Arachnids, insects and crustaceans are similar in that they are all invertebrates. This means that they do not have an internal skeleton. Instead, the members of these groups have a hard external case or exoskeleton. However, there are many differences between arachnids, insects and crustaceans that set these classes apart from one another. Within the groups, there are also many different orders, families, genera and species that have characteristics that are very different from those of other members of their class.
What Is an Insect?
An insect is a six-legged animal that has three body parts: a head, thorax and abdomen. The legs are attached to the middle portion of the body, the thorax. An insect also has compound eyes that help it see and antennae that help it feel its way around in the world. Most insects hatch from an egg and moult a number of times before going through metamorphosis into an adult insect. During a moult, insects remove their old exoskeleton to reveal a new one underneath.
Differences Between Insects
Among the insects, life cycles vary extensively. Some insects go through complete metamorphosis. This sort of insect begins its life as an egg, moves into a larval stage, creates a case and emerges as an entirely different-looking animal. A caterpillar goes through this sort of metamorphosis. In incomplete metamorphosis, an insect changes gradually as it sheds its exoskeleton, moving from an egg to a nymph to an adult. A dragonfly goes through this sort of metamorphosis.
What is an Arachnid?
An arachnid is an animal with at least eight legs. Unlike an insect, an arachnid has two body parts: the abdomen and the cephalothorax. The most commonly known arachnid is the spider, but other animals such as scorpions also fall into this category. Unlike the insects, arachnids do not have wings or antennae.
Differences Between Arachnids
Within the arachnids there are a number of different groups such as the harvestmen, scorpions, ticks and mites. A scorpion is an arachnid with a long abdomen that has a stinger at the end. A tick is very flat and has a small head, while spiders have a larger head and a rounder abdomen. A harvestman looks like a spider with extremely long legs, and it is one of the groups of invertebrates that bears the name “Daddy Longlegs.”
What Is a Crustacean?
A crustacean is an animal with limbs that have two parts. Crustacean larvae are also different from the larvae of other invertebrates. Crustacean larvae look very different from the adults of the same species and may look very similar to other crustacean larvae. These larvae use the head appendages to move around. The Class Crustacea includes animals such as the crab, crayfish, lobster and pillbug.
Differences Between Crustaceans
Size is one of the greatest differences in the Class Crustacea. Some crustaceans are extremely small, such as one parasitic crustacean species that is less than a 1/10 mm in size. Other species in the class are extremely large for an arthropod. The Japanese Spider Crab can be over 12 feet wide.
Although the crustaceans are often considered to be a class of aquatic animals, some crustaceans also live on land. The woodlouse or pillbug is a gilled animal that lives in wet wood.