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Do you know how to identify a beetle?

In my recent search for more knowledge regarding beetles or Coleoptera, I ran into many websites, PDFs, and books that told me not to identify beetles using shape and color. I was astounded because I used color and shape for identification since childhood, only to realize that this simply hints at a possible family of beetles. I will be the first to admit that I was wrong and that identifying Coleoptera requires a fine-tuned method passed on by the scholar Carolus Linnaeus. Carolus Linnaeus was a Swedish scholar who invented a way to classify living things in the 1700s. The method passed on to us is called taxonomic classification, which follows an organizational approach to classifying life from the largest group of all living things (Domain) all the way down to the most specific classification (species).

Beetles belong to Coleoptera, a taxonomic rank called order. To get here, we have to go down the list:

  • Domain - Eukarya or a cell with a true nucleus

  • Kingdom - Animalia

  • Phylum - Arthropoda or an invertebrate with a segmented body and jointed limbs

  • Class - Insect

  • Order – Coleoptera or sheathed wings

  • Suborder – Explained below

  • Family – Explained below

  • Genus – Explained below

  • Species – Explained below

As you may have noticed, we have yet to reach the smallest group used in classification. That is where the fun begins. The first step is to determine if a beetle would fall under the suborder Polyphaga or Adephaga. These suborders refer to where the hind coxa cavities are compared to the first sternite on the abdomen. I know this seems a little intimidating, admittedly I thought so too. Still, determining a suborder can easily be explained in the pictures below.

Fig. 1. Suborder Adephaga – 1st visible sternite is divided by coxa. Photo by N. Morrison/MYOPScience

Fig. 2. Suborder Polyphaga – 1st sternite is not divided by coxa. Photo by N. Morrison/MYOPScience

Once you have identified the suborder, you are now ready to move onto the family. We will continue with the Polyphaga example from Figure 2. Family can be determined by body shape, color, antennae shape (serrate, club, lamellate), number of antennae segments, tarsal formula, and several others.

Fig. 3. Tarsal Formula – 4-4-4 is the number of tarsi segments located at the end of the tibia and is associated with the family Cerambycidae. Photo by N. Morrison/MYOPScience

Tarsal Formula refers to the number of segments an insect has in its feet or tarsi. The Scarabaeidae family has a Tarsal Formula of 5-5-4 or five tarsi segments in the front, five in the middle, and four in the back. Different tarsal formulas can give hints to which family a beetle may fall into.

Fig. 4. Antennae – Serrated with 12 segments. Photo by N. Morrison/MYOPScience

Now that we have figured out the family is Longhorn beetle (Cerambycidae), based on the body shape, tarsal formula, and antennae features; we are ready for Genus. If we looked through an insect guide, we would find a genus under Cerambycidae described as a brown or black beetle with 12 or more segmented, strongly toothed antennomeres on their large antennae; this would be determined as the genus Prionus.

Lastly, we identify species by location and features that differ from one species to the next. Sometimes the differences between species prevent the two from physically mating. There are only one Prionus species in California that I am aware of, and it is Prionus californicus.

We have identified our first beetle. This is a somewhat simplified version of identification due to 2 missing suborders, many more terms to describe body shape, antennae shape, disregarding genitalia anatomy, and ignoring genetics. Overall, it is a good start to the identification of Coleoptera, and I’m glad you took the journey with me because I’m learning as well.

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