What’s the difference between insects and arachnids?
Of the 1.5 million animal species scientists have catalogued to date, about two-thirds are insects.
Insects come in a variety of shapes and sizes and exhibit a variety of behaviors. But they all share a defining set of characteristics. What do ants, flies and beetles have in common? They all have antennas, 6 legs and bodies consisting of 3 parts: a head, a thorax and an abdomen.
They are also immensely successful. For example, ants. Some renowned biologists predict that there may be about 10 billion on Earth. That’s 10,000,000,000,000,000,000… or about 1.4 million ants per human on the planet!
The arachnids are named after Arachne, the daughter of the shepherd and master weaver of Greek mythology who became a spider. In fact, spiders are the largest order in their class. Other known arachnids include scorpions, ticks, mites and solifuges (also known as camel spiders).
Arachnids are often referred to as insects. Although the two classes are similar in many ways, they must be differentiated. Insects and arachnids are arthropod invertebrates with exoskeletons, articulated legs and segmented bodies.
The difference is in the numbers: Arachnids have 8 legs and only 2 segments in their bodies. Their head and thorax are combined to form something called a cephalothorax.
Let’s start, surely, with the best adapted predator of the world of insects: the religious mantis. Taking advantage of the camouflage and its vegetal structure, the religious mantis are the masters of the ambushes.
They may be small but they will attack almost anything that is within the reach of their toothed legs. Not only other insects (like the bees in the video accompanying this paragraph), they also feed on small rodents, frogs, lizards and even small birds!
In addition to stealth, these formidable hunters have incredible vision. Not only do they have two large compound eyes on the sides of the head that allow them to see 360 degrees, but they also have three additional small eyes for specific functions.
Mantids are able to turn their heads 180 degrees to expand their field of vision. No other order of insects can do this.
What adds to their insatiable reputation is participation in sexual cannibalism. Female mantises never miss a meal. Females eat males alive during or shortly after mating.
The banana spider is, in fact, the name of two distinct species: Phoneutria feray Phoneutria nigriventer. They are also sometimes referred to as banana wandering spiders or Brazilian spiders because of their well-documented talent for appearing in people’s fruit bowls.
Why should that matter? Well, both species are on the list of the world’s deadliest spiders. Their venom is highly toxic, but the severity of their effect depends on how much venom has been inoculated. Generally, the female banana spider produces a greater amount of venom.
Their venom attacks the nervous system, causing unbearable pain, loss of muscle control, and respiratory problems. In male victims, a bite can lead to priapism, a continuous and painful erection of the penis. And at high concentrations, the venom can cause paralysis and suffocation.
The next one is a family of really huge arachnids. There are 83 species of hunting spiders, including the world’s largest spider, the Australian Heteropoda maxima (giant hunting spider), which has a wingspan of 30 cm. Although often referred to as tarantulas, they actually belong to the Sparassidae family.
Hunting spiders are not as poisonous as banana spiders, nor are they particularly aggressive. Although their tusks are long and sharp enough to pierce human skin, it is unlikely that a hunter spider bite will cause more than pain and swelling.
Even without posing a deadly threat to humans, these highly effective predators deserve inclusion on this list. As their name suggests, instead of weaving webs and waiting for prey to arrive, hunting spiders prefer to stalk them.
What makes them so effective? To begin with, they are very fast. It is not easy to measure the speed of a spider, but it has been estimated that the fastest hunter species can run 42 body lengths per second.
Not only are they fast, but they are also absolutely powerful. Check out the video by dragging a mouse along the side of a fridge!
All the invertebrates we have seen so far are capable of capturing prey twice their size, but none alter the natural order of things like the larvae of the Epomis beetle. These white worms, which measure no more than 20 mm, feed exclusively on amphibians.
It is a rare inversion of the predator-prey relationship. You can think of it as David and Goliath, only in this case David kills and eats giants regularly!
How do they do it? As you can see in the video, the larva attracts the frog, simply resembling the prey. When the frog attacks, the larva evades the attack.
The prey becomes a predator and the larva bites the throat or lower part of the amphibian with its double hook jaws. Parasitic behavior occurs, the larvae adhere to the frog and begin to eat it.
There is something disturbing about this role reversal, but it is certainly a fascinating adaptation.