BEETLES: CHARACTERISTICS, FEEDING, REPRODUCTION - BIOLOGY - 2024

The beetles or coleoptera (Coleoptera) are a group of terrestrial or flying insects that have a pair of modified wings. The rigid, sclerotized first pair of wings, called elytra, protects the abdomen and the second pair of membranous wings.

With more than 380,000 species, the coleopterans constitute the order of the Animalia kingdom with the largest number of described species. Commonly called beetles, they have a wide morphological diversity and are adapted to most terrestrial habitats.

Insect of the order Coleoptera. Source: flickr.com

These insects have a rigid and compact body, covered by an exoskeleton, being of variable sizes, from a few mm to 10 cm long. From dark colors to compositions with bright, metallic and iridescent tones.

The body is segmented into the head, thorax and abdomen, even though they appear to be divided into two parts, due to the presence of the pterothorax. The chewing mouth apparatus, a pair of compound eyes, and a pair of antennae are located on the head.

The wings, formed by the elytra and the membranous pair, are inserted next to the three pairs of legs on the thorax. Some species have protrusions of different shapes such as horns, jaws, antennae or spines that come out of the exoskeleton that covers the body.

The vast majority of beetles are herbivores, and different species are pests of agricultural crops. Indeed, larvae are the main agents that cause most of the agricultural and forestry damage.

characteristics

Coleoptera or beetles are commonly known as catangas, cascarudos, bubute, tacas, fireflies, light bugs, vaquitas, ladybugs, among others. It is the order of the most numerous class Insecta, more than 380,000 species have been identified, and they are located in various ecosystems.

The distinctive character of the coleopterans are their forewings or elytra that protect the second pair of membranous wings. The membranous wings have a longitudinal and transverse folding system that facilitates flight.

Like all insects, its body is divided into three segments: head, thorax, and abdomen, and it has three pairs of legs. They are of variable coloration, which often results from the environment where they live, from black and dark, to bright and metallic tones.

Its body presents a high degree of sclerotization, due to the accumulation and hardening of chitin in the exoskeleton. The size varies depending on the family, small like the Staphylinidae (2-10 mm) or large like the Scarabaeidae (2-180 mm).

Most of the coleopterans are herbivores, however, there are carnivores that feed on other insects or snails. The oral apparatus is chewing, both in larvae and in adults, with some species with a sucking or licking system.

Coleoptera are holometabolic insects, of complete metamorphosis, in addition their reproduction is of the sexual type, females and males must intervene. They are oviparous, the female lays the eggs, of different shapes, sizes and colors, on particular structures of the plant.

Coleopterans generally do not cause any harm to human beings, they even recycle organic matter or are used in biological pest control. However, some species are considered agricultural, forest, domestic and stored product pests, causing great economic losses.

Taxonomy

The beetles or coleopterans are an extensive group of insects belonging to the order Coleoptera Linnaeus (1758). The term Coleoptera comes from the Greek κολεός = koleos "box-case", and πτερον = pteron "wing", which means wings in a case.

At the taxonomic level, coleopterans are included in the Animalia kingdom. This classification includes those eukaryotic, heterotrophic, multicellular and tissue organisms.

These specimens are assigned to the most numerous and varied phylum in the animal kingdom, the phylum Arthropoda. That is, invertebrate organisms with an exoskeleton and articulated appendages, including insects, arachnids, crustaceans and myriapods.

The next taxonomic level includes the beetles in the class Insecta. This classification describes the organisms that have a body divided into a head, thorax and abdomen, have a pair of antennae, three pairs of legs and two pairs of wings.

Regarding the subclass, they are included in the Pterygota, a group of winged insects, that is, they exhibit a pair of wings in the second and third thoracic segments.

Coleoptera are under the superorder Endopterygota. This grouping includes holometabolic insects or with complete metamorphosis, whose development goes through egg, larva, pupa and imago.

