With more than 450,000 described species, the world of beetles (order Coleoptera) is an astonishing realm of biological diversity. In the face of this enormous variation, which includes beetles ranging from microscopic feather beetles in the family Ptiliidae (a few tenths of a millimetre) to giants such as the giant carpenter beetle (Titanus giganteus) measuring tens of centimetres, the dichotomous key is the most accurate and reliable tool for identifying a particular individual - from family to species. Let us now leave aside modern molecular methods, based on DNA analysis, which are still inaccessible to amateur entomologists.
Dichotomous keys (from the Greek dichotomia, "division into two parts") are a fundamental pillar of taxonomy and applied entomology. They are not simply a list of possible identities, but function as a structured, hierarchical algorithm that guides the user step by step to the identity of an unknown organism through a series of binary decisions. Without these keys, the determination of most species in such a large and morphologically complex order would be almost impossible, as random comparisons with encyclopaedias are not only impracticable but also unreliable in practice. The keys effectively translate the observed morphology into a logical sequence that is based on relationships and leads to an unambiguous conclusion.
The essence of a dichotomous key lies in its strictly binary structure, which is based on the principle of elimination. Each step of the key, known as a couplet, represents a choice between two opposing statements (often labeled 1a and 1b, 2a and 2b, etc.), which are called leads. These statements describe specific, mutually exclusive morphological features that the user must verify on the specimen.
Structure and usage practice:
The user must carefully examine the specimen and decide which of the descriptions exactly matches the observed specimen. The process of keying requires not only a good eye, but also a precise knowledge of anatomical terminology to avoid confusion of terms (e.g., mistaking the tibia for the femur, or the tarsomeres for the penultimate article of the antenna). A misinterpretation in the very first steps of the key - which often divides beetles into large groups (e.g. Adephaga vs. Polyphaga) - inevitably sends the user to the completely wrong taxonomic branch, from which it is difficult to return.
For example, a step in the key might look like this:
1a. The trusses (Elytra) are truncated and the rump is exposed. (Example: Rattlesnakes, Staphylinidae). . . . . . . . . . . . Go to step 2
1b. The elytra covers extend along the entire length of the abdomen.(Example: Most families) . . . . . . . . . . . . . . . . Go to step 5
A more advanced step that might follow could involve oral health details, for example:
5a. The upper lip (Labrum) completely covers the incisors (Mandibulae); the antennae are lamellate. (Example: Beetles). . . . Go to step 6
5b. Antennae are visible even when the mouth is closed; antennae are saw-like or filiform. (Example.) . . . . . . . . . . . . Go to step 12
After selecting one of the options, the user is directed either to the next, finer step in the taxonomic hierarchy (e.g., from family to subfamily or tribe) or directly to the specified taxon (e.g., family, genus, or species name). The whole process is thus a series of logical eliminations, whereby only one correct identity is gradually selected from the many possible identities.
In the case of beetles, the dichotomous key has an irreplaceable role, as it relies on morphological features that are relatively stable within a taxon and easily observable with the aid of optics. These characters, also called diagnostic characters, have been evolutionarily verified and embody the true phylogenetic and kinship relationships between beetle groups. Success in beetle determination is therefore strongly correlated with the ability to accurately interpret key characters. Among the most important characters often used in keys that reflect basic evolutionary lineages are:
- The tarsal formula (tarsal formula)
The number of links on the sole (tarsus) of each pair of feet (e.g., 5-5-5, 5-5-4, 4-4-4). This formula is essential and is often already used to differentiate at the family level. Variation in the number of cells (called heteromerism) is the basic principle for classifying many large groups. Also important is the distinction between true tetameria (four articles) and so-called cryptopentameria, where the fifth article is so dwarfed that it is hidden in the notch of the fourth article (e.g. in the Curculionidae or Chrysomelidae). - The shape of the antennae
Stalked, saw-shaped, lamellate, filiform, ... - Antenna shape is critical for distinguishing families because it reflects adaptation to specific sensory tasks. For example, the lamellate antennae of the Scarabaeidae are adapted for detecting pheromones in the soil or air, whereas the saw-shaped antennae of the Elateridae and the thread-like antennae of the Cerambycidae are primarily for touch and orientation. - Shape and characters of the shield (Pronotum)
The presence of grooves, lateral pits, spines, or its general outline and proportions relative to the head and trunks. Shield features tend to be very constant in many groups. For example, the narrow, elongate shield of the covariates is diagnostic, as are the thick teeth or spines on the sides of the shield in some of the bowfins. - Characteristics of the trunks
The presence and arrangement of grooves (striae) and punctation, as well as the arrangement of veins (venation) in blanitic wings under the trusses (if the key requires it). The number and depth of grooves on the trusses are often used to distinguish species within a genus.
However, accurate and effective use of the dichotomous key requires appropriate technical support. Further information regarding the appropriate technical equipment used in determination can be found in the following sections of this Lexicon:
- Topics ⟶ Collection ⟶ Tools for Creating a Collection
Here you will find mainly descriptions of the tools, including pictures, that are used when working with dichotomous keys. These are mainly a good magnifying glass and/or a microscope. - Topics ⟶ Collection ⟶ Determination and organization of the collection
Here you will find a description of the actual work procedure for determining insects, descriptions of other tools and equipment, and perhaps even a list of the most common mistakes made by entomologists when making determinations.
The Dichotomous Key is thus not just a collection of descriptions, but a precise, standardized scientific tool that allows us to navigate the complex taxonomic "landscape" of beetles. Its role in identifying beetles is absolutely primary; it is a bridge between the immense diversity of life and its categorization. Mastery of these keys, supported by a deep knowledge of morphology and the right optical equipment, is a fundamental and indispensable skill for any entomologist and forms the basis for reliable scientific knowledge in the order Coleoptera.
An overview of dichotomous keys applicable for the determination of Coleoptera representatives in the Czech Republic: