Insects are becoming smaller: miniaturization

According to different studies, multicellular organisms tend to become smaller and smaller through time. This phenomenon is called miniaturization and is considered one of the most significative evolutionary trends among insects. Miniaturization is a driving force for diversity and evolutionary novelties, even though it must deal with some limitations.

Learn more about this phenomenon and met some of the most extreme cases of miniaturization among insects through this post.

Why are animals becoming smaller?

For some years now, multiple studies suggest there is a widely extended trend to miniaturization among multicellular animals (i. e. organisms composed by more than one cell).

Miniaturization is a remarkable natural phenomenon headed to the evolution of extremely small bodies. This process has been observed in different non-related groups of animals:

• Shrews (Soricomorpha: Soricidae), mammals.
• Hummingbirds (Apodiformes: Trochilidae), birds.
• Diverse groups of insects and arachnids.

To know more about giant insects, you can read “Size matters (for insects)!“

Diversification and speciation processes have given place to lots of new species through time, all of them constantly competing for limited space and food sources. This scenario turns even more drastic in tropical regions, where diversification rates are extremely high.

Learn about the ecological niche concept by reading “The living space of organisms“.

Facing the increasing demands of space and resources, evolution has given place to numerous curious phenomena such as miniaturization to solve these problems: by becoming smaller, organisms (either free-living or parasites) gain access to new ecological niches, get new food sources and avoid predation.

Despite many animals tend to miniaturization, this phenomenon is more frequently observed among arthropods, being one of their most remarkable evolutionary trends. Moreover, arthropods hold the record of the smallest multicellular organisms known to date, some of which are even smaller than an amoeba!

Guinness World Record of the smallest insects

The smallest arthropods are crustaceans belonging to the subclass Tantulocarida, which are ectoparasites of other groups of crustaceans, such as copepods or amphipodes. The species Tantulacus dieteri is still considered the smallest species of arthropods worldwide, which barely measures 85 micrometers (0,085 millimeters), thus being smaller than many unicellular life beings.

However, insects do not lag far behind.

Mymaridae

Mymaridae (or fairyflies) are a family of wasps inside the superfamily Chalcidoidea from temperate and tropical regions. Adults, ranging from 0.5 to 1 millimeter, develop as parasites of other insects’ eggs (e. g. bugs, Heteroptera). For this reason, fairyflies are very valuable as biological control agents of some harmful pests. Also, they are amongst the smallest insects worldwide.

Currently, the one holding the record as the smallest known adult insect is the apterous (wingless) male of the species Dicopomorpha echmepterygis from Costa Rica, with a registered minimum size of 0.139 millimeters. They neither have eyes nor mouthparts, and their legs endings are deeply modified to get attached to the females (somewhat bigger and winged) time enough to fertilize them. They are even smaller than a paramecium, a unicellular organism!

You can read “Basic microbiology (I): invisible world” to know more about unicellular organisms.

Male of D. echmepterygis. Link.

Fairyflies also include the smallest winged insects worldwide: the species Kikiki huna from Hawaii, with and approximate size of 0.15 millimeters.

Trichogrammatidae

Like fairyflies, trichogrammatids are tiny wasps of the superfamily Chalcidoidea that parasite eggs of other insects, especially lepidopterans (butterflies and moths). Adults of almost all the species measure less than 1 millimeter and are distributed worldwide. Adult males of some species are wingless and mate with their own sisters within the host egg, dying shortly after without even leaving it.

The genus Megaphragma contains two of the smallest insects worldwide after fairyflies: Megaphragma caribea (0.17 millimeters) and Megaphragma mymaripenne (0.2 millimeters), from Hawaii.

A) M. mymaripenne; B) Paramecium caudatum. Link.

Trichogrammatids also have one of the smallest known nervous systems, and that of the species M. mymaripenne is one of the most reduced and specials worldwide, as it is composed by only 7400 neurons without nucleus. During the pupae stage, this insect develops neurons with functional nuclei which are able to synthetize enough proteins for the entire adulthood. Once adulthood is reached, neurons lose their nuclei and become smaller, thus saving space.

Ptiliidae

Ptiliidae is a cosmopolitan family of tiny beetles known for including the smallest non-parasitic insects worldwide: the genera Nanosella and Scydosella.

