locations on plants, such as the underside of a leaf.

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. Many insect species, such as Vine weevils, some stick insects, and, most notably, aphids, are parthenogenetic, meaning they do not require males to fertilize their eggs. 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.