Crabs and lobsters belong to a group called crustaceans, often referred to as the “insects of the ocean”1, so it is understandable that they might be confused as true insects.
Although crabs and lobsters might look like scorpions or spiders they are actually more related to insects such as moths and beetles. Phylogenetically crustaceans and insects are considered sister groups.
An exoskeleton is a rigid structure covering the outside of their bodies and serves many of the same functions that the internal skeleton serves in vertebrates (reptiles, birds, and mammals, including humans).
Both crustaceans and insects are grouped as arthropods and they have a lot in common, including jointed legs, antennae, and an exoskeleton.
Spiders, scorpions and ticks, however, are arachnids that are not more closely related to crustaceans than they are to millipedes!
It supports their structure and movement and protects them from mechanical harm and evaporative water loss (in terrestrial species).
Other arthropods with exoskeletons include millipedes, centipedes, scorpions and spiders.
Are crabs related to spiders?
Spiders are arthropods, so they are classified as arachnids along with scorpions and ticks, whereas most crabs are crustaceans. However, the term “crab” can include true crabs, hermit crabs, and horseshoe crabs.
While true crabs and hermit crabs are crustaceans, horseshoe crabs are more closely related to spiders than they are to other crabs2.
Unlike other crabs, horseshoe crabs don’t have antennae and have six pairs of legs instead of five, and a tail3.
Additionally, their mouth is situated in the middle of their body, between their legs, and each leg has a pincer on the end to carry food to their mouth.
Did crabs evolve from spiders?
Crabs did not evolve from spiders. Instead, crustaceans and arachnids probably had the same water-dwelling arthropod ancestor.
Interestingly, crabs and lobsters are actually more closely related to flies, moths, and butterflies than they are related to spiders and scorpions!
The first known terrestrial arthropod was a millipede-like creature living around 425 million years ago4, whereas aquatic arthropods, like Trilobites lived 521 million years ago5 and horseshoe crabs 480 million years ago6.
In contrast, the first known terrestrial arachnid lived 419 to 359 million years ago7.
Are Lobsters Considered Bugs?
Since a lobster is a crustacean, not an insect, it is not technically a bug. However, the word “bug” is not a scientific term and is often used loosely to refer to any small arthropod8, 9.
Technically, true bugs are insects with four wings and sharp, sucking mouthparts from the order Hemiptera, such as stink bugs and aphids.
Lobsters and other crustaceans do not fit that description very well!
However, it would not be strictly wrong to call a crab or lobster a bug although they are not insects.
Why are Crustaceans Confused with Insects?
There might be many good reasons why people often confuse crustaceans and insects – they are, after all, sister groups in a phylogenetic sense (see figure at the end!).
Their small size, together with their exoskeletons, antennae, and jointed legs, can make crustaceans and insects appear similar.
Most people don’t know enough about either group of organisms to tell them apart. Once you know what to look for, though, it is easy to tell them apart.
How are insects similar to crustaceans?
Insects and crustaceans are similar in many ways. Most of them are small (less than 0.22 pounds or 0.1 kg), except for some crab species that can grow up to 14 pounds (6.4 kg)7.
Since the exoskeleton is rigid, both insects and crustaceans need to shed their exoskeleton (molt) to grow. Without the protection of their exoskeleton, arthropods are very vulnerable during their molting phase.
In addition to their exoskeleton, all insects and crustaceans have a body divided into three parts: a head, thorax, and abdomen, although in some species some of these parts can be reduced or fused.
For example, in crabs and lobsters, the head and the thorax are fused. In contrast, a beetle has a very distinctive head, thorax, and abdomen. In both insects and crustaceans, their body is divided into segments and each segment contains one pair of legs.
In most insects and some crustaceans, the abdominal appendages have been lost and they only have legs on their thorax.
Shrimp, on the other hand, have walking legs on their thorax, but small swimmerets on their abdomen.
How are Crustaceans Different from Insects? (How to tell them apart?)
Despite their similarities, you can tell crustaceans apart from insects by asking yourself the following questions:
- Where does it live?
- Can it fly?
