Most of insects are dioecious, reproduce sexually by mating and lay eggs. However, as a group they have developed many other reproductive strategies.
Discover them through this article!
Types of reproduction
Sexual reproduction involves the participation of specialized sexual cells or gametes originated in the sexual organs by meiosis. It is the most common type of reproduction among arthropods and insects.
In amphygony, two types of gametes are generated, which lead to the formation of the embryo once they fuse. Most of amphygonic insects are unisexual or dioecious, so each organism generates only one type of gamete. In fact, only a few cases in which a single organism generates more than one type of gamete (hermaphroditism) are currently known; i. e. Icerya purchasi (Hemiptera), Perla marginata (Plecoptera) and several species of the family Termitoxenidae (Diptera).
Finding mate and courtship
In dioecious organisms, the fusion of the gametes takes place once they find a mate. Insects develop diverse and complex strategies to find a proper mate: emission of pheromones, light, sounds and vibrations, development of an attractive coloration pattern, amongst others (of which we talked widely in this post about insects’ communication).
Once they get a mate, courtship usually takes place; however, only successful courtships end in copulation. Courtship behavior and strategies include the performance of nuptial dances, gifts (i. e. food, as occurs in some scorpionflies (Mecoptera)) or the formation of swarms (nuptial flights, as in Hymenoptera), amongst others. In some cases, females will not mate with the male if he does not possess a wide territory or a suitable food source.
In the following video, we can enjoy the honeybee nuptial flight:
The fertilization or syngamy is the process through which the gametes fuse to form the embryo. This process takes place both in dioecious and hermaphrodite organisms.
- Internal fertilization
Following with the dioecious organisms, the most frequent mechanism among “modern” insects to guarantee gametes meeting is mating (internal fertilization). When mating, males usually transmit his gametes (spermatozoa) directly to the female body, inside which male gametes meet with the female ones (ovules).
- External fertilization
In some insects and related groups, fertilization does not need a direct contact of male and female sexual organs (external fertilization). In this case, males produce a spermatophore, a packet or capsule containing sperm, manufactured by the accessory glands of the male reproductive system; it is usually covered by a lipoprotein film that prevents it from dehydration. Usually, the spermatophore is considered an intermediate step between aquatic and terrestrial reproduction.
Spermatophore is produced by hexapod related groups, such as Myriapoda (millipedes, centipedes); also, by basal hexapods, like Collembola, Diplura and Protura; basal insects, such as Archaeognatha and Zygentoma (bristletails and silverfishes); and some groups of “modern” insects, like Orthoptera, Psocoptera, Coleoptera, Neuroptera, Mecoptera and some Hymenoptera. Sometimes, the male produces a spermatophore and leaves it over a surface, waiting the female to take it (as in Collembola); in other groups, the male offers it directly to the female as a nuptial gift, or leads the female where it has been deposited (Zygentoma and Archaeognatha).
Internal fertilization is considered an evolutive adaptation to terrestrial life. However, there are still some insects that carry on internal reproduction that conserve the genetical information to produce a spermatophore; in these cases, the male introduces the spermatophore inside the female’s body, which serves to her as an additional nutritional source for her eggs.
Parthenogenesis is the generation of offspring through unfertilized eggs. Usually, parthenogenesis is classified among asexual reproductive strategies; however, it is more like a special type of sexual reproduction since female gametes generated by meiosis are involved in the process.
Parthenogenesis can be:
- Accidental: occasionally, an unfertilized egg gives birth to a larva; i. e. Bombyx mori (silkworm butterfly).
- Facultative: while some eggs are fertilized, others not.
- Obligated: eggs only develop if they are unfertilized. It occurs in many species with alternant parthenogenetic and amphygonic generations.
Moreover, depending on the chromosomic number of the ovule, parthenogenesis can be:
- Haploid (n) or arrhenotoky: unfertilized eggs (n) generate males and fertilized eggs (2n), females. It takes place in bees and other Hymenoptera, in some Coleoptera and Zygentoma, and it is always facultative. Sex determination at birth is a key process in the evolutive history of colonial structures in social insects.
- Diploid (2n) or thelytoky: unfertilized eggs (2n) always give birth to females with the same genetic number as the progenitor female (clones). It takes place in aphids (Aphididae, Hemiptera), cockroaches, scale insects (Coccoidea, Hemiptera) and in some curculionid beetles; it tends to be an obligated parthenogensis. This type of parthenogenesis has the potentiality to generate hundreds of descendants in a short lapse as a detriment to the genetical variability. In aphids, parthenogenetic generations alternated with amphigonic generations allow them to undergo demographical explosions at specific times.
Sometimes, parthenogenesis occurs in immature stages (larval or pupal). In the pedogensis or paedogensis, immature forms can generate offspring by parthenogenesis; it takes place in gall midges (Diptera) and in a species of beetle, Macromalthus debilis, amongst others. It must not be confused with neoteny, in which a larva develops traits and reproductive structures typical of an adult (as occurs in some scale bugs).
In the asexual reproduction, the generation of offspring occurs without the participation of any type of gamete.
It is very uncommon in insects, being represented only by a single and odd strategy called polyembryony. Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg by scission. Even though it takes place an initial fertilization, offspring is generated asexually. It occurs just in a few species of gall midges and in a few chalcidid hymenopterans (parasitoids), through which they undergo population explosions.
There exist different strategies through which insects generate their offspring:
Oviparous insects lay eggs. It is the most common reproductive strategy.
Fertilized eggs are incubated inside the reproductive ducts of the female. It happens in some cockroaches, aphids, scale bugs and flies (Muscidae, Calliphoridae and Tachinidae), in some beetles and trips (Thysanoptera). The eggs hatch immediately before or after being laid.
Females give birth to larvae. There exist different types of viviparity in insects:
- Pseudoplacental viviparity: female develops eggs containing little or no yolk, so she must nourish them through a placental-like tissue. It occurs in many aphids and Dermaptera, in some Psocoptera and in Polyctenidae (Hemiptera).
In this video of Neil Bromhall, we can see a group of aphids giving birth:
- Hemocelous viviparity: embryos develop freely inside the female’s hemolymph (the internal liquid of insects, similar to blood), from which they obtain nutrients by osmosis. It occurs only in Strepsiptera and in gall midges. In some gall midges, larvae feed on their progenitor, which is also a larva (extreme case of larval pedogenesis).
- Adenotrophic viviparity: larvae are underdeveloped, so they must keep feeding on liquids excreted by accessory glands located on females’ reproductive ducts (‘mammary glands’). Once they reach the optimal size, larvae pupate immediately after being laid. This type of viviparity takes place in flies of the families Glossinidae (tsetse fly), Hippoboscidae (horse or dove flies), Nycteribidae and Streblidae (bat flies).
In this video of Geoffrey M. Attardo (AAAS/Science), we can see a tsetse fly giving birth to its larva:
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Who said that the (a)sexual life of insects was simple? Do you know any curious data? Leave your comments below!
Montés, F. J. 2013. El universo de los insectos. Mundi-Prensa Libros.
- “Reproductive System” and “Survival Strategies” in General Entomology. NC State University web.
- Entomology: Internal Anatonomy/Physiology: Reproduction. School of Boological Sciences. University of Sydney.
Main picture property of Irene Lobato Vila (the owner of this post).