Beetle morphology

Text © Christoph Benisch, 2007

1.  Basic scheme of beetle morphology

The morphology of the beetle follows a quite uniform scheme. In the diagram on the left side the basic morphology of a beetle is shown (Carabus auratus, dorsal view). A typical characteristic of beetles are the hard elytra. The scientific (Latin) name of the order is Coleoptera, which translates to "sheathed wings".

Like in all insects, the body of the beetle is segmented in three main parts: Head (Caput), thorax and abdomen. The sense organs are attached to the head, namely the eyes (Oculi), antennae and mouthparts. Behind the head is the pronotum, which is the top sclerite of the prothorax. The center of the pronotum is also called dorsum. At the base of the pronotum, between the elytra the small triangular mesoscutellum can be seen. The abdomen is normally covered by the elytra. The elytra are very import in beetle identification, as they often display characteristic shapes or maculation.

In the ventral view, the thorax can be seen, which consist of three segments: prothorax, mesothorax and metathorax. The prothorax is always clearly separated and bears the front pair of legs. The articulation is separated from the thorax by the so called episternum. The part of the thorax directly located behind the articulation is called epimerum. Attached to the prothorax is the mesothorax. It bears the middle pair of legs. The last segment of the thorax is the metathorax. It bears the rear pair of legs.

The abdomen is the 3rd main segment of the body. The upper side is covered by the elytra, the lower side forms the venter. The abdomen consists of 8-9 segments. The dorsal segments are called tergite, the ventral segments sternite. The segments are more heavily chitinized and flexibly connected to each other, allowing the abdomen more flexibility than the head and thorax. The apical tergite is usually visible in dorsal view and is called pygidium. The apical sternite is also called anal sternite. At sides of the tergites tiny holes are located, the so called spiracles Through these pores oxygen-rich air can diffuse into tracheal tubes.

Despite this uniform basic scheme, the beetles can differ considerably in their appearance. In the body length alone rather large differences are found: The smallest Central European species are only 1 mm long (e.g. Scydmaenidae, Leiodidae), whereas the largest Central European beetle, the stag beetle (Lucanus cervus) can reach up to 8 cm of body length.

2.   Head and mouthparts

The head is more or less flexibly attached to the thorax by the cervix. The head is composed of a rigid capsule and contains the eyes (oculi), antennae and mouthparts. On the sides of the head below/in front of the eyes the cheeks (genae) are located. Those parts located on the side behind the eyes are called temple (tempus). The upper side of the head in front/between the eyes is the forehead (frons), the upper part posterior to that is called vertex.

The front part of the head is called clypeus and is often separated from the head by a rigid suture, the frontoclypeal suture. Attached to the clypeus is the upper lip (labrum), which is separated from the clypeus by the somewhat flexible clypeolabral suture. In some beetle species, the labrum is covered by the clypeus and not visible in dorsal view. On the anterior end of the head the mouthparts are located, which consist of several parts. Species feeding on plants use their mandibles to bite off and chew their food, predators usually have edged and pointy mandibles to capture and retain their prey. In some species, the mandibles are not functional for food intake, e.g. in the males of the stag beetle Lucanus cervus. Their mandibles are formed like antlers, which are used for fights between male opponents. On the lower front side of the head the maxilla are located. At their sides segmented palps (palpus maxillaris) are attached. They are usually visible from dorsal view. The labium or lower lip finally is a fused structure consisting of several parts (submentum, mentum, glossa). The segmented palps attached to the mentum are called palpus labialis.

3.   Antennae

The antennae vary greatly among the beetles and are of high relevance for the determination of many species. The serve for sensory perception and can detect motion, odor and chemical substances. They are segmented and usually consist of 11 parts, the first part is called scape (scapus), the 2nd pedicelle (pedicellus). The other segments are jointly called flagellum. There are many different types of antennae in beetles. Antennae with a thread-like shape are called filiform [A], gradually clubbed antenna are called clavate [B], such abruptly clubbed at the end are called capitate [C]. Antennae with a saw-toothed shape are called serrate [D], antenna formed like a comb are called pectinate [E]. If the first segment is longer and the antenna is bent, it is called geniculate [F]. Antennae ending in nested plates are known as lamellate [G].

types of antennae

The different types of antenna are highly relevant for the determination of beetles. Ground beetles (Carabidae) do most have filiform antennae (type A), geniculate antennae (type F) are found in Lucanidae and Curculionidae. Click beetles (Elateridae) often have serrate (type D) or pectinate (type E) antennae. Lamellate antennae (type G) are typical for scarabaeid beetles (Scarabaeidae).