Ptiliidae eggs are very large in comparison with the adult female size, so they can develop a single egg at a time. Other species undergo parthenogenesis.

Learn some more about parthenogensis by reading “Immaculate Conception…in reptiles and insects“.

Currently, the smallest Ptiliidae species known and so the smallest non-parasitic (free living) insect worldwide is Scydosella musawasensis (0.3 millimeters), from Nicaragua and Colombia.

Scydosella musawasensis. Link (original picture: Polilov, A (2015) How small is the smallest? New record and remeasuring of Scydosella musawasensis Hall, 1999 (Coleoptera, Ptiliidae), the smallest known free-living insect).

Consequences of miniaturization

Miniaturization gives rise to many anatomical and physiological changes, generally aimed at the simplification of structures. According to Gorodkov (1984), the limit size of miniaturization is 1 millimeter; under this critical value, the body would suffer from deep simplifications that would hinder multicellular life.

While this simplification process takes places within some groups of invertebrates, insects have demonstrated that they can overcome this limit without too many signs of simplification (conserving a large number of cells and having a greater anatomical complexity than other organisms with a similar size) and also giving rise to evolutionary novelties (e. g. neurons without nucleus as M. mymaripenne).

However, getting so small usually entails some consequences:

• Simplification or loss of certain physiological functions: loss of wings (and, consequently, flight capacity), legs (or extreme modifications), mouthparts, sensory organs.
• Considerable changes in the effects associated with certain physical forces or environmental parameters: capillary forces, air viscosity or diffusion rate, all of them associated with the extreme reduction of circulatory and tracheal (or respiratory) systems. That is, being smaller alters the internal movements of gases and liquids.

So, does miniaturization have a limit?

The answer is yes, although insects seem to resist to it.

There are several hypotheses about the organ that limits miniaturization. Both the nervous and the reproductive systems, as well as the sensory organs, are very intolerant to miniaturization: they must be large enough to be functional, since their functions would be endangered by a limited size; and so, the multicellular life.

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Multicellular life reduction seems to have no limits. Will we find an even smaller insect? Time will tell.

Main picture: link.

Forensic entomology: arthropods at the crime scene

Unavoidably, every organism’s life comes to an end sooner or later. But where the cycle of life ends for some, others will find their opportunity to start a new life. Insects and other arthropods are some of the organisms that take advantage of dead animal rests to develop, and their study offers us valuable information to set the time, place and circumstances of someone’s death (something of special interest for criminologists). How is this information obtained from the study of arthropods? Keep reading to find out the answer.

Origins of forensic entomology

Forensic entomology is a branch of applied entomology that uses insects and other arthropods as scientific evidences to aid legal investigations; however, its most well-known use is the medicolegal one. Medicolegal forensic entomology is focused on the study of insects and other arthropods that inhabit decomposing remains or corpses to determine the time passed after their death, as well as to clear up the circumstances and determine the location where it took place. It’s a useful tool for criminologists since it can help to verify the alibi of a suspected assassin or to help in the identification of a victim.

Human skull with dermestid beetles. Public domain.

Forensic entomology is not a modern discipline. The concept of forensic entomology and the first case resolved by applying this discipline dates to at least the 13th century in China: the identity of the assassin of a farmer was discovered when all the suspected were gathered and forced to leave their sickles on the ground; then, flies were attracted only to one sickle, because it still had remains of blood and tissues on its edge.

Back then, the use of forensic entomology was anecdotal and their bases were not well-known. It was not until the 17th century that Francesco Redi refuted the “spontaneous generation” theory, which defended the idea that life comes from non-life (organic and inorganic matter) and that no causal agent is needed. Through different experiments, Redi proved something that seems us logical nowadays: life comes through life, so that insects that develop on corpses were already there before we noticed them (either as eggs or larvae).

Unconsciously, Redi’s experiments revealed some more facts: for example, that both location and climatology agents which a corpse is exposed to determine the composition and abundce of insect’s populations. This is very interesting since it helps us to find out the exact location where a death took place and if the corpse was moved to another place.

