Is a spider a type of insect?
Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They live mainly on land but are also found in fresh water and in all marine environments, except for the open ocean. There are over 100,000 named species, including many species of spiders, scorpions, daddy-long-legs, ticks, and mites (Figure below). There may be up to 600,000 species in total, including unknown ones.
(left) A daddy-long-legs spider. (right) Various diseases are caused by bacteria that are spread to humans by arachnids, like the tick shown here.
Characteristics of Arachnids
Arachnids have the following characteristics:
- Four pairs of legs (eight total). You can tell the difference between an arachnid and an insect because insects have three pairs of legs (six total).
- Arachnids also have two additional pairs of appendages. The first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, help the organisms feed, move, and reproduce.
- Arachnids do not have antennae or wings.
- The arachnid body is organized into the cephalothorax, a fusion of the head and thorax, and the abdomen.
- To adapt to living on land, arachnids have internal breathing systems, like a trachea or a book lung.
- Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals.
- Several groups are venomous. They release the venom from specialized glands to kill prey or enemies.
- Several mites are parasitic, and some of those are carriers of disease.
- Arachnids usually lay eggs, which hatch into immature arachnids that are similar to adults. Scorpions, however, give birth to live young.
The arachnids are divided into eleven subgroups. Below are the four most familiar subgroups, with a description of each (Table below).
|Subgroup||Representative Organisms||Approximate Number of Species||Characteristics|
|Acarina||Mites and ticks||30,000||
- Arachnids have four pairs of legs, specialized appendages, and a fused head and thorax.
- Arachnids include spiders, daddy-long-legs, scorpions, and ticks.
Use the resource below to answer the questions that follow.
- Spiders at http://www.youtube.com/watch?v=ALsnmm-ghlA (4:03)
- How do spiders differ from insects?
- What are some of the uses of their silk?
- How do spiders keep from becoming stuck in their own webs?
- What role do spiders play in their ecosystem?
- List three types of arachnids.
- What are two key features of arachnids?
- Arachnids also have two additional pairs of appendages. Describe these two pairs.
- List two specific features of spiders.
- List two specific features of scorpions.