CAROS: CHARACTERISTICS, HABITAT, REPRODUCTION, FEEDING - BIOLOGY - 2024

The mites (Acari) are a subclass of arachnids which are characterized by small size, which can sometimes be microscopic. They are found all over the world and are even found on the body surface of some hosts, mainly mammals.

Within this group a total of 50,000 species have been described. However, specialists express that there are many more that remain unknown. Perhaps the small size of each specimen is what has made its discovery and description difficult.

Mite seen under the electron microscope. Source: Pixabay.com

Among the mite species, there are known parasites, both of animals and plants, that can become a serious health and nutritional problem, since they sometimes affect large areas of food crops.

characteristics

Mites are eukaryotic and multicellular organisms, made up of cells that have specialized in various functions such as nutrition, reproduction and excretion, among others.

Likewise, during the early stages of their embryonic development they present three germ layers: ectoderm (external), mesoderm (intermediate) and endoderm (internal). From these layers the different tissues and organs that make up the individual are formed.

Similarly, mites are coelominated animals. This is also related to embryonic development, since during this an internal cavity called coelom is formed.

Likewise, from another embryonic structure, the blastopore, both the mouth and the anus are formed simultaneously, which makes them protostomized animals.

Regarding symmetry, mites are animals with bilateral symmetry, since they are made up of two halves that are exactly the same, taking into account the longitudinal plane.

Mites are dioecious and reproduce through sexual mechanisms, with internal fertilization and indirect development. In this same order of ideas they can be oviparous, ovoviviparous and viviparous.

Taxonomy

The taxonomic classification of mites is as follows:

• Domain: Eukarya
• Animalia Kingdom
• Phylum: Arthropoda
• Subphylum: Chelicerata
• Class: Arachnida
• Subclass: Acari

Morphology

Mites are organisms that generally have a small size, even being microscopic. It can reach up to 3 cm.

-External anatomy

As with all organisms belonging to the arthropod phylum, the body of mites is divided into segments or tagmas, which receive different names, depending on the type of mite.

In this sense, in parasitiform mites, the anterior region is known as the gnatosome, while the posterior region is called the idiosome. On the other hand, in acariform mites, the anterior region is the proterosome and the posterior region is the hysterosome.

Anterior segment (gnatosoma - idiosome)

It is the smallest segment of the body of a mite, compared to the second segment. It is separated from this by a suture that is very evident. In this tagma is where two characteristic elements of chelicerates are found: chelicerae and pedipalps.

With regard to chelicerae, these are generally made up of three segments and are modified according to the function they fulfill according to the species of the animal. This is how they can perform functions of biting, chopping and sawing, among others. Likewise, chelicerae end up in clamps most of the time.

With respect to the pedipalps, they are made up of six segments, of which the coxa, the initial segment, is fused with the gnatosome. They can also be modified to fulfill various functions.

Posterior segment (idiosome - hysterosoma)

This is the largest segment of the animal. It is shaped like a sack. Likewise, some species of mites have simple eyes and stigmas, the latter being holes through which they breathe.

Several articulated appendages, the legs, are detached from this segment. They are found in pairs, specifically four pairs.

Ventral and dorsal view of a mite of the order Mesostigmata. Source: SE Thorpe

The legs are made up of a total of seven joints, from distal to proximal: pretarsus, tarsus, tibia, genu, femur, trochanter and coxa. This constitutes a characteristic element of this group, since they are the only arthropods that have so many knots on their extremities.

On the legs there are a series of sensory organs, these being more abundant in the front legs. Likewise, some legs are modified at their distal end, developing a kind of claw.

Likewise, on the ventral surface of this segment of the animal there are two orifices: the anal and the genital, which have the function of eliminating waste substances and the reproduction process.

-Internal anatomy

Nervous system

It is of the ganglionic type. It is mainly made up of two nerve cords that occupy a ventral position in the body.

They also present a series of structures that are responsible for capturing sensory stimuli. These structures are compound eyes and sensitive hairs that are distributed over the entire surface of the animal.

Digestive system

The digestive system of mites corresponds to that of arachnids. It is made up of three regions: the stomodeum, mesodeo, and proctodeo.

The stomodeum is the initial part of the digestive tract. It is made up of the mouth, the pharynx, and the esophagus. The mesodeo is made up of intermediate structures, essentially the intestine, which is where most of the absorption of nutrients takes place.

Lastly, the proctodeum is the final segment of the digestive system. It contains the anus, which is where waste from the digestive process is eliminated.

Respiratory system

The type of respiratory system that predominates in mites is the tracheal. This is made up of a series of tubes that are known as tracheas.

The tracheas branch out inside the animal's body, forming smaller and smaller tubes called trachealas. These reach each of the cells of the animal to bring them the oxygen they require to develop.

Likewise, the tracheae open outward through holes called spiracles.

Excretory system

The excretory system of the mites is quite rudimentary and simple. It is made up of a series of tubes known as Malpighi tubules. These collect the waste and flow to the proctodean level.

Likewise, it is possible that, like all arachnids, they present coxal glands that open at the level of the coxae of the animal's appendages.

Circulatory system

It is of the open type, also known as a lagoon. The most important organ is the heart, which is located at the level of the second segment of the mite's body (idiosome or hysterosoma).

