Arxiu d'etiquetes: adaptive radiation

Evolutionary adaptations of feeding in insects

Over millions of years, insects have got adapted to countless ecological changes. On previous articles, we talked about flying adaptations in insects and how flying made them more diverse. In this new article, we explain you the origin and evolutionary changes of insects’ mouthparts and therefore of feeding diversification throughout their evolutionary history.

Introduction: Entognatha vs Ectognatha

Before talking about feeding evolution of insects, we must state the differences between the terms “insect” and “hexapod”. Insects constitute the major and most diverse class of the subphylum Hexapoda. This class includes the best known families of insects: lepidopteran, hymenopteran, coleopteran, dipteran, etc. However, this subphylum also includes three orders of wingless arthropods which together constitute the class Entognatha: Collembola (springtails), Protura and Diplura.

So, the subphylum Hexapoda includes two classes: Insecta and Entognatha. Which is the main difference between them? Essentially, the position of their mouthparts: on one hand, Entognatha (ento- ‎(“inside”) +‎ Ancient Greek gnáthos (“jaw”)) have their mouthparts protected inside the head and they only project them during feeding; on the other hand, Ectognatha or Insecta (ecto- ‎(“outside”)) always have external mouthparts.

ectognatha-vs-entognatha
Mouthparts of a beetle (Ectognatha, left) and anterior view of a springtail, with mouthparts retracted within the head (Entognatha, right). Source: beetle by Fyn Kynd Photography, CC; springtail by Gilles San Martin, CC.

Mouthparts of insects or Ectognatha

Both mouthparts diversification and feeding diversification are the result of a long evolutionary process. So, it’s expected that there exist ancestral and derived feeding structures.

The most ancestral mouthparts and those which has also suffered less adaptive modification are the mandibulate or chewing ones. These type of mouthparts are linked to solid food-based feeding and they can be currently observed in a lot of groups: crickets and grasshoppers; dragonflies and damselflies; beetles; cockroaches and mantis; mecopterans, neuropterans… and also in larval stages of some insects that develop a different type of mouthparts when reaching adulthood (e.g. butterfly larvae).

Mandibulate mouthparts are often used as a model to explain the evolution of mouthparts in insects due to their ancestral origin. The most used chewing model is the one observed on orthopterans (such as locusts or grasshoppers).

ortopthera-mouthparts-chewing
Mandibulate or chewing model of an orthopteran. Source: John R. Meyer, North Carolina State University. Link.

Based on this model, insect’s mouthparts are made of 5 main structures: labrum, mandibles, maxillae, hypopharynx and labium. Mandibles, maxillae and labium are considered true or appendicular appendages because they develop from metameres (also known as somites; segments in which their body is divided) during the embryonal development; thus, these three structures are considered equivalent to locomotor appendages from a morphological point of view. On the contrary, labrum and hypopharynx aren’t true appendages because of their non-metameric origin, although they are also considered buccal appendages due to their essential role in feeding.

What’s the function of each of these structures?

Knowing original functions of these structures on the mandibulate model lets us to understand the changes that have undergone the different adaptive forms emerged throughout the evolution of insects’ feeding:

chewing-mouthparts-beetle-grasshopper
Examples of mandibulate or chewing mouthparts: beetle (left) and locust (right). Source: John R. Meyer, North Carolina State University. Link.
  • Labrum. A plate-like sclerite located before de rest of feeding structures, protecting them. Its size varies among species and it helps to contain the food. The posterior surface is known as epipharynx.
  • Mandibles. A pair of jaws for crushing or grinding the food. They operate from side to side.
  • Maxillae. A pair of appendages which are divided in three parts: cardo, which articulates with the head; stipes, which supports a sensory palp; galea and lacinia, which act as fork and spoon to manipulate the food.
  • Hypopharynx. A little process located behind mandibles and between maxillae that helps mix food and saliva.
  • Labium. Unlike mandibles and maxillae, the two original appendages forming the labium have fused together along the middle. The labium is also subdivided in two parts: postmentum, pieces which articulate with the head; prementum, distal pieces which support a pair of sensory palps and divide apically forming four lobes: glossae and paraglossae.
mouthparts_mandibles_maxilla
Mandible, maxilla, labium and hypopharynx (Davies, 1991).

Evolutionary adaptations of mouthparts

How did they evolve?

It’s considered that all models of mouthparts originally evolved from an ancestral mandibulate form. However, it’s more than probably that this process took place in different groups simultaneously when insects started to expand in range, food became more accessible and new sources of food appeared. This is an excellent example of adaptive radiation (when two or more populations, exposed to different selective pressures, diverge from a common ancestor).

