LOXOSCELES: CHARACTERISTICS, CLASSIFICATION, NUTRITION, REPRODUCTION - BIOLOGY - 2025

Loxosceles is a genus of spiders in the family Sicariidae of medium size, generally between 5 and 13 mm in length, with a pyriform cephalothorax. They have two pairs of lateral eyes and a pair located anteriorly, distributed in the shape of a triangle. The anterior or frontal eyes are larger than the lateral eyes.

The spiders of this genre are known by the name of violinist spiders because they generally have marks on the thorax in the shape of said musical instrument. They are also called corner spiders, because they tend to live hidden in hard-to-reach corners and crevices.

Laxosceles laeta, macro in front view. Taken and edited from: Ken Walker, Museum Victoria.

The genus is represented by more than 100 species of wandering spiders distributed worldwide. The greatest diversity of species is found in Latin America, especially in Mexico (41 species) and Peru (19 species). Depending on the species, the preferred habitat can range from temperate forests to deserts or dunes.

Fiddler spiders are part of a small group of spiders whose venom can kill humans. Together with black widows (genus Latrodectus) they are the largest and most dangerous responsible for poisoning by spiders worldwide. Its venom is proteolytic and necrotoxic and produces a series of symptoms known as loxoscelism.

characteristics

Spiders of the genus Loxosceles are medium-sized organisms, generally not exceeding 15 mm long, with larger females and a more developed abdomen (opistosoma).

The prosoma or cephalothorax is pyriform, ornamented with a series of violin-shaped spots. In the anterior part there are six eyes arranged in three pairs in recurring transverse rows, the anterior pair larger than the rest and located frontally.

The prosoma or cephalothorax is pyriform, ornamented with a series of violin-shaped spots. In the anterior part of it there are six eyes arranged in dyads, the anterior pair is larger than the others and is located frontally, while the remaining two pairs are located laterally.

The chelicerae or buccal appendages are united on their inner margin by a membrane up to half their length. The lamina where these appendages articulate (cheliceral lamina) presents the bifid apex.

It has two tarsal claws armed with a single row of teeth, in females the pedipalp lacks claws. The relative size of the legs varies by species, but the third pair is always the shortest.

The larger bullous glands have a slit-shaped opening, the posterior median rows (spinerettes) lack a spit, while the posterior lateral ones have modified setae.

Females lack external sexual organs, that is, they are haplogins and present a wide gonopore surrounded by mushrooms that leads internally to two storage receptacles. The male copulatory organ is represented by a simple bulb and a plunger with a thin tube without accessory structures.

Spiders of this genus are nocturnal and their degree of activity is closely related to the ambient temperature; its presence is accentuated in the warmer months.

Taxonomy

Fiddler spiders are taxonomically located in the order Araneae, suborder Araneomorphae, family Sicariidae. In this family, until recent years, taxonomists only recognized two genera, Loxoscheles and Sicarius, although currently some authors have resurrected the genus Hexophthalma, proposed by Karsch in 1879.

The genus Loxoscheles was erected by Heineken and Lowe in 1832 and is currently made up of 116 species, most of which are present in the Neotropics.

Both the family Sicariidae and the genus Loxoscheles have been shown to be monophyletic, however, some authors suggest that the species L. simillima could belong to a new genus not yet described.

Habitat and distribution

In natural environments, Loxoscheles species occupy a great diversity of habitats. They can be found in temperate forests, rain forests, savannas, chaparral, and deserts. They prefer dark places like leaf litter, under decaying logs, in hollows of trees or in pieces of bark.

Some species are synanthropic, that is, they have adapted very well to environments modified by man, being able to live in houses or in peridomiciliary environments. Inside the houses they can live in attics, under furniture, behind pictures or in any corner or crevice that is difficult to access.

The genus Loxoscheles is globally distributed, with a greater number of species described in the Neotropics (87 species). The greatest diversity has been reported for Mexico with 41 species.

At least 23 species are known to inhabit the Arctic regions (13 in the Nearctic and 10 in the Palearctic), while only two species are considered cosmopolitan.

Nutrition

Fiddler spiders are primarily insectivorous carnivorous animals. These spiders build webs whose short hammock shape is inefficient for capturing their prey, but their sticky condition and resistance allow them to trap some organisms.

They are also active hunters, mainly during night hours. When they capture a prey, they quickly inoculate it with the poison and after its death they inject digestive juices that dissolve the prey's tissues.

After digestive juices digest the prey, the spider sucks up the nutrients and continues digestion inside its body.

