Arxiu d'etiquetes: neoteny

The (a)sexual life of insects

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

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.

1. Amphygony

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).

Icerya purchasi (left; picture property of Vijay Cavale, CC 3.0) and Perla marginata (right; picture property of gailhampshire en Flickr, CC 2.0).

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).

Grasshoppers of the species Romalea microptera from the United States, mating. Picture property of, CC 3.0.
  • 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).

Sminthurus viridis (Collembola); behind, the spermatophore. Modified picture; original picture property of Gilles San Martin on Flickr, CC 2.0.
Orthoptera (female) grabbing the spermatophore laid by a male. Modified picture; original picture property of Sandrine Rouja on Flickr, CC 2.0.

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.

2. Parthenogenesis

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.
Silkworm butterfly (Bombyx mori). Occasionally, some of its unfertilized eggs give birth to a larva. Picture property of Nikita on Flickr, CC 2.0.

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.
In honeybees, fertilized eggs give birth to females (workers or queen depending on the diet they are given during the larval stages) and unfertilized eggs, to males. Pictures by Alex Wild and figure by Ashley Mortensen (web of the University of Florida).


  • 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.
Aphis nerii (aphids). Picture property of Andrew C, CC 2.0.

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).

Asexual reproduction

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.

Offspring generation

There exist different strategies through which insects generate their offspring:


Oviparous insects lay eggs. It is the most common reproductive strategy.

Praying mantis lay or ootheca (left; picture property of Scot Nelson on Flickr, CC 2.0) and lay of the butterfly Pieris brassicae (right; picture property of Walter Baxter, CC 2.0).


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!


Main picture property of Irene Lobato Vila (the owner of this post).

Frogs, toads and newts: the last amphibians

With about 7000 living species, amphibians currently occupy almost all the habitats on Earth. While in the last entry we explained the origin of the first tetrapods and how those gave rise to the different groups of primitive amphibians, in this entry we will explain in more detail the characteristics of current amphibians, the so-called lissamphibians.


The term “Lissamphibia” (“smooth amphibian”) is used to name current amphibians and it’s useful to tell them apart from the rest of fossil amphibians, while the term Amphibia (“double life” referring to the aquatic larval stage of most species), is used to name all tetrapods except the amniotes (reptiles, birds and mammals).

Most authors consider lissamphibians a monophyletic group (a group which includes all the descendants of a common ancestor) which includes the different groups of modern amphibians. The main characteristics of this group are:

Dermal characteristics

  • Smooth, scaleless, permeable skin that allows gas exchange (both pulmonary and cutaneous respiration) and the absorption of water (most amphibians usually do not need to drink water). This makes them susceptible to skin infections like the one from the Batrachocytrium dendrobatidis fungus.
FrogSkinSection through frog skin by Jon Houseman. A: Mucous gland, B: Chromophore, C: Granular poison gland, D: Connective tissue, E: Stratum corneum, F: Transition zone, G: Epidermis, and H: Dermis.
  • Two types of skin glands: mucous (the majority, to maintain humidity) and granular (less numerous, secrete toxins of different intensity).

Skeletal characteristics

  • Pedicellate and bicuspid teeth.
teethPhoto of pedicellate teeth, in which the crown and base are made of dentine and are separated by a narrow layer of uncalcified dentine.
  • A pair of occipital condyles.
  • Short, stiff ribs not encircling the body.
  • Four digits on the front limbs and five digits on the hind limbs.
10050622254_8cffbfb0e4_oSkeleton of giant salamander in which we can see some of the characteristics of lissamphibians. Photo by Graham Smith.

Auditory characteristics

  • Papilla amphibiorum, a group of specialized cells in the inner ear which allow them to hear low frequency sounds.
  • Stapes-operculum complex which are in contact with the auditory capsule, improve reception of aerial and seismic waves.

