Insects vs Arachnids
Arthropods are composed of few classes, but insects and arachnids are two of the most important classes in the phylum. All the arthropods have distinguishing characteristics from other animals, but the members of these two classes have exhibited very distinctive physiognomies and those are good enough to identify insects from arachnids. They are different in their taxonomic diversity, morphological features, and in many other aspects.
Insects
Insects are the largest group of animals with an expected number of species between six to ten million. So far, there are about 1,000,000 described species of insects. Insects can sustain in almost all the ecosystems due to their extreme adaptability. This extremely higher number of insect species in the world lifts their importance. Some of the very common insects are butterflies, ants, bees, weevils, paddy bugs, crickets, grasshoppers, leaf insects, mosquitoes etc.
Insects have three specialized segments in the body known as tagma, consisting of the head, thorax, and abdomen. Basically, the head is designed for feeding and sensory functions, thorax mainly for locomotion, and the abdomen functions mainly for reproduction. There are three pairs of legs originating from the thorax. The head has two compound eyes and two antennae for sensory functions. In the abdomen, anus opens the oviduct and rectum to the exterior (i.e. they have only one opening for defecation and reproduction). Somehow, this thriving group of animals are considered the most successful in the Kingdom: Animalia.
Arachnids
Arachnids are a group of arthropods including spiders, mites, ticks, harvestmen, scorpions etc. There are more than 10,000 described species of arachnids, and almost all of them are terrestrial. The most prevailing and standing out feature of the arachnids is the presence of four pairs of legs (eight legs). However, some of those legs have become sensory appendages in some arachnid species. In addition to their legs, arachnids have two very distinctive appendages that act like arms with cutting and feeding abilities. In fact, these enlarged appendages are the chelicerae, which are additionally used in feeding and defense. The presence of pedipalps is another important feature of arachnids, which is useful in locomotion and reproduction.
The body organization of the arachnids is composed of cephalothorax and abdomen, aka prosoma and opisthosoma. Arachnids are wingless creatures, unlike most of the arthropods. The absence of antennae could be used as another distinguishing feature of them. One of the most important physiological characteristics of arachnids is the lack of extensor muscles; instead, they have a hydraulic pressure system to extend their legs with elastic thickenings at joins, as in spiders and scorpions. They have a special gas exchanging system that has been evolved from the book lungs. Their feeding is carnivorous mainly. Their sensory hairs and trichobothria are sensory structures additional to the compound eyes and ocelli. With the presence of internal fertilization for reproduction, arachnids could be regarded as a well-developed group of animals.
What is the difference between Insects and Arachnids?
• Insects are more diversified with over million species than the arachnids who have only 10,000 species.
• Insects have six pairs of legs, but there are eight pairs of legs in arachnids.
• Insects have wings at least in a stage of their lifecycle, but arachnids are always wingless creatures.
• Insects are found in almost all the habitats, whereas arachnids predominantly prefer the terrestrial habitat.
• Chelicerae are enlarged in arachnids more than in insects.
• Gas exchange system is evolved from book lungs in arachnids but not in insects.
• Arachnids show internal fertilization but rarely or not in insects.
• Insects have extensor muscles but not in arachnids.
• Insects have antennae but not in arachnids.
• Arachnids are predominantly carnivorous, but insects could be carnivorous, omnivorous, or herbivorous.