OPILIONES: CHARACTERISTICS, TYPES, DIET AND SPECIES - BIOLOGY - 2025

The opiliones are a group of animals that belong to the phylum Arthropoda and the Arachnida class that are characterized by the extreme length of the legs in most of the species that comprise it.

This order was first described in the mid-19th century by the Swedish zoologist Carl Jakob Sundevall. It is believed to have originated in the Paleozoic era, specifically the Devonian period. This is so because the oldest fossils that have been recovered from this group date from that period.

Exemplary of opiliones. Source: JonRichfield Within the order of the opiliones there are approximately 6500 species, which are widely distributed by almost all the ecosystems of the planet.

Taxonomy

The taxonomic classification of the opiliones is as follows:

Domain: Eukarya

Animalia Kingdom

Phylum: Arthropoda

Class: Arachnida

Order: Opiliones

characteristics

Opiliones are animals whose cells are considered eukaryotic, since their DNA is packaged within the cell nucleus forming chromosomes. They are also made up of different types of tissues, with cells specialized in various functions such as nutrition, absorption and reproduction.

Despite belonging to the group of arachnids (such as spiders and scorpions), they do not have glands that synthesize venom, so they lack this mechanism to capture and immobilize their prey.

In addition, the opilions are triblastic, coelomed, protostomized and present bilateral symmetry. This is explained in the following way: during their embryonic development, they present the three germ layers (ectoderm, endoderm and mesoderm) and from an embryonic structure called blastopore, the mouth and anus originate simultaneously.

This type of arachnid has an internal cavity known as a coelom, in which the different organs that make up the animal develop. Likewise, the opiliones are made up of two exactly equal halves, taking the longitudinal axis of the body as the imaginary dividing point.

Opiliones are animals that stand out within the group of arachnids for their long limbs and for their eating habits, since their diet includes small vertebrates, fungi, plants and even decomposed organic matter.

Morphology

As with the rest of the arachnids, the body of the opiliones is divided into two segments or tagmas: prosoma (cephalothorax) and opistosoma (abdomen). The most marked difference with respect to the other arachnids is that the delimitation between the two segments is not very clear or noticeable.

Similarly, opiliones have six pairs of articulated appendages: two chelicerae, two pedipalps, and eight legs.

- Prosoma or Cephalothorax

It is the anterior segment or tagma of the animal's body. It has an average length of 15 mm. It is made up of approximately six segments. The prosome is covered by a kind of protective layer with a hard and resistant consistency known as the prosomic shield.

In the prosomic shield it is possible to appreciate several holes. In the central part, it has a protuberance in which the animal's organs of vision are housed. Likewise, in the lateral area it presents openings into which glands characteristic of this order of arachnids flow, which are known as repugnant glands.

The animal's articulated appendages originate from the prosoma. The ventral portion of the prosoma is almost entirely occupied by the coxae of the legs.

On the other hand, on the dorsal surface of the prosomal shield, structures called suprachelyceric laminae can be seen between the chelicerae birth and the front edge.

Cheliceros

The chelicerae of the opiliones do not have venom glands. They are also made up of three joints called distal, middle and basal. They are short in length and end in a clip.

The main function of the chelicerae is the capture and immobilization of the prey, in those species that have carnivorous habits.

Pedipalps

They are the second pair of appendages of the opiliones. Unlike other arachnids, these do not differ much from the legs, that is, they are thin and of great length. In some species they end in a nail.

They are made up of six joints, from distal to proximal: tarsus, tibia, patella, femur, trochanter, and coxa.

Legs

It is one of the characteristic elements of this order of arachnids. They are very thin and long, and can even exceed the length of the animal's body. They can measure more than 12 cm.

From a structural point of view, the legs are made up of the following joints: coxa, trochanter, femur, patella, tibia and tarsus. The difference between pedipalps and legs is that in the latter, the tarsus is divided into the tarsus and the metatarsus.

The function of the legs is related to locomotion. Although many think that the length of the legs can hinder the movement of the animal, this does not happen, since these animals can move quite quickly.

Exemplary of opiliones. Note the length of its legs. Source: LiCheng Shih

- Opistosoma or abdomen

The opistosome has a clear segmentation pattern. It is made up of a total of 10 segments.

In this part of the opilion is where most of the structures that make up the different organic systems are housed.

It also has a kind of rigid cover, which is made up of the union of different smaller plates, called sternites.

It is important to note that some openings belonging to different body systems are located on the surface of the opisthosoma. For example, in sternites number 2 and 3 there are, in a lateral position, the spiracles into which the respiratory passages open.

In this same order, towards the anterior end, the genital opening, better known as the vaginal operculum, is located. At the posterior end is the anal operculum.

- Internal anatomy

Circulatory system

The circulatory system of these animals is quite rudimentary and simple. The main organ is a heart, which is cylindrical in shape and also has seven ostioli. The heart is housed in a space called the pericardial cavity.

With respect to the blood vessels, here a single aorta artery emerges from the heart and begins to branch into increasingly narrow arterioles, which reach the cells of the animal

The fluid that circulates through the opilions is not blood but hemolymph.

Nervous system

The opiliones have a very rudimentary nervous system, basically made up of accumulations of neurons that in turn constitute nerve ganglia.

As with all arachnids, in opiliones there is a ganglion that acts as the brain. Likewise, the different ganglia that make up the nervous system are closely related and communicated with structures of the digestive system such as the esophagus and the intestine.