Historically, it was not until the 19th century that Bergeret, a French doctor, along with the discoveries of Orfila (who listed and described more than 30 insects and other arthropods which colonize dead bodies) and Redi, expanded and systemized forensic entomology. However, it’s considered that the true birth of this discipline took place at 1894, when J. P. Mégnin published his most famous paper: ‘La faune des cadavres: Application l’entomologie a la medicine legale’.

Uses and application of forensic entomology

When criminologists face a crime, they ask themselves three basic questions: ‘How’, ‘When’ and ‘Where’. Forensic entomology can respond correctly to those of the moment and place of death.

When?

From a legal point of view, it’s essential to estimate the time elapsed since the victim’s death. This time lapse is known as Post-Mortem Interval (PMI). In human corpses, this interval can be estimated through three methods: histological (temperature, stiffness, cadaveric lividity…), chemical (measurement of the level of different chemicals) and zoological (animal action and insects’ invasion). Also, we must consider the deterioration level of plastic tissues, clothes, etc. However, after 72h the most efficient method to estimate de PMI is forensic entomology.

There exist two ways to estimate de PMI using arthropods:

• Establishing the age and development rate of larvae. This method is mainly used at the first decomposition stages of dead bodies.

Calliphora sp. larvae on a dead body. Author: Hans Hillewaert, CC.

• Establishing the composition and level of development of arthropods’ communities to be then compared with the natural patterns observed on near habitats. This method is mainly used at advanced stages of corpse decomposition.

Where?

The place of the death strongly determines the species of arthropods we can find in a dead body as well as the succession patterns of their communities. Among the most determinant parameters of arthropods’ communities’ composition we highlight the biogeographical region (species from tropical and temperate regions are almost never the same), the season (at median latitudes, seasonality has an important role on biological cycles) and the specific traits of the habitat (moisture, solar radiation, accessibility and exposition degree, etc.), which can make more or less easier the colonisation of arthropods and, consequently, to alter the PMI estimation.

Along this article, you will notice we only refer to terrestrial arthropods: this is due to the difficulty and complexity to determine the PMI and the place of death at marine habitats.

The main actors: the arthropods

Classification

Arthropods we can find in a corpse are classified in four main groups:

• Necrophagous: they feed directly on dead remains and constitute the main group of arthropods we can find in a corpse. Necrophagous include basically dipterans (families Calliphoridae, Sarcophagidae, Muscidae, Phoridae…) and coleopterans (families Silphidae, Dermestidae…) .
• Predators and parasites of necrophagous: they’re the second most relevant group of arthropods in a corpse. It is formed by coleopterans (families Silphidae, Staphylinidae, Histeridae), dipterans (families Calliphoridae, Stratiomydae) and parasitic hymenoptera of larvae and eggs of dipterans (e.g., Ichneumonidae) which were previously on the dead body.
• Omnivorous: wasps, ants and coleopterans which feed both on body remains and remains of other arthropods.
• Accidental species: those species that use the dead body as an extension of their own habitat, so that they greatly vary according the external conditions (collembola, spiders, millipedes and centipedes, mites, etc.).

To know more about organisms’ relationships, you can read ‘Symbiosis: relationships between living beings‘.

The corpse colonisation step by step

Despite the specific variations depending on each case, colonisation and succession of populations of arthropods in a corpse follow a quite constant pattern.

1. Immediate principles degradation
   

Some dipterans (Calliphoridae and/or Sarcophagidae) feel attracted to gases expelled by the body during the first stages of decay (ammoniac, sulfhydric acid, nitrogen, carbon dioxide). Then, they lay their eggs inside natural holes (eyes, nose and mouth), in wounds or on the surface in contact with the substract, which has a high moisture level due to the accumulation of bodily fluids. However, their sense of smell is so developed that they sometimes land on the body when the person is still alive, especially when it has open wounds!

You can learn more about insects communication and senses through the article “How do insects communicate?“.

We rarely see these two families coexisting at the same time in a corpse, probably due to the fact Sacrophagidae larvae prey on the ones of Calliphoridae.

Calliphora vicina (left); Sarcophaga carnaria (right). Authors: AJC1, CC; James K. Lindsey, CC.

Is essential to know the development degree of larvae and pupae of each species, as well as their length of their life cycles and their specific traits to estimate the PMI. These parameters may vary among the species, also due to the external conditions and/or the death circumstances; moreover, their presence is so common that their absence can result informative as well.

