Going to try something a little different today and cover a terrestrial Arthropod for the first time in this series, Arthropods include a wide range of animals such as Arachnids, Insects, Crustaceans and many more invertebrates.
I figure it might be a good idea to start with an animal that divides public opinion, which of course means I’m going to be writing about a certain genus of Spider, and we may as well start with a group of Spiders that likely terrifies a large portion of the public, Jumping Spiders.
I feel the fear of Spiders is unjustified in most cases but it is understandable, if you’re taught to fear something from a young age that fear will stay with you, and you’ll eventually pass your fear on to your children, thus creating a never ending cycle of largely unwarranted fear, for the most part we have very little to fear of these 8-legged critters who play an important role in determining the overall health of an ecosystem, some of them even yield mildly intriguing and complex lives.
The Jumping Spider Family
The family group Salticadae, alternatively known as the Jumping Spiders make up the largest family group of all Spiders (Araneae), all in all there are seven sub-families comprising of roughly 600 separate genera containing over 5800 species, the vast majority of species fall into the sub-family Salticinae which makes up around 90% of all known Jumping Spider species.
Jumping Spiders are present on all continents apart from Antarctica and can be found as far north as Iceland, they make up roughly 13% of all of the known spider species on earth (44,000+ total species) and have adapted to a wide variety of habitats, climates and altitudes, for such a large number of species their range in size is not overly large, the smallest Jumping Spiders measure just 0.04 inches and the largest reach just 1 inch in length.
Most species can be found in tropical forests which are known to be the most biodiverse biomes on the planet, Jumping Spiders also exist in desert regions such as the species Phidippus californicus to other extremes such as the species Euophrys omnisuperstes which inhabits the slopes of Mount Everest and has been observed at altitudes of 6700 metres above Sea level.
A key trait of the Jumping Spider is that they do not construct webs to entangle prey, they do however construct web structures known as “pup tents” that they use to find refuge from wet weather as well as to moult, store egg cases and to sleep in at night, they are diurnal predators and their eyes are only useful in daylight which means Spiders in equatorial regions will likely spend half of their day within their lairs, all jumping spiders are carnivorous aside from a few exceptions that feed on pollen and nectar.
We are of course not going to write about 5300 species of Spider so we will instead cover what exactly makes a Jumping Spider a Jumping Spider, before covering Genera that I find quite interesting, hopefully by the end of the article you may have a different opinion on these animals and may opt to take the approach of “save not squish” when confronted with one in the future.
Specialised Adaptations
Jumping Spiders are easily recognisable from a few visual features, namely the shape of their Cephalothorax, the length of their front four legs in ratio to their hind legs and lastly their eyes, of which there are always eight.
A Cephalothorax is the term used to describe the feature of a fused head and thorax, this is a feature observable in all Spiders apart from those within the family group Archaeidae (Assassin Spiders), aside from their extra limbs this is a key difference between Arachnids and Insects (Spiders also have an Endoskeleton as well as an Exosekelton for a random fact).
The Cephalothorax of the Jumping Spider is often a rectangular/square shape and is always shorter than the abdomen, it is also commonly adorned with hair-like or scale-like setae that give the Spider a fuzzy and sometimes cuddly appearance… though that last bit might just be my own opinion, be the judge of that for yourself!
All Jumping Spiders have eight legs, as do all Arachnids, the front legs of the Jumping Spider are often longer (there are many exceptions) than their hind legs though not as dramatically as in the Spider family group Thomisidae (Crab Spiders), unlike the Crab Spiders their legs are rarely outstretched and are often tucked close to the body.
Despite being larger than their hind legs, the front legs play a less important role in the Spiders jumping ability and are used predominantly as a grasping aid when grappling with their prey, their shorter hind legs play a more important role in their locomotion.
Unlike certain Insects such as Grasshoppers which rely on well-muscled legs to jump effectively, the Jumping Spider instead relies on a different mechanism, each of the Spiders legs are split in to seven segments which are the coax, trochanter, femur, patella, tibia, metatarsus and the tarsus, so in total there are six joints within each limb.
Spiders have flexor and extensor muscles just as we do which are used to control the movement at the joints, you might think well… why can’t I jump like the Jumping Spider then?
Well, the Spider does differ from us in that it has no extensor muscles at two of their six joints, specifically the femur-patella joint and the tibia metatarsus joints, this inevitably means that the Spiders aren’t able to extend these segments using muscles, but for the Jumping Spider to jump it must fully extend its legs which is a seemingly impossible task without the required muscles.
To traverse this problem the Jumping Spider instead opts to use their head, but not in a mental capacity, they instead rapidly change the blood pressure within the Cephalothorax, they do this by contracting the muscles that connect the upper and lower plates of the Cephalothorax, this reduces the available volume within the Cephalothorax and forces blood in to their legs.
This instantaneous blood flow to their limbs causes the limbs to rapidly extend and “snap” straight, from prone this causes the Jumping Spider to be launched in to the air allowing some species to jump up to 50 times their body length, to see this phenomenon in action, here’s a video with slow-motion footage showing the largest species of Jumping Spider in action, they grow to a size of 1 inch in length.
https://www.youtube.com/watch?v=FhpL5h8sEHo