Currently the order Coleoptera is divided into four suborders, identified as Adephaga, Archostemata, Myxophaga and Polyphaga. The Adephaga suborder is a large group of more than 40,000 species, where the families Caradidae and Dytiscidae stand out.

About 50 surviving species have been described from the Archostemata suborder. Some aquatic coleopterans have been described in relation to the suborder Myxophaga.

In the Polyphaga suborder, the largest number of coleopterans (350,000 species) is found. This suborder is in turn divided into the five infraorders Bostrichiformia, Cucujiformia, Elateriformia, Scarabaeiformia and Staphyliniformia.

Of the infraorders the main families are: Bostrichiformia (Dermestidae), Cucufiformia (Curculionidae), Elateriformia (Elateridae). Scarabaeiformia (Scarabaeidae), and Staphyliniformia (Staphylinidae).

Classification

The Coleoptera order is made up of a diversity of families categorized by their morphology and habits. These families include: Elateridae, Buprestidae, Cucijidae, Coccinellidae, Meloidae, Tenebrionidae, Bostrichidae, Anobiidae, Scarabaeidae, Cerambycidae, Chrysomelidae, Bruchidae, Dermestidae, Lyctidae, Anthribidae, Curculionidae and Scolytidae.

One of the most used classifications due to its economic importance for man is that based on the predatory activity of coleopterans. This classification includes the families Anabiidae, Chrysomelidae, Curculionidae, Nitidulidae and Scolytidae.

Families of economic importance

Family Anobiidae

• Carcomas or Queras: larvae of coleopterans of the Anobiidae family that pierce the processed wood of beams and furniture.
• Death Clock Beetle (Xestobium rufovillosum): wood-boring xylophagous beetle, it is one of the most damaging pests in wooden works and constructions.

Anobiidae family (Xestobium rufovillosum). Source: businessdiary.com

Family Chrysomelidae

Coconut Leaf Beetle (Brontispa longissima): a species that defoliates young leaves of seedlings in coconut seedlings.

• Dorífora or Potato Beetle (Leotinotarsa ​​decemlineata): it is a pest for solanaceous crops. The larvae of the insect defoliate the young leaves of the crop, completely eliminating the plants.
• Elm galeruca (Pyrrhalta luteola): beetle that feeds on elm leaves, in its larval stage and as an adult.

Chrysomelidae family (Brontispa longissima). Source: biolib.cz

Family Curculionidae

• Cotton Weevil (Anthonomus grandis): A tiny beetle that feeds on the tender tissues, shoots and flowers of cotton.
• Red Weevil (Rhynchophorus ferrugineus): beetle that in its larval stage attacks palm trees, making galleries inside them. In a high degree of infestation, the plant turns yellowish and suffers wilting, causing the death of the affected stem.
• Black Weevil or Agave Weevil (Scyphophorus acupunctatus): pests of the genus Agave, both in crops for the production of fiber and beverages, as well as in ornamental production. Likewise, it attacks Yuca plantations.

Family Curculionidae (Rhynchophorus ferrugineus). Source: flickr.com

Family Nitidulidae

• Small Hive Beetle (Aethina tumida): Considered a serious problem in commercial bee hives. Aethina larvae and adults consume the hive panels, consuming the honey, pollen, and young.

Family Nitidulidae (Aethina tumida). Source: wikimedia.org

Family Scolytidae

• Elm Bark Beetles (Scolytus multistriatus and Hylurgopinus rufipes): pests that cause damage to the foliage, it is also a source of transmission of the fungus Ophiostoma, which causes elm graphiosis, which destroys trees by invading conductive tissues such as xylem.

Family Scolytidae (Scolytus multistriatus). Source: wikimedia.org

Habitat and food

This group of insects is found in all habitats on earth, even underwater, with the exception of the open sea. They are species of diurnal and nocturnal habits, that live in colonies or are solitary and wandering.

They live on the ground, under stones and among the fallen leaves of the forests. On the plants in the stems, the foliage, flowers and fruits; on or under fresh water, and in deserts.