 2. Butyric fermentation of fat

When fermentation of fats begins, there appear the first coleopterans (Dermestidae) and lepidopterans (e.g. the moth Aglossa pinguinalis), being very common in one month old corpses. While Dermestidae life cycle lasts 4-6 weeks (larvae feed on fats and moults of preview arthropods), the one of lepidopterans such as A. pinguinalis can last until the next spring if external temperatures are not adequate for their development.

Dermestes maculata (left); Aglossa pinguinalis (right). Authors: Udo Schmidt, CC; Ben Sale, CC.

 3. Caseic fermentation of proteins

During this stage of the body decay, there appear dipterans which are also very common during other fermentation processes, such as the one of cheese or ham (Piophila sp., Fannia sp., as well as Drosophilae, Sepsidae and Sphaeroceridae genres). There also appear coleopterans of the genus Necrobia.

Piophilia casei (left); Necrobia violacea (right). Authors: John Curtis, Dominio Público; Siga, CC.

 4. Ammoniacal fermentation

During this stage, there appear the last dipterans (genus Ophira and family Phoridae, essentialy), which can be found inside bird nests or lairs feeding on animal remains, excrements and organic wastes from their hosts. There also appear different groups of necrophagous coleopterans of the genera Nicrophorus, Necrodes and Silpha, very common during advanced decay stages, and predator coleopterans of the families Staphylinidae (genera Coprohilus, Omalium and Creophilus) and Histeridae (genera Hister and Saprinus).

Nicrophorus humator (left); Coprophilus striatulus (right). Authors: Kulac, CC; Udo Schmidt, CC.

 5. Disappearance of rests

After more than 6 months, the corpse is almost totally dry. In this moment, there appear a huge number of mites of different species that feed on mildew and fungi covering the rests. Posteriorly, there also appear coleopterans that feed on hair rests and nails (Dermestes, Attagenus, Rhizophagus, etc.), some Dermestidae species present on previous stages and some lepidoterans.

After more than a year, the scarce rests are sometimes attacked by coleopterans of the genera Ptinus, Torx and Tenebrio.

Tenebrio obscurus. Author: NobbiP, CC.

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Forensic entomology is just one example of how useful can be to study insects and other arthropods both from a taxonomical and an ecological point of view. However, there exist many more applications. Do you know them or want to learn about them? You can leave your suggestions and curious facts on the comments’ section below.

References

• Entomología Forense. Colegio de Postgraduados.
• Joseph, I., Mathew, D. G., Sathyan, P., & Vargheese, G. (2011). The use of insects in forensic investigations: An overview on the scope of forensic entomology. Journal of forensic dental sciences, 3(2): 89.
• Magaña, C. (2001). La entomología forense y su aplicación a la medicina legal. Data de la muerte. Boletín de la Sociedad Entomológica Aragonesa, 28(49): 161.

Main photo made by the author of this post using different images (fly vector dessigned by Freepik on www.flaticon.com with a licence CC 3.0 BY).

What are parasitoid insects and what are they useful for?

Almost everybody could explain you more or less accurately what both parasites and predators are. But could everybody say you what a parasitoid is?

Animals (and especially insects) set up a lot of different symbiotic relationships, but often we find organisms whose relationship is somewhere between one and another (this is not a matter not of black or white!). In the case of parasitoid insects, we talk about organisms that establish a symbiotic relationship with traits of both predator-prey relationships and a parasitic ones.

Read this article to find out what parasitoid insects are, which is their origin and which kind of parasitoid insects exist. They are more useful than they seem to be!

Parasites, parasitoids and predators

Parasitoids are not exclusively insects, but the greater part of parasitoids belong to the subphyllum Hexapoda. For this reason, I will focus my explanation on parasitoid insects.

Before giving you further explanations, we must make the differences between parasitoids, parasites and predators clear.

In a parasitic relationship, parasites benefit at the expense of other organisms, the hosts, which are damaged in result. But despite of hurting it, parasites try to keep their hosts alive as long as possible in order to keep on benefiting from them, so parasites rarely kill their hosts.

Aedes albopictus female (tiger mosquito or forest mosquito) biting its host (Public domain).