Thanks to fossil records (insects preserved in amber, coprolites and evidences of attacks on plants) we know that the appearance of all models of mouthparts took place in at least 5 periods 420-110 myr ago. Eventually, some groups changed from a solid-based diet to a liquid-based diet: exposed liquids (e.g. nectar), tissue liquids (e.g. sap or blood) or even suspended particles. For those which adopted a liquid-based diet, these changes involved a great adaptive advantage during the expansion of angiosperms (flowered plants) in the Cretaceous period.

img_1558-min-min
The adoption of a liquid-based diet by some insects, as the one seen in butterflies, involved a great adaptive advantage for these organisms during the expansion and diversification of flowering plants. Moreover, this gave room to the start of a coevolutive process between insects and plants. Author: Irene Lobato.

Types of mouthparts

On the basis of the mandibulate type, let’s see a summary of the main adaptive modifications observed in different types of mouthparts:

MANDIBULATE-LAPPING TYPE

Mandibulate-lapping mouthparts are linked to a liquid-based diet (e.g. nectar), even though in some cases they conserve the chewing function. They’re typical of hymenopterans. Sawflies or suborder Symphyta, considered the most ancient group of hymenopterans, conserve almost all original structures and functions of mandibulate mouthparts. Both wasps and bumblebees have undergone a reduction of both mandibles and maxillae and a massive development of labial glossae, forming a kind of tongue for drinking liquid food; however, they can still chew. Finally, bees have mandibles not for feeding, but for other purposes (such as fighting, grooming theirselves or working wax scales into honeycomb), and both maxillae and labial glossae lengthen giving room to a hairy tongue with an internal duct (the salivary duct), so their diet is exclusively liquid-based.

bee_mouthparts_eng
General scheme of lapping mouthparts of a bee (left; image by Xavier Vázquez, posteriorly modificated by Siga, CC) and mouthparts of a bee of the species Colletes willistoni (right; public domain image). Md: Mandibles; mx: Maxillae; lb: labium.

SUCKING-LAPPING TYPE

In this kind of mouthparts, mandibles undergo a massive reduction (and if present, they’re not for feeding purposes), even disappearing in some cases; so, insects with sucking-lapping mouthparts have a diet exclusively based on exposed liquids. There exist two main variations of this model: the ‘maxillar sucking’ or siphoning type typical of evolved lepidopterans and the ‘labial sucking’ or sponging type typical of flies and other dipterans.

In flies, mandibles are totally absent, maxillae are only represented by maxillary palps and posterior part of labium massively increase, forming two lobes which are sponge-like organs called the labella. The labella is a complex structure consisting of many grooves which sops up liquids much like a sponge does.

fly-mouthparts
Sponging mouthparts of flies (left; source: Educational Media Group (EMG), RMIT University, 2002-06-01, Fly mouthparts illustration [Online, Image Illustration], Educational Media Group (EMG), Melbourne, Vic) and mouthparts of a fly of the species Gonia capitata (right; by Richard Bartz, CC).

In evolved lepidopterans, mandibles and labium are almost absent (only labial palps are visible), while maxillary galeae develop forming a long proboscis also known as ‘haustellum’ with a central alimentary duct for sucking liquids.

proboscis-butterfly
Siphoning mouthparts of a butterfly (left; by tdlucas5000, CC) and electron microscopy image of the proboscis (right; public domain image).

PIERCING-SUCKING TYPE

This type of mouthparts appears in different groups of insects with independent evolutionary lineages, so there exist lots of variations. Let’s see some examples:

  • Heteroptera (bugs): they’re the only ones which possess this type of mouthparts since the very moment of birth. Both maxillary and labial palps are absent in these organisms, and labium forms a duct that encloses 4 stylets: two maxillary stylets and two mandibular stylets. This structures configure the beak or ‘rostrum’. Maxillary stylets delimit a salivary duct and a food duct, and together with mandibular ones allows the organism to pierce different tissues and then soak up their liquids: sap in phytophagous forms and blood in predatory ones.
heteroptera-min
Scheme of the piercing-sucking mouthparts of an heteropteran (left; image from Baker, 2011) and mouthparts of a predatory bug of the family Reduviidae (right; image property of John R. Meyer, North Carolina State University. Link).
  • Mosquitoes: their mouthparts are very similar to the ones of bugs; however, they possess one more stylet, corresponding to the hypopharynx, which contains the salivary duct (through which they inject different substances to their hosts, such as anticoagulants). Labrum and hypopharynx together form the food duct, and labium has only an assistant function of supporting the stylets.
mosquito
Scheme of mosquito’s mouthparts (left; by Xavier Vázquez, posteriorly modified by Siga, CC) and a mosquito female (right; by Grzegorz “Sculptoris” Krucke, CC). Lr: labrum; hp: hypopharynx; mx: maxillae; md: mandibles; lb: labium.
  • Phthiraptera and Siphonaptera (lice and fleas): their mouthparts are formed by the epipharynx, both labial palps and both laciniae of maxillae. Maxillary palps are well developed and are always situated before the rest of the structure. Lice and fleas use their mouthparts to parasite their hosts, piercing their tissues and then sucking their blood.
head_of_the_flea_lens_aldous_tagged-min
Mouthparts of Siphonaptera (fleas): 1: eye; 2: labial palps; 3: maxillar stylet (lacinia); 4: hypopharynx; 5: maxillary palps; 6: maxilla (galea). Source: public domain.
  • Thysanoptera (thrips): these tiny insects usually appear as pests in agricultural crops, sometimes even being vectors of different plant viruses. Their mouthparts present right-left asymmetry and the piercing structure is formed by the labium, the labrum and maxillae. Delimited by all these structures, there are also two maxillary stylets and only one mandibular stylet (the other one become atrophied). Thrips scratch the plant surface and then pierce it by their stylets, through which they suck plant fluids.
trips-min
Scheme of thrips’ mouthparts (lefts; image form personal notes from the course “Biology and Diversity of Arthropods”, Universitat Autònoma de Barcelona) and frontal view of a thrip (right; property of John W. Dooley, USDA APHIS PPQ, Bugwood.org, CC).