Other characteristics

  • Fat bodies associated with gonads.
  • Presence of green rods in the visual cells (these allow the perception of more colours).
  • Presence of a muscle elevator of the eye (called levator bulbi).
  • Forced-pump ventilation system (their short ribs do not allow pulmonary ventilation, so they pump the air through their mouth).
Two_stroke_buccal_pumpingExplicative diagram about buccal ventilation in lissamphibians, by Mokele.


Nowadays only three living amphibian orders persist: the order Salientia or Anura (which includes frogs and toads), the order Caudata or Urodela (salamanders and newts) and the order Gymnophiona or Apoda (caecilians). The second name of each order refers to the current species and their recent ancestors, while the first name refers to the whole order since the separation of each order.

There are two hypotheses regarding the relationships between the three orders. The most accepted both by anatomic and molecular analyses is that Salientia and Caudata are grouped together into the clade Batrachia, while the other one is that Caudata and Gymnophiona together form the clade Procera.

Batrachia proceraTwo hypothetical evolutionary trees by Marcello Ruta & Michael I. Coates (2007), showing the Batrachia and Procera hypotheses on the relationships between Salientia (S), Caudata (C) and Gymnophiona (G).

Currently there are three groups of hypotheses of the origin of lissamphibians: the temnospondyl hypotheses, the lepospondyl hypotheses and the polyphyletic hypotheses.

Temnospondyls are the main candidates to be the ancestors of lissamphibians, as they share many characteristics, such as the presence of pedicellated, bicuspid teeth, and short, stiff ribs. Authors defending these theories say that lissamphibians suffered during their evolution a process known as paedomorphosis (retention during the development of juvenile characteristics), this way explaining why temnospondyls reached such large sizes while lissamphibians are much smaller and usually have lighter and less ossified cranial structures.

temnospondyliDrawings from Marcello Ruta & Michael I. Coates (2007) of skeletons belonging to Celteden ibericus (left, a lissamphibian) and Apateon pedestris (right, a temnospondyl) to show similitudes in skeletal structure.

Hypotheses regarding a lepospondyl origin for lissamphibians do not have such a strong support as the temnospondyl hypotheses. However, recently some statistical studies combining anatomic and molecular data have given some support to these hypotheses.

Nevertheless, there is a third group of hypotheses we must consider, the ones that say that lissamphibians are a polyphyletic group (with different origins for the different orders). According to one of these theories, frogs and salamanders (clade Batrachia) would have a temnospondyl origin, while caecilians (order Gymnophiona or Apoda) would have originated from lepospondyl ancestors, many of which had already suffered a limb reduction process.

 Lissamphibian_phylogenyModified outline of the three different hypotheses regarding the origins of the lissamphibians; 1. Lepospondyl origin, 2. Temnospondyl origin, 3. Polyphyletic origin.

Still, most authors support a monophyletic and temnospondyl origin for lissamphibians, but alternative hypotheses shouldn’t be discarded.


With up to 4750 species, frogs and toads form the most diverse lissamphibian order. The first known Salientia is Triadobatrachus, which, despite having a tail, already presented some typical characteristics of modern frogs, such as a short spine with few vertebras and the hind limbs longer than the front limbs.

TriadobatrachusInterpretation by Pavel Riha, of the ancient Salientia, Triadobatrachus massinoti.

The anatomy of modern anurans is unique among the animal kingdom. Their skeleton seems totally dedicated to allow these animals to jump (even though many species move simply by walking). Some of their characteristics are:

  • A short and stiff trunk (less than 12 vertebras), an especially long pelvic girdle and the vertebras of their posterior end (that in other amphibians form the tail) are reduced and fused forming the urostyle.
  • Long hind limbs, with the tibia and fibula fused together (to aid in impulse during jumping) and short and strong front limbs (to resist the impact on the landing).
3888291918_f779053a0a_oPhoto of a pig frog (Rana grylio), a typical american anuran.

Also, of all current amphibians frogs are the ones with the most developed hearing apparatus and vocal organ. Males, usually present specialized structures to amplify sound during the mating season.