In a predator-prey relationship, predators feed on a lot of organisms (the prey) throughout their life cycle in order to keep on developing. Unlike parasite organisms, predators don’t try to keep their prey alive so long, because the purpose of preying on other organisms is to obtain energy as faster as possible (for example, mantids, dragonflies…).

Mantis eating a prey (Picture by Avenue, CC).

Finally, between parasitism and predation we find parasitoid organisms: insects with a parasitic larval stage that develop by feeding on a single host, which is usually another insect or arthropod. In contrast with parasites, parasitoids larvae kill their hosts to complete their life cycle; so, in which sense are they different from predators? The answer is that parasitic larvae only need to feed on a single host to reach adulthood. While parasitoid larvae are a parasitic life form, parasitoid adults tend to be herbivores or predators.

Caterpillar of the lepidopteran species Hasora badra surrounded by wasp cocoons of the family Braconidae (Picture by SoonChye ©).

Origin and diversity of parasitoids

Parasitoid insects are present in many insect orders (Coleoptera, Diptera..), but the greater part of them is located in the Hymenoptera order (bees, wasps and ants). Because of that, in this section I will focus on talking only about the origin and diversity of hymenopteran parasitoids.

The most important and also evolved group of hymenopterans is the suborder Apocrita, which includes wasps, bees and ants. In turn, the suborder Apocrita is divided in two artificial groups:

• Aculeata: they don’t have a parasitic larval stage. The ovopositor (an organ that females use to lay their eggs) has been transformed into a sting that inoculates venom (organisms of this group are also called “stinging wasps and bees”).

Sting of a female bee (Apidae) (Public domain).

• “Parasitica”: they have a parasitic larval stage. Adult females of the group Parasitica have a long and sharp ovopositor they stab into different surfaces (wood, another insect…) so they can lay their eggs inside. In contrast with Aculeata, Parasitic hymenopterans don’t sting (they’re not venomous).

Parasitoid female bee of the species Megarhyssa macrurus, family Ichneumonidae, with its long and sharp ovopositor she use to lay their eggs (Picture by Bruce Marlin, CC).

About 77% (66.000 species more or less) of parasitoid insects known nowadays belong to the Parasitica group, and most of them are wasps.

Origin of hymenopteran parasitoids

To understand the origin of certain morphological, anatomical or conductual traits of an organism, we often have to study the traits of a “sister taxon” or “sister group”, i.e. a group more closely related to the group in question than any other group (they share the most recent common ancestor).

The sister group of Apocrita is the family Orussidae (from the Symphyta suborder), which is also considered the most ancient groups of hymenopterans.

Orussus coronatus (Fam. Orussidae) (Public domain).

It’s believed that the common ancestor of Apocrita and Orussidae had first developed a parasitic life form among hymenopterans. This conclusion is based on the studies about ecologic traits of current Orussidae specimens: some of these organisms establish a positive relationship with some symbiotic xylophagus fungi (i.e. fungi that feed primarily on wood); these fungi usually develop inside a sort of tiny baskets located over the surface of ovopositors, so they can be inoculated inside the wood the Orussidae feed on when laying. Thus, fungi process wood to obtain a product that can be digested by Orussidae. However, there exist Orussidae specimens which don’t establish this kind of symbiotic relationship and parasite other specimens instead (especially the ones that possess symbiotic fungi). Thus, these parasitic Orussidae obtain nutrients by feeding on other Orussidae members and obtain more energy in result.

So, this being an ancient group it’s believed that the observed behavior in some current Orussidae members could be a reflection of the ancient origin of parasitism and parasitoids among the Hymenoptera order.

Types of parasitoids

Even if there are many ways to classify parasitoids, we can divide these organisms mainly into two groups: the ones that stop host’s development when laying inside it and the ones that don’t stop host’s development. Let’s talk about these two groups:

Idiobionts

Idiobiont parasitoids paralyze or prevent further development of hosts when laying, so parasitoid larvae could have a reliable and immobile source of food at their birth.

Usually, idiobionts attack hosts that are concealed in plant tissues (for example, wood) or exposed hosts that possess other kinds of physical protections, so female parasitoids have developed long and sharp ovopositors that allow them to pierce these barriers.