AN EXTREME CASE: THE ATROPHY

Adult forms of some insects, such as mayflies (Ephemeroptera) or some dipterans, suffer a total reduction of their mouthparts. In these cases, the only function of adults is down to reproduction, so they lose all feeding functions and structures when metamorphose.

.           .            .

There’s no doubt that insects form the most diverse group of organisms all over the world, showing not only a huge amount of species, but a big range of forms of mouthparts.

Do you know any other curious feeding structures in insects? Feel free to share your opinion or contributions in the comments. 

REFERENCES

There have also been consulted the personal notes taken from the subject “Biology and Diversity of Arthropods” given during the course 2013-2014 at the Universidad Autònoma de Barcelona.

Main photo, from left to right: 1) Lisa Brown, CC, 2) Public domain and 3) Richard Bartz, CC.

Difusió-anglès

The living space of organisms

We all have our own living space, the place where we feel comfortable, like we were at home. We also have our routines, habits and that list of preferences that make us unique. Each of us, ultimately, have our own ecological niche, an extensive concept for each species that share the Earth with us. From it comes an important ecological processes such as competition or speciation, a key concepts for understanding the assembly and dynamics of natural ecosystems.

INTRODUCTION

When you are asked how you would describe close people, the first thing that comes to your mind is their way of being when you’re with them and what they loves to do. We know what is the first thing they always ask in a restaurant, what annoys them, what sites they like to frequent, what they like to do when they have free time and even how they behave when they like someone. If we have also lived with them, we could guess almost their daily routine since they wake up until they go to bed. Although we do not always have the same behaviour, there are many traits, hobbies and routines that characterize and differentiate us. Each of us have our comfort zone, our hobbies, food preferences and people with whom we love spending our free time.

7852026050_3fd72271f8_b
The dietary preferences of each of us and our routines and hobbies serve as a comparison to illustrate the diversity of ecological niches in the natural world. Source: Flickr, George Redgrave.

THE ECOLOGICAL NICHE OF A SPECIES

This “living space” that all of us have and in which we feel identified, is also comparable to the ecological niche of the organisms. The ecological niche of a species is a concept that always has been presented us as the “occupation”, “profession” or “work” that an organism carries up in the place where it lives (Wikipedia or CONICET), but the definition includes more than that. Hutchinson (1957) defined it as: ” n-dimensional hypervolume, where the dimensions are environmental conditions and resources, that define the requirements of a species to persist over time.” Despite the confusing definition, it is interested to point out the term “n-dimensional” as the ecological niche is based on this idea. An ecological niche is nothing more than all those multidimensional species requirements. In other words, the ecological niche of a species would be everything that involve the species and make it to prosper and survive where it is. Refers, ultimately, to all those variables that affect them in their daily lives, both biological variables -the contact with other species- and the physical and chemical ones-the climate and the habitat where they live-. An ecological niche of a species would be the spectrum of food it eats or can consume, the time of the day in which it is active to perform its functions, the time of the year and the way it carries out the reproduction, the predators and preys, the habitat it tolerates and all those physical and chemical factors that allow this species to remain viable.

warblers
These 5 species of warblers of North America seem to occupy the same habitat (the fir), but actually not. The truth is that each warbler occupies a different position in the tree. Source: Biology forums.

