FOR JUMPING SPIDERS, IT IS ALL ABOUT THE EYES

Some spiders are famous for being big, like tarantulas, others are famous for being venomous, like black widows, but the ones I am going to talk about today are famous for very different reasons. With 6,000 different species, they are the biggest family of all spider, containing 13% of the known species. The members of the Salticidae family, also known as jumping spiders, have very complex visual organs, amazing jumping ability and a face perpendicular to the ground. All these unique characteristics makes them some of the most interesting spiders in the world. If you want to learn about some of their secrets, make sure to read this article until the end.

Their vision is the sense from which most of their characteristics are based. Unlike most spiders, they have an excellent sight, and each of their 4 pairs of eyes have a different function. As you can see in the in the diagram, they have two anterior and two posterior pairs. Depending on their position in the head, they are surnamed lateral or medial.

Photo by: David Edwin Hill
https://en.wikipedia.org/wiki/File:Jumping_spider_vision_David_Hill.png

The fourth pair of eyes, known as posterior lateral eyes, are wide-angle movement detectors. They cover the rear part of their view, as well as the sides. Their third pair, formed by the posterior medial eyes, is less complex. Unlike all the other eyes, they do not form images and basically work like ocelli. This means they can only detect if there is light coming in and its direction. The second pair is the most complex of all the secondary eyes. It is formed by the two anterior lateral eyes, which form images with a quite good level of details. Their two vision angles overlap, and thanks to that, they have stereoscopic vision.

The anterior median eyes are the primary eyes of these spiders. They have excellent vision and they are the only ones that can move to look at an objective. They form very detailed three-dimensional images and can calculate distance, despite not having stereoscopic vision. They can do this using a technique called image defocus. It basically consists in focusing on something with a layer of their eyes and then seeing how fuzzy it is in the others. The level of fuzziness allows them to estimate distance.

Thanks to their amazing sight, these spiders have based their courtship mechanism on visual stimuli. Males have colorful, plumose and sometimes iridescent hairs in their legs. Some have taken this to the extreme, like the peacock spider in the photo. They use these features to attract females, but that is only the beginning. To gain the attention of the females, the males perform a courtship “dance”. Although it is different in every species, is basically consist on showing the flashy parts, most also point their front legs upwards and they perform lateral and zig-zag movements around the female. It has also been recently discovered that males make a vibrating, buzzing sound as well. Finally, if the female is passive, the male touches her with the front legs (never if she is aggressive). If she is receptive, the mating can start.

Their visual precision and fabulous distance estimations has allowed them to develop the ability that names them. They can jump distances of several times their body length, but they don’t do it thanks to muscular legs, like other arthropods. Instead, they do it thanks to their body fluids. They have developed a hydraulic system that allows them to essentially squeeze the fluids from the rest to the body onto their rear legs. This increases the pressure of fluid in their rear legs and causes a fast and abrupt extension. In case they miscalculate the jump and don’t reach their objective, they have a safety measure. Just as a bungee jumper, they attach a safety line to their starting point, only that instead of a rope, it is a spider silk line.

This jumping ability, along with their sight, has defined the way in which jumping spiders hunt. Their method consists in a series of meticulous steps that try to increase the level of success as much as possible. First, they face their eyes and body to the prey and estimate the distance to it. Then, they attach the safety silk line and jump to catch the prey. If they miss or want to enjoy their meal at their starting point, they climb back up the line.

Jumping spiders are beautifully colored, small and have the amazing ability of jumping very far. But we must realize that all of those features are only possible thanks to an amazing sense of sight that has caught the interest of scientist during many years and that continues to be researched. Few animals are as unique as them so, the next time you see a little arthropod in a leave, make sure to look closer, because it might be one of these.