Liotryphon caudatus female (Hymenopteran of the family Ichneumonidae, superfamily Ichneumonidea) with her long and sharp ovopositor (Picture by CNC/BIO Photography Group, Biodiversity Institute of Ontario, CC).

Idiobiont parasitoids can be both ectoparasitoids and endoparasitoids (i.e. if larvae attack hosts from outside or inside host’s body), although mostly are ectoparasitoids. Moreover, parasitoid larvae feed on hosts only on the last development stages until the moment they reach adulthood.

Ectoparasitoid idiobiont females first inject venom into the host, to induce temporary or permanent paralysis, and then ovoposits on or near the immobilized host. In some cases, females that have just layed their eggs stay near the lay to protect it and also to prevent host to be eaten by other organisms.

Femella d’un himenòpter de la subfam. Pimplinae (fam. Hymenopteran female from the subfamily Pimplinae (family Ichneumonidae) stabbing her ovopositor in a trunk surface to lay eggs (Picture by Cristophe Quintin on Flickr, CC).

Generally, idiobiont adult females don’t have any preference when looking for a place to proceed on egg laying, so larvae feed on a wide variety of organisms.

Koinobionts

Most of parasitoid insects (and especially hymenopterans, dipterans and coleopterans) are koinobionts.

Unlike idiobionts, almost all koinobionts are endoparasitic and lay their eggs directly inside the host, which can be both exposed and concealed. However, the trait that truly differentiates koinobiont parasitoids from idiobiont parasitoids is the fact that koinobionts allow the host to continue its development while feeding on it. Thus, the parasitic larvae feed on the host while growing inside host’s body without causing it any damage…until the moment larvae reach the adulthood, when they emerge from the body of the host, causing its death.

Aleiodes indiscretus female (Hymenopteran from the family Braconidae, superfamily Ichneumonoidea) inoculating eggs inside the body of a gypsy moth larvae (Lymantria dispar) (Foto de domini públic).

Once the parasitic larvae are inside host’s body begin to grow to reach the pupal stage. Until this moment, larvae use different mechanisms to avoid or block the immune response of the host (for example, by placing eggs in hosts tissues where immune system doesn’t work). So, larvae can develop by feeding on host’s nutrients until the moment they metamorphose, when adult parasitoids emerge from inside the body of the host, killing it consequently.

Due to the close relationship established by parasitoids and hosts, koinobiont parasitoids tend to be less generalist than idiobionts when looking for a suitable host.

Ecological function of parasitoids

Parasitoids, like predators or parasites, perform an important ecological role because they act as natural regulators of other organisms populations. So, parasitic larvae kill a lot of organisms that could damage the environment or even other organisms if their populations grow excessively. Thus, the disappearance of parasitoids (just like predators or parasites) could entail an excessive increase of some animal populations (especially other insects populations).

For that reason, parasitoids are considered as a great biological control agent against different plagues in gardens and crops.

Tobacco hornworm (Manduca sexta) being attacked by a parasitoid wasp of the superfamily Braconidae. In this picture, the larvae of the wasp have reached the pupal stage (white rice-shaped cocoons) and, at the end of pupation, adults will emerge, killing the hornworm. Tobacco hornworm is considered a harmful plague for plants of the family Solanaceae (like tobacco, tomato and potato) (Foto de R.J Reynolds Tobacco Company Slide Set).

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References

• Notes from the subject “Biology and Biodiversity of Arthropods” taken during my Biology studies at Universitat  Autònoma de Barcelona (UAB).
• Timothy M. Goater, Cameron P. Goater, Gerald W. Esch (2013). Parasitism: The Diversity and Ecology of Animal Parasites. Ed. 2. Cambridge University Press.
• Vincent H. Resh, Ring T. Cardé (2009). Encyclopedia of Insects. Ed.2. Academic Press.
• Donald L. J. Quicke (2014). The Braconid and Ichneumonid Parasitoid Wasps: Biology, Systematics, Evolution and Ecology. John Wiley & Sons.
• http://bugs.bio.usyd.edu.au/learning/resources/Entomology/internalAnatomy/imagePages/parasitoidWaspLarvae.html

Main image by Ton Rulkens (Flickr, CC).