To give an illustrative example, let us place ourselves in the African savannah. The main grazing ungulates and those which perform mass migrations are compound by zebras, wildebeest and Thomson’s gazelles. At first glance, you might think that their ecological niche is very similar: same habitat, same routine, same predators and same food. The same food? Absolutely not. During migration, zebras go ahead, devouring tall grass, which is the worst quality. They are followed by wildebeest, which eat what remains standing, and these are followed by Thomson gazelle, which eat the high-quality grass, which is starting to grow again.

picg8
Although at first glance it may seem that feed on the same food, each species focuses on a different part of the plant. Source: Abierto por vacaciones.

CAN TWO SPECIES LIVE TOGETHER WITH THE SAME NICHE IN THE SAME PLACE?

The competitive exclusion principle, proposed by Gause (1934), states that two species occupying the same niche can not coexist in the long term as they come into competition for resources. Thus, in a competitive process for the same ecological niche, there is always a winner and a loser. In the end, one of the competitors is imposed by another, and then two things can happen: the extinction of the loser one (image A) or a traits displacement in order to occupy another niche (image B). In fact, the competitive exclusion principle is behind the current problems with invasive species. Invasive species niche is very similar to native species niche and, when they converge in the same habitat, the invasive species end up displacing native species, as they are better ecological competitors. It also often happens, of course, the opposite: the exotic species is worse than its counterpart and the competitor fails to thrive in the new environment.

20_08_competitvexclusion-l1335993938015
Image A | This study was conducted in order to observe the effect of competitive exclusion in two species of protists. Both species occupy almost identical ecological niches, but they are not living together in nature. The density of one falls sharply when they are forced to share the same space, until it eventually disappears. This same process occurs with invasive species. Source: Jocie Broth.
pinzones
Image B | When the 3 species of Darwin’s finches (in different colors) coexist on the same island, a trait displacement occurs by competitive exclusion. Individuals from the ends tend to have very similar bill depths to those of the other species, resulting in a niche overlap and subsequent competition. The final boundaries are established thanks to this process. Source: Nature.

THE FUNCTIONAL EQUIVALENCE

We have seen that to share ecological niche is synonymous of having conflict between species. However, there is a situation in which problem do not take place. The hypothesis of functional equivalence proposed by Hubbell proclaims that if the niches are identical and the species life parameters (fertility, mortality, dispersion) are also the same, none of them has a competitive advantage over the other, and the battle ends in tables. This fact seems to occur only in a very stable ecosystem in a Panama rainforest island (Barro Colorado). Different species of trees, as having almost identical parameters of life, do not compete between them and are distributed randomly, as if the individuals of different species belong to the same species. Furthermore, it seems that speciation in this kind of rainforest could also occur by chance, which would have caused the high density of species that harbor these forests.

1503897694_2ab5f7ba2e_b
Tropical forests have a tree species density unique in the world. One hectare of tropical forest may contain up to 650 tree species, more than the number of tree species present in both Canada and continental US. Will Hubbell’s functional equivalence theory be behind the explanation for this curious fact? Source: Flickr, Jo.

NEW NICHES, NEW SPECIES

Speciation, or the creation of new species, usually occurs when new ecological niches are created or the existing become unoccupied. In both cases, to occupy a new ecological niche imply a gradual differentiation from the initial population to become a genetically distinct species. As an example of formation of new ecological niches we have the case of the emergence of angiosperms. Their booming opened many new possibilities, thanks both to increasing diversity of seeds and fruits (which, in turn, increased the number of specialized species) and the emergence of complex flowers, which allowed the explosion of many pollinators (facilitating the emergence of new insectivores). As an example of unoccupied niche, there is the famous case of the extinction of non-avian dinosaurs. Dinosaurs dominated a lot of niches, from land to air ecosystems, and even the aquatic environment. Those empty niches was occupied by many mammals, thanks to their high fertility and plasticity (flexibility to adapt into different habitats). That eventually led large ratios of speciation in a short time, what is known as adaptive radiation.

Eomaia_NT
This is Eomaia scansoria, an extinct species of mammals that lived at the same time as the dinosaurs. The extinction of the dinosaurs opened up a wide range of possibilities to mammals, which, although they were expanding, remained in the background. Their great plasticity led them to colonize many habitats, by occupying the free ecological niches left by the dinosaurs. Source: Wikipedia.

ASSEMBLY OF COMMUNITIES

As we have seen, the ecological niche is behind fundamental ecological and evolutionary processes. All living communities today have been formed thanks to the niches of different species. Through competition, species niches were overlaping, and the communities were assembled like a puzzle. When a piece disappears, another takes its place, playing the role that the other had in the community. However, knowing the whole ecological niche of a species is arduous and, in most cases, impossible. As in human relationships, an exhaustive knowledge of everything that influences the life of a species (or the living space of a person) is of great importance in order to ensure their long-term preservation.

REFERENCES

Ricard-anglès