Regarding the sensory organs that the opiliones have, it is said that they have simple eyes, which are not capable of distinguishing sharp images. These only manage to differentiate light from dark.

In addition, they do not have specialized sensory receptors, since they do not have trichobotrians or sensory structures in the extremities.

Digestive system

The digestive system of the opiliones is complete, very similar to that of other members of the Arachnida class, although with some well-marked differences. Among these differences, the most representative is that they do not have a proper stomach.

First, the digestive tract is made up of a mouth opening that opens into a cavity, which communicates directly with a very short cylindrical tube, the esophagus. This is continued with the so-called midgut, which finally culminates with the exit orifice, the anus.

It should be mentioned that at the level of the oral cavity there are cells that specialize in the secretion of digestive enzymes, which are of great help in the degradation and processing of the food that the animal ingests.

Finally, unlike other arachnids, opiliones lack a hepatopancreas.

Respiratory system

The type of breathing that opiliones have is tracheal. Taking this into account, your respiratory system is made up of a series of branching tubes known as tracheas.

As they enter the body of the animal, the tracheae branch into smaller and smaller tubes called trachealas, which reach the cells carrying the oxygen they need.

In the tracheoles is where the gas exchange takes place. The tracheas communicate with the exterior through holes called spiracles. Through these the oxygen-laden air enters and leaves with carbon dioxide as a waste from respiration.

Reproductive system

Opiliones are diodic organisms. This means that the sexes are separated, which is why there are male individuals and female individuals.

Males have a copulatory organ that has the peculiarity of being protractile. This means that it can project outward during the moment of copulation.

In the case of females there is also an organ that is protctile, the ovipositor. This has structures known as seminal receptacles that serve to store the sperm after the copulation process.

Classification

The Opiliones order is made up of a total of four suborders: Laniatores, Cyphophthalmi, Dyspnoi and Eupnoi.

Laniatores

It includes organisms whose exoskeleton has certain elements such as spines and protrusions. In addition, their legs are not as long as those of other species of opiliones.

Cyphophthalmi

They are characterized because their legs do not exceed the body in length. They are small in size, measuring no more than 8 mm. They are present on all continents, except Asia.

Dyspnoi

They are almost exclusive to the northern hemisphere, specifically the temperate zones. The largest opiliones belong to this suborder.

Eupnoi

Members of this suborder are characterized by prominent eyes, very long legs, and noticeable spines on their pedipalps. They are distributed throughout the world geography, preferring mostly temperate areas.

Distribution and habitat

This is a group of animals that are widely distributed throughout the world. The only place where specimens have not yet been found is on the Antarctic continent.

Now, the opiliones have developed capacities to adapt to the different types of ecosystems that exist on the planet. This is why they can be found in deserts, forests and jungles. Their favorite places are under rocks or stones, in caves, in the litter and even in the detritus.

Opiliones are animals that tend to stay grouped, so it has been possible to find populations with a large number of individuals.

Set of opiliones together in their natural habitat. Source: Luis Fernández García

Feeding

This group of animals is an exception among arachnids. This is because their diet is not purely carnivorous, but they can also feed on decaying plants, fungi and even organic matter. Species have been recorded that also feed on the carrion of other animals.

The digestive tract is small in size, which is why they cannot ingest very large food particles.

When the food is large, the animal secretes a series of digestive enzymes that begin to degrade it. Once turned into a kind of porridge, the animal ingests it. This passes to the esophagus and then to the middle intestine where the absorption of nutrients takes place. Finally the waste is expelled through the anus.

Not all species have this feeding mechanism. There are some that manage to ingest food and then the digestive process takes place entirely inside the animal's body.

Reproduction

The type of reproduction that opiliones have is sexual. For it to occur, the fusion of a female gamete with a male one is necessary. The opiliones have internal fertilization, indirect development and are oviparous.

The fertilization process occurs when the male introduces his copulatory organ into the ovipositor, depositing the sperm there.

After fertilization occurs, the female proceeds to lay the eggs. However, this is not a process that occurs immediately in all cases. There are species in which egg laying can be delayed for up to a few months after fertilization.

When the eggs hatch, the individual that hatches is in larval form. Later it experiences a series of molts. From the first molt they go from being larvae to being nymphs. The average of molts is six. Once they reach maturity, they do not experience any more molts.

Representative species

The Opiliones order includes approximately 6,500 species.

Leiobunum politum

It belongs to the suborder Eupnoi and to the Sclerosomatidae family. It has a brown body and very long legs that become thinner as they move away from the body. It is located mainly in North America.

Pantopsalis listeri

They are animals with a totally black body, belonging to the Eupnoi suborder. They also have chelicerae covered by irregular teeth. Its legs are very long, greatly exceeding the length of the body.

Pantopsalis listeri. Source: Christopher Taylor

Pelltonichya sarea

Known to some as an albino spider, this species is characterized by its pale color and developed pedipalps, which have teeth that they use to capture prey.

References

1. Barrientos, JA (ed.). 2004. Practical course in Entomology. Spanish Association of Entomology, CIBIO and Autonomous University of Barcelona. 947 pp.
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
3. García, A. and Medrano, M. (2015). Order Opiliones. Chapter of the book: The Arthropods of the Río Ñambi Nature Reserve. National university of Colombia.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill
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