4.   Elytra and alae

The elytra are the stiff and strongly sclerotized forewings of the beetles, modified to protect the sensitive hindwings (alae) when at rest. They are not any longer used for flying. In most cases, the elytra cover the abdomen, however there are beetle species with shortened elytra (e.g. rove beetles (Staphylinidae)), in which the tergites are at least partially visible. The elytra meet along the elytral suture, a distinctive longitudinal line across the abdomen. Adjacent to the elytral suture in many cases a sharp thin line can be observed, the so called stria suturalis. In some species the elytra are fused together and form a closed carapace. In those cases also the membranous wings (alae) are reduced or totally missing and these are flightless (e.g. some ground beetles).

The elytra often exhibit characteristic structures, punctures and colorations. The structure can be regarded as remains of the framework of veins of the membranous alae, from which the elytra have developed during evolution. The elytra can also bear setae, hairs or scales, which are also relevant for determination. In the diagram below the characteristic elytral structures of four closely related ground beetles of the genus Carabus (C. violaceus, C. purpurascens, C. problematicus and C. intricatus):

When not in use, the membranous wings (alae) are folded and tucked under the elytra. In flight the elytra are lifted and separated and the membranous alae are unfolded. After the flight the alae are again tucked under the elytra as illustrated below for the stag beetle Platycerus caraboides. In a few cases, e.g. in some Scarabaeidae the elytra stay fused in flight and during lift-off the elytra are just lifted a little and the alae unfurl from the sides.

The alae consist of a diaphanous membrane, stabilized by veins. The longitudinal veins contain the wing tracheae, which supplies the alae with hemolymphe. The transversal veins do not contain tracheae, but simply stabilize the membranous wing.

Alae For the determination of several orders of insects, e.g. hymenopterans and dipterans the veins are highly relevant, however in the determination of beetles the alae and their vein structure are used very seldom. The basic principle of a membranous beetle wing is illustrated in the diagram on the left. The costal vein (C) is the front vein and is located on the front edge of the wing. This vein is unbranched. Behind the costal vein the subcostal vein (Sc) is located, which ends at the edge of the wing as well. The following longitudinal veins are called radial vein (radius, R), median vein (media, M), cubital vein (cubitus, C) and anal vein (analis, A). These veins are branched. Together with the transversal veins they enclose segments, which are also referred to as cell. Those cells enclosed by veins on all sides are called closed cell, those located at the edge of the wing are called open cell.

5.   Legs

The beetles, like all other insects have 3 pairs of legs. One pair of legs is anchored in the pro-, meta- and mesothorax. The leg is segmented in six parts: The coxae are articulations, anchoring the leg firmly into the thorax, yet allowing front-and-back movement. The trochantin is attached to the coxae and often barely visible. The next segment is the thigh, also called femur. At the apical end of the femur the shin (tibia) is attached. The last segment is called foot (tarsus) and consists of 2-5, mostly 4-5 parts. They can be broadened and carry sticky pads of packed setae (e.g. in males of ground beetles or in longhorn beetles).

The legs are used for locomotion and are specifically adapted to the way of life of the respective species. Predatory ground beetles have long and slender legs, enabling them to run after prey. Dung beetles or histerid beetles have fossorial, rake-like legs used for digging. Aquatic species exhibit legs adapted to swimming. In diving beetles (Dytiscidae) the tarsi are long, broad and equipped with long seta, acting like an oar and allowing them swimming and diving at amazing speed. In some species within the leaf beetles (e.g. subfamily Halticinae) and weevils (subfamily Rhynchaeninae) the hind legs, especially the femora are increased and used for jumping. These beetles can make very large jumps, compared to their small body size.