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#6: What defines an insect?



A general overview of arthropod body plans and phylogeny (relationships) of the four extant groups. <https://dev.biologists.org/content/129/5/1225/F1>

 Litinsects #6: What defines an insect?

I realize I’ve been throwing around words like arthropod and insect but maybe not everyone knows how these groups are related. Today I'm going through some classification basics.

First off, insects are ARTHROPODS, meaning they have paired JOINTED LIMBS. Arthropods include 5 main subgroups: (Future posts will go into each of the other arthropod groups in more detail...)
    • (1) Trilobites, which are extinct
    • (2) Chelicerates, which include spiders, scorpions, and mites
    • (3) Myriapods, consisting of millipedes and centipedes
    • (4) Crustaceans, including lobsters, crabs, and barnacles
    • (5) Hexapods, the group to which insects belong

  • Arthropods have a hard EXOSKELETON made of chitin that supports and protects the body. All arthropods grow by MOLTING, a process by which they shed their exoskeleton known as ECDYSIS

There are some defining anatomical features that ALL INSECTS have in common:


  • Different groups of arthropods vary in the number of leg pairs. Insects belong to the subgroup HEXAPODA since they have 6 LEGS!

  • The hexapods are divided into 2 groups, differing in the position of their mouthparts. All insects, aka Ectognatha from ecto “outside” and gnathos “jaw”, have EXTERNAL MOUTHPARTS. The other hexapod group is Entognatha, from ento “inside” and gnathos “jaw”, which have mouthparts recessed into a pouch in their heads. These include Collembola, Diplura, and Protura, all of which are apterous (wingless)

  • The insect body plan includes 3 basic units: 
    • (1) the HEAD, which include the eyes, antennae, and mouth parts
    • (2) the THORAX, to which the 6 legs and wings attach
    • (3) the ABDOMEN, which contains most of the reproductive, digestive, respiratory, and excretory structures

  • Each body unit is divided into segments. The word INSECT comes from the Latin insectum meaning “cut up” or “segmented”. With the word ENTOMOLOGY, it’s basically the same thing only in Greek; entomon means “cut into pieces” which then became the Greek word for insect.

I hope this cleared up some of the relationships among these groups and gave you a better idea of what actually defines an insect. TAXONOMY, or the classification of organisms, is a very interesting but very complex field. Some evolutionary relationships are still not fully understood or are constantly changing with new data. Scientists do their best to classify species based on most recent knowledge but these are not necessarily set in stone.

Thanks for reading... more to come tomorrow!
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