Article by K Griffiths
A common misconception I often encounter is that spiders, slaters and other creepy crawlies such as millipedes or anything people like to refer to as 'bugs' are Insects. But this is not the case. Let us take a look at why.
What exactly is an Insect?
In order to understand why spiders are NOT Insects, we must first
define what and Insect actually is. In the animal kingdom, creatures are
classified as belonging to various groups, comprising of those most related to each other. In the past this was done by morphology, that is animals which shared similar physical characteristics. Nowdays DNA sequencing can provide further insights as to which groups species belong to and is still leading to the reclassification and reshuffiling of various species as new information comes to light.
Kingdom Animal is divided into large groups called Phyla. The largest of these is Phylum Arthropoda, the Arthropods, which account for close to 90% of all Animal species.1 Phylum Arthropoda is then broken down into various classes, one of these being the Insects, Class Insecta. Within the class insects are broken down into various orders such as bees & wasps, ants, beetles and so on (Figure 1).
define what and Insect actually is. In the animal kingdom, creatures are
classified as belonging to various groups, comprising of those most related to each other. In the past this was done by morphology, that is animals which shared similar physical characteristics. Nowdays DNA sequencing can provide further insights as to which groups species belong to and is still leading to the reclassification and reshuffiling of various species as new information comes to light.
Kingdom Animal is divided into large groups called Phyla. The largest of these is Phylum Arthropoda, the Arthropods, which account for close to 90% of all Animal species.1 Phylum Arthropoda is then broken down into various classes, one of these being the Insects, Class Insecta. Within the class insects are broken down into various orders such as bees & wasps, ants, beetles and so on (Figure 1).
Charateristics of Class Insecta
All insects have several distinct characteristics which separate them from
other classes of Arthropods. All insects possess 3 pairs of legs (6 legs), 3
separate body divisions (not always obvious), one pair of antennae and they are the only arthropods to posses wings. "But a catapiller doesn't have wings!", I hear you say. Wings are adult charateristics and are not always obvious. For instance there are wasps where only the males posses wings and the female does not, often leading them to be mistaken for ants. These species are found in the Family Mutillidae and are often known as 'Velvet ants' owing to the fact they resemble fuzzy ants.
other classes of Arthropods. All insects possess 3 pairs of legs (6 legs), 3
separate body divisions (not always obvious), one pair of antennae and they are the only arthropods to posses wings. "But a catapiller doesn't have wings!", I hear you say. Wings are adult charateristics and are not always obvious. For instance there are wasps where only the males posses wings and the female does not, often leading them to be mistaken for ants. These species are found in the Family Mutillidae and are often known as 'Velvet ants' owing to the fact they resemble fuzzy ants.
Where do spiders fit in?
Like Insects, spiders are also Arthropods. But they don't belong to Class Insecta, instead belonging to the Arachnida. One of the dead give aways is the fact they have 8 legs (four pairs) and not 6 (3 pair), a requirement if you are an insect. Below are several other classes of Arthropods, which possess similar features and are sometimes confused as insects, and the defining characteristics.1
Class Arachnida: spiders, scorpions, ticks and mites
2 body divisions
4 pairs of legs (8 total)
No antennae
No wings
Class Myriapoda: centipedes and millipedes
Number of legs can vary but is never less than 9pairs of legs (18 total).
Many body segments
One pair of antennae
No wings
Class Crustacea: includes crabs, shrimps and slaters
Number of legs can vary but never less than 4 pairs (8 total)
2 pairs of antennae
Body shape can vary, but never with 3 main divisions (unlike insects)
No wings
So there you have it. Bare in mind some things such as body divisions can be confusing, as 2 body segments may appear to be fused as one. But a good rule of thumb if you are unsure if something is or isn't an insect is to look at the number of legs and the number of antennae. Insects have 6 legs and only 1 pair of antennae!
Class Arachnida: spiders, scorpions, ticks and mites
2 body divisions
4 pairs of legs (8 total)
No antennae
No wings
Class Myriapoda: centipedes and millipedes
Number of legs can vary but is never less than 9pairs of legs (18 total).
Many body segments
One pair of antennae
No wings
Class Crustacea: includes crabs, shrimps and slaters
Number of legs can vary but never less than 4 pairs (8 total)
2 pairs of antennae
Body shape can vary, but never with 3 main divisions (unlike insects)
No wings
So there you have it. Bare in mind some things such as body divisions can be confusing, as 2 body segments may appear to be fused as one. But a good rule of thumb if you are unsure if something is or isn't an insect is to look at the number of legs and the number of antennae. Insects have 6 legs and only 1 pair of antennae!
References
Zborowski, P. & Storey, R. (2010) A field guide to Insects in Australia (3rd ed.) Australia: New Holland