Reproduction

Spiders of the genus Loxoscheles are all dioecious (they have separate sexes), sexually dimorphic in size, with females larger than males. Fertilization is internal and the eggs develop in ootheques.

Before mating there is a courtship, in which the male jumps and dances around the female and sometimes gives her a prey as an offering. If the male is accepted, the female raises the cephalothorax and the male introduces the pedipalps (which have been modified as secondary copulatory organs) into the female's gonopore.

Loxosceles rufescens adult male. Taken and edited from: Antonio Serrano.

Sperm are released in packages called spermatophores and deposited in receptacles in the female (spermatheca). The female releases the eggs and these are fertilized by the sperm. Once the ovules are fertilized, the female places a variable number of eggs, depending on the species, in an ootheca.

Each female can produce several ootheca in each reproductive period. The eggs take between 30 and 60 days to hatch, depending on the species and the incubation temperature.

Poison

Loxosceles venom is complex in composition, in which many components are involved, including: sphingomyelinases, metalloproteinases, alkaline phosphatase, and serine proteases.

It is essentially proteolytic and necrotic in action and produces a series of effects that together are called loxoscelism. Loxoscelism can be cutaneous, affecting skin cells and tissues; or visceral or systemic, in which the venom enters the bloodstream and is transported to different organs of the body.

Sphingomyelinase D is the main responsible for necrosis, as well as the hemolysis caused by this poison.

When the venom penetrates the tissues it causes an inflammatory reaction involving arachidonic acid and prostaglandins, which causes severe vasculitis. With occlusion of the local microcirculation, hemolysis, thrombocytopenia, and disseminated intravascular coagulation (DIC) can also occur.

Loxoscelism can also include tissue necrosis, coagulation, and kidney failure. Although there is a specific treatment against loxoscelism, the antidote is only effective when given in the first hours after the bite. Otherwise it loses effectiveness until it becomes completely ineffective.

Other treatments include the application of ice, immobilization of the affected limb, pain relievers, application of high pressure oxygen or electric current, colchicine, antihistamines, corticosteroids and dapsone, all of them with contradictory results.

Representative species

Loxosceles laeta

It is native to South America, common in Chile, Peru, Ecuador, Argentina, Uruguay, Paraguay, and Brazil. It has been accidentally introduced in different countries in North and Central America, as well as in Australia, Finland and Spain.

It is one of the larger Loxosceles species (up to 15 mm) and wider (or robust) than its congeners of other species. It is one of the most dangerous fiddler spiders due to the power of its venom and it is also the most widely distributed species in the American continent.

Anaphylactic shock can occur between 5% and 20% of cases, with mortality occurring in about 1/3 of these cases.

Loxosceles rufescens

It is a species native to the Mediterranean area of ​​the European continent, although it has been accidentally introduced in various countries of the Asian continent, as well as in Australia, Madagascar and North America. It is also present on some islands in the Pacific and Atlantic.

One of the characteristics of Loxosceles rufescens is the presence of a large cardiac spot, although sometimes inconspicuous. Its coloration ranges from brown to light reddish, tending to be slightly lighter than other species of the genus.

This species, which can reach 20 mm, is the most poisonous spider in the Mediterranean area and is very prolific, as it can lay up to 300 eggs in a single ootheca.

Loxosceles reclusa

It is endemic to North America and is widely distributed in the United States and northern Mexico. It lives mainly among rocks and among pieces of wood or firewood. In Oklahoma (USA) there are about 100 annual cases of poisonings by this species.

Macro, antero frontal view of Loxosceles reclusa. Taken and edited from: Insects Unlocked.

It is relatively small for the genus, reaching up to 9 mm in length. Its abdomen is covered by short mushrooms that give it a smooth appearance. The female lays an average of 50 eggs per ootheca.

References

1. LN Lotz (2017). An update on the spider genus Loxosceles (Araneae, Sicariidae) in the Afrotropical region, with description of seven new species. Zootaxa.
2. A. Rubín (2019). Loxosceles laeta: characteristics, habitat, nutrition, reproduction. Recovered from: lifeder.com.
3. Loxosceles. On Wikipedia. Recovered from: en.wikipedia.org.
4. Fiddler or corner spider. Recovered from: anipedia.org.
5. Loxosceles reclusa. On Animal Diversity Web. Recovered from: animaldiversity.org.
6. AR de Roodt, OD Salomón, SC Lloveras, TA Orduna (2002). Poisoning by spiders of the genus Loxosceles. Medicine (Buenos Aires).
7. SR Vetter (2008). Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations. The Journal of Arachnology.