- How many antenna does it have?
- How many legs does it have?
Where you find them will be your first clue. Crustaceans are generally aquatic, while insects are mostly terrestrial.
Secondly, if they can fly, they are insects since crustaceans do not have wings.
There are, however, many wingless insects, so this is not a fool-proof test. If they have pincers on their front legs, like in crabs and lobsters, they are crustaceans.
Again, some crustaceans, like woodlice, do not have pincers.
Insects only have two antennae, whereas crustaceans have four. However, in crustaceans, not all their antennae are necessarily the same length, so it might be easy to miss the smaller pair of antennae.
While the above-mentioned characteristics can point you in the right direction, the most definitive way to tell insects and crustaceans apart is to count their legs.
So next time you see a creepy-crawly, try this trick to count its legs and determine what it is:
If it has six legs, it is an insect. If it has eight legs, it is a spider or scorpion. Ten or more legs mean it is a crustacean. If it has more legs than you can easily count, it is a centipede or a millipede.
This is a good general rule to tell the different groups apart!
In addition, some more technical ways to tell insects and crustaceans apart would be from the thickness of their exoskeleton, the way they breathe, and the looks of their offspring!
Crustaceans and insects both have an exoskeleton made of chitin. However, most crustaceans have thicker and larger plates than most insects do.
Crustaceans also typically breathe using gills, while insects breathe using a tracheal system.
Finally, the young of most crustacean species resemble their parents more closely than the young of most insect species do, as these are often larvae before growing a full exoskeleton.
How Closely Related Are Crustaceans and Insects?
Insects and crustaceans are more closely related to each other than to any other arthropods, including spiders and millipedes.
The most recent studies of their genetics and brain morphology have established that insects evolved from crustacean ancestors10 approximately 400 million years ago11.
This means that the crustaceans and insects evolved from the same animal around 400 million years ago – an ancestor that was neither an insect nor a crustacean!
The tree above also really shows how the horseshoe crab evolved from something that looked like a trilobite and that some of its relatives went onto land and became spiders, scorpions, and ticks!
We have looked into the relatedness of insects and crustaceans and as you know by now, it is not entirely unjustified to confuse insects with crabs, lobsters, shrimp and crayfish but they are not the same!
However, they do belong to two different groups, that are again different from that of spiders, ticks and scorpions.
And during the course of history, it is more likely that the common ancestor of all bugs looked more like a crab than a spider!
So when you consider that insects evolved from crustaceans, then referring to crustaceans as “insects of the ocean” is really the wrong way around.
Instead, it would be more accurate to refer to insects as “shellfish of the land” as their ancestors came from the sea!
- Kindersley D. 2007. DK Nature: Crustaceans. Fact Monster.
- Tyrrell KA, 2019, Study confirms horseshoe crabs are really relative of spiders, scorpions. Phys.org News.
- National Oceanic and Atmospheric Administration Office for Coastal Management worksheet on horseshoe crabs.
- Dunham W. 2020. Millipede from Scotland is world’s oldest-known land animal. Reuters.com.
- Oxford University Natural History Museum. What were trilobites?
- Bicknell RDC, Pates S. 2020. Pictorial atlas of fossil and extant horseshoe crabs, with focus on Xiphosurida. Frontiers in Earth Science: 98. https://doi.org/10.3389/feart.2020.00098
- Barnes RD. 2021. Arthropod. Brittanica. https://www.britannica.com/animal/arthropod
- Collins Dictionary. 2005. Bug. https://www.collinsdictionary.com/dictionary/english/bug
- Merriam-Webster Dictionary. 2022. Bug. https://www.merriam-webster.com/dictionary/bug
- Tamone SL, Harrison JF. 2015. Linking insects with crustacea: Physiology of the pancrustacea: An introduction to the symposium. Integrative and Comparative Biology, 55 (5): 765-770. https://doi.org/10.1093/icb/icv093
- Ronshaugen M, McGinnis N, McGinnis W. 2002. Hox protein mutation and macroevolution of the insect body plan. Nature. 415 (6874): 914-917. https://doi.org/10.1038/nature716