Like any heart, its function is to pump blood to all the cells of the animal. For this it uses a series of conduits, analogous to the veins of other living beings. The fluid that circulates in the mites is the hemolymph, in which cells called amoebocytes can be found, which perform various functions such as coagulation.

Reproductive system

Mites are dioecious organisms, which means that there are female and male organisms.

However, the configuration of the reproductive organs can vary. There are species in which they are arranged in pairs, while there are others in which they can be found fused into a single structure.

The reproductive organs flow into an orifice that is in the ventral area of ​​the animal.

It is also possible to find other variations; for example, there are species in which females have a structure called spermatheca, in which they store the male's sperm.

In the case of males, there are species in which they have a copulatory organ with which they can directly transfer sperm into the female's body.

Habitat and distribution

Mites are a fairly diverse group that is widely distributed throughout the earth's geography. As worthy representatives of arthropods, they have managed to colonize practically all existing ecosystems.

It is possible to find species of mites in aquatic ecosystems, both marine and freshwater, and even in those whose environmental conditions could be hostile to other living beings, such as in the desert or the poles.

In the case of mites that are parasites, they must be found in the host's body in order to survive, be it a plant or an animal.

Classification

The Acari subclass is made up of a total of 3 superorders, which in turn comprise the orders:

Acariforms

Also known as actinotrichids. They are characterized by having extensions on the body surface known as silks, which are made up of the polysaccharide chitin.

This chitin has the ability, when seen under polarized light, to unfold the light beam into two rays perpendicular to each other. It covers the orders Oribatida, Trombidiformes and Astigmata.

Parasitiforms

Some of the most recognized mites such as ticks belong to this group. The coxae of these animals are free. Furthermore, its anterior segment is surrounded by a sclerotic ring, from which the first appendages emerge: pedipalps and chelicerae. It includes three orders: Mesostigmata, Holothryrida, and Ixodida.

Opilioacariformes

It is characterized because they are among the largest mites that exist. In addition, the length of their limbs greatly exceeds that of the body, being disproportionate. It encompasses a single order, the Opilioacarida.

Reproduction

Mites reproduce sexually, with the fusion of male and female gametes being evident.

In the reproductive process, the first thing that happens is that the male introduces the sperm into the female's body during copulation. This causes fertilization to take place within the female's body.

Once fertilization occurs, the female proceeds to lay the eggs. Depending on the species, the number of eggs can be high or low. It is even known of species in which the female lays about 300 eggs in each laying. The female begins to lay her eggs 3 or 4 days after fertilization.

When the eggs hatch, larvae emerge from them that are immature, that is, they do not have the characteristics of adult individuals of the species. This is why they need to go through a series of larval stages until they finally acquire the characteristics of adult individuals.

The period of sexual maturation of each specimen (male and female) is different. Males become sexually mature 2 days after the egg has hatched, while in females this is a much slower process.

These organisms have a different short life expectancy for the male and for the female. In the case of the former (male) they barely live for about two weeks, while the females live for a few more weeks.

Feeding

Depending on the species, mites can have different food preferences. This is how there are mites that are herbivores, other carnivores and also other blood-sucking.

Carnivores feed on other arthropods and even mites. Likewise, herbivores feed on nectar from flowers, leaf fragments and fruits, among others. Those that are hematophagous latch onto an animal, be it a mammal, amphibian or reptile, and periodically suck its blood.

Tick ​​after feeding. The increase in their body size is evident. Source: Pixabay.com

When eating food, it is subjected to the action of various digestive enzymes, which degrade it. At the level of the mesodeo, the absorption of nutrients is carried out and the waste is released by the proctodean, specifically through the anal orifice.

Representative species

Mites encompass a total of 50,000 described species. Among these there are some that are more recognized and identifiable by the general population. These are:

Dermatophagoides pteronyssinus

It is the famous dust mite. It is one of the mites most studied by specialists. They measure approximately 350 microns. The male is usually smaller than the female.

They are frequently found on mattresses, rugs, and pillows, among other places in the home. Despite being considered harmless, they can be the cause of triggering allergic reactions in susceptible people.

Sarcoptes scabiei

At the health level it is widely known, since it is the mite responsible for scabies, better known as scabies. It is characterized by having a bulky body with four pairs of equally bulky forelimbs.

Sarcoptes scabiei. Source: W. Linsenmaier

It is also known as the scabies mite. It owes its name to the fact that the females open tunnels in the stratum corneum of the skin, releasing certain toxins and their eggs as it moves. This causes a lot of itching in the host.

Amblyomma americanum

It is one of the species of tick. It is widely distributed along the east coast of the United States and Central America, in addition to the north of the South American continent.

They are characterized by being the largest mites, in addition to being able to duplicate it when they fill with the blood of their host. It generally affects mammals, although ticks have also been described in other classes of animals.

Ticks are known vectors for diseases like Lyme disease.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
4. Lebrun, P., De Saint, D., Van Impe, G. and Wauthy, G. (1991). The Acari. Chapter of the book "The Acari".
5. Pérez, T., Guzmán, C., Montiel, G., Paredes, R. and Rivas, G. (2014). Biodiversity of mites in Mexico. Mexican Journal of Biodiversity. 85
6. Vogel, P., Morelo, S. and Juárez, N. (2015). Mites and the consequence on human health. Hospital Nutrition. 31 (2).