There are a good number of species that are agricultural pests, of stored grains and at the domestic level, causing enormous economic losses. For example, Chrysomelids are defoliator beetles, Curculionidae are processed food pests, and Derméstids feed on textile products.

The feeding of this immense group of individuals, including both the larva and the adult, is as wide as their distribution. Its preferred food is live plants: root, stem, leaves, flowers, pollen, fruits and seeds, causing defoliation, wounds and mutilated.

There are even species such as Hypothenemus hampei, a beetle that pierces and multiplies within the coffee fruit, causing severe damage. On the other hand, species such as Scarabaeus laticollis, which feed on decomposing organic matter, favor the dynamics of the ecosystem.

Feeding types

A classification of coleopterans based on their eating habits includes:

• Philosophers: beetles that feed on the foliage of plants. In this group the different pests of agricultural crops are included. For example: Family Chrysomelidae.
• Rhizophagus: species that feed on the root system of plants. For example: Family Elateridae.
• Polynivores or Polinífagos: they destroy the flowers, since they feed on pollen. For example: Family Oedemeridae.

Polynivore of the Family Oedemeridae. Source: wikimedia.org

• Antóphagos: they feed exclusively on flowers. For example: Subfamily Cetoniinae.
• Frugivores or Carpophagi: they feed on the fruits. For example: Subfamily
• Xylophages: they cause damage to forest areas, since the larvae of certain species feed on wood. For example: Cerambycidaeo Family.
• Spermophagi: they feed on the seeds. For example: Family Bruchidae.
• Fungivores or Mycophagi: they feed on fungi. For example: Family Ciidae.
• Predators: beetles that catch and eat other species of insects, or snails, slugs and worms, being useful for biological control. For example: Family Staphylinidae.
• Saprophagous: they feed on decaying litter and plant debris. For example: Family Scarabaeidae.
• Dung: they feed on mammalian excrement. For example: Subfamily Scarabaeinae.
• Ghouls: they feed on the corpses of other species. For example: Subfamily Silphidae.

Reproduction

Sexual reproduction is characteristic of the coleopterans. However, in some species parthenogenesis may occur, that is, the formation of a new individual without fertilization occurring.

Courtship and mating

Courtship is a set of acts that identifies the behavior of beetles before starting to mate. This type of activity ensures that only the fittest and strongest have a chance to reproduce.

Courtship and mating in Coleoptera. Source: flickr.com

The pre-mating ritual is essential for the cupola to occur; the female emits a pheromone that attracts the male. Female pheromones are chemical compounds (fatty acids, amino acids or terpenoids) that affect the behavior of the male in this case.

Other coleopterans, such as fireflies, use bioluminescence during courtship, a biochemical process that occurs in special organs located in the abdomen. The male flies or dances over the female, slaps her back, and finally caresses her with his antennae until she accepts the union.

Generally, the mating is short except for some species where it can last several hours. In this activity, fertilization occurs, the male transfers the sperm to the female to fertilize the egg.

After mating and fertilization of the female, within a few days the eggs are ready to be laid. The females deposit the eggs in an appropriate substrate that guarantees food for the larvae at birth.

Lifecycle

The order Coleoptera as a member of the superorder Endopterigota, is characterized by experiencing a complete metamorphosis or holometaboly. The transformation process that occurs in beetles happens through four phases or stages: egg, larva, pupa and adult.

Egg

Each female has the ability to fertilize thousands of eggs during her life cycle. The eggs have a soft and smooth, semi-permeable shell, with a size, shape and color depending on each species.

Coleoptera eggs. Source: backyardnature.net

The oviposition of the eggs is characteristic of each family of coleopterans. Some can be placed loose in the substrate, individually or grouped on the underside or bundle of the leaves, or buried in the ground.

In fact, some species provide special care to ensure food and the survival of new organisms. Some beetles build galleