why Arthropoda are most successful animal
Answers
An arthropod is generally defined by five characteristics:
- Bilateral symmetry;
- Segmentation;
- Hardened exoskeleton;
- Limb pairs;
- Jointed limbs.
Bilateral symmetry ensures that every appendage is mirrored on the left or right side, so everything is doubled. The hardened exoskeleton is a guaranteed defence, which can be gotten rid of by evolution (e.g. in spiders).
By themselves, these two characteristics are cool, but not so unique: most animals are bilaterally symmetric (the main exceptions are sponges, cnidarians, and echinoderms). Hardened cuticles, shells, and other built-in physical defences are featured in many animal phyla.
The real innovation that arthropods have is their modularity, granted by segmentation and their limbs.
Segmentation alone is an enormous advantage, which you can also see in the enormous diversity of annelids. Unlike annelids, arthropods also have limbs - and those limbs are also segmented.
This extreme amount of segmentation leads to a lot of compartmentalisation, which gives evolution free rein to basically do whatever it wants. The genetic and developmental architecture that enables this segmentation also happens to be very nicely ordered and compartmentalised, meaning evolution will not have a difficult time modifying the arthropod body. It’s so easy that even a rookie in the lab can make legs instead of antennae grow on the head of an arthropod.
From the earliest arthropods, the frontal body segments were turned into sensory and feeding regions, the middle segments into locomotor regions, and other functions relegated to the hind segments. Because there are multiple segments involved, there is also a lot of redundancy in this system.
So if you don’t like this arrangement, you can just squish together the mid- and hind-segments and make a spider. Want to fight? Modify your frontal appendages to raptorial ones. Think you have too many legs? Just limit them to three pairs, like in insects. Don’t like swimming? Get rid of the gills, drill a few holes in the exoskeleton, and walk on land. Don’t like walking? Grow a pair of wings.
These are very basic descriptions, to the point of sounding ridiculous, but that is simply how these successful transitions happened, and they explain why arthropods are so successful: their anatomy and developmental genetics gives them an extreme potential for evolvability.
Coincidentally, their modularity extends to their life cycle too. Gene expression networks change in developmental time in arthropods, enabling very flexible life cycles - from larval-adult life cycles to the extreme found in holometablic insects (larva-pupa-adult). Each life stage has a different set of environmental adaptations, meaning that every arthropod is potentially adapted to two environments, not just one. For example, odonates (dragonflies and damselflies) are master freshwater predators as larvae, and master aerial predators as adults - one species of odonate can thus be the master of two completely different habitats.
There is no other animal phylum where this perfect storm of modularity and potential for evolvability exists.
Arthropods are after all insects ,which is present in all the countries . It forms the largest percentage of the world organism.