Fiddler Spiders in Mexico: An Underrecognized Insect

Most spiders have venom glands, but do you know what they use them for? The small venom they transmit and the size of their mouthparts make them harmless to us.

Fiddler Spiders in Mexico: An Underrecognized Insect
A spider hit the flip-flop! The small arachnid that has been stigmatized and feared by many, is harmless most of the time to humans. Photo by Nathan Dumlao / Unsplash

Almost all spiders indeed have venom glands, but do you know what they use them for? To paralyze or kill their prey, other insects such as crickets, cockroaches, flies, etc. Due to the little venom they transmit and the size of their mouthparts, they do not represent any danger to us.

In Mexico, there are two spiders whose bites can cause damage to our health: black widows (Latrodectus) and fiddler spiders (Loxosceles). The latter is present in Mexico, there are about 40 species distributed in all states, with sizes ranging from 8 to 15 mm in length. As its name implies, the fiddler spider is recognized by a violin-shaped figure on the carapace (with the tuning fork pointing to the rear), by the pear shape of this carapace, and by the three pairs of eyes present in a recurved transverse row.

Human bites by this spider are painless and occur accidentally because the spider is only trying to defend itself. But it is dangerous to be bitten by one of them because its venom causes tissue death, as well as internal damage to our kidneys and liver, mainly caused by the action of the enzyme sphingomyelinase D, which causes the clinical condition called "loxoscelism".

These Mexican fiddler spiders can be found in dry habitats (hillsides, tree bark, rocks, etc.), low jungles or thickets, caves, cracks in walls, and also in our homes or in urban areas where there is little movement (cellars, closets, behind furniture, between roof tiles, etc.), and which serve as a refuge. The key to keeping them away is simply a constant cleaning of these places and shaking clothes and shoes before use to avoid accidents.

In Mexico, there are few studies of these spiders, and most of them are 37 years old, so errors may occur in the identification of the species. For this reason, in the laboratory of the Institute of Biology of the UNAM, Tlaxcala branch, research was carried out using molecular tools (DNA) to recognize and differentiate between the species of fiddler spiders that exist in the central-western region (Mexico City, Colima, State of Mexico, Guerrero, Hidalgo, Jalisco, Michoacán, Morelos, Nayarit, Puebla, and Tlaxcala).

By Claudia Isabel Navarro-Rodríguez, Laboratory of Arachnology Tlaxcala (LATLAX), Institute of Biology, National Autonomous University of Mexico (UNAM)