Litoria_chloris_callingRed eyed tree frog (Litoria chloris) showing the vocal sac, used to amplify the sound of its calls.

Size in anurans varies from 3 kg in weight and 35 centimetres in length of the goliath frog (Conraua goliath) to the 7, 7 millimeters long recently discovered Paedophryne amanuensis, currently the smallest known vertebrate.

Paratype_of_Paedophryne_amauensis_(LSUMZ_95004)Photo from Rittmeyer EN, Allison A, Gründler MC, Thompson DK, Austin CC (2012)  of Paedophryne amanuensis, the smallest known vertebrate in the world on a US dime.

With such a diversity, vital strategies of anurans vary greatly and it’s difficult to generalize on their reproductive biology, even though most show indirect development (born as tadpoles and passing through a metamorphosis process) and they mate and lay their eggs in an aquatic medium.

BufoBufoTadpolesTadpoles of common toad (Bufo bufo) from northern Germany by Christian Fischer.


The urodeles or caudates are the order of lissamphibians which externally most resemble primitive amphibians. This group includes salamanders and newts, most of which have a long body, a well-developed tail and four relatively short legs. Most urodeles are terrestrial and are distributed mainly in the northern hemisphere, with a few species inhabiting the tropics.

Salamandra_TigrePhoto of an eastern tiger salamander (Ambystoma tigrinum) from the House of Sciences, Corunna - Spain. Taken by Carla Isabel Ribeiro.

Most species present internal fertilization and are oviparous. Most also present indirect development (larvae, metamorphosis, adult), and the larvae usually resemble miniaturized adults with external, ramified gills. Various groups of salamanders suffer neoteny phenomenon, in which individuals, even though sexually developing into adults, externally keep larval characteristics.

Joung_and_very_large_larva_of_Salamandra_infraimmaculata,_Ein_Kamon,_IsraelYoung and very large larvae of near eastern fire salamander (Salamandra infraimmaculata), Ein Kamon, Israel. Photo by Ab-Schetui.

Currently, urodeles are classified into three suborders: the Sirenoidea, the Cryptobranchoidea and the Salamandroidea. Sirenoideans are urodeles with both specialized and primitive characteristics, such as the loss of hind limbs and the presence of external gills. Cryptobranchoideans are large primitive salamanders (up to 160 centimetres) which present external fertilization, while salamandroideans are the most numerous group of urodeles (with more than 500 species) and the most diverse, with most species being terrestrial and having internal fertilization using packs of sperm called spermatophores.

20090924201238!P_striatus_USGSPhoto of a northern dwarf siren (Pseudobranchus striatus) a sirenoidean from the United States.


The most ancient known member of the order Gymnophiona is Eocaecilia micropodia, an amphibian about 15 centimetres long with a considerably long body, a short tail and really small limbs.

Eocaecilia_BWRestoration by Nobu Tamura of Eocaecilia micropodia an ancient Gymnophiona from the early Jurassic.

Current caecilians (order Apoda) have completely lost any trace of limbs, girdles or tail, due to their adaptation to a subterranean lifestyle. That’s why they also suffered a process of cranial hardening and their eyes are extremely reduced. They also present a series of segmentary rings all along their bodies, which make them look somewhat like earthworms.

Ichthyophis kohtaoensis, ca 12Yellow-striped caecilian (Ichthyophis kohtaoensis) from Thailand, by Kerry Matz.

There are currently about 200 species of caecilians divided into 10 families. Their size varies from about 7 centimetres in the species Idiocranium russelli from Cameroon, to up to 1,5 meters of Caecilia thompsoni from Colombia. They present a pantropical distribution, internal fertilization and a great variation in their development (there are viviparous and oviparous species and some which endure metamorphosis while some have direct development).

KONICA MINOLTA DIGITAL CAMERAPhoto of Gymnopis multiplicata an american caecilian. Photo by Teague O'Mara.


The following sources have been consulted in the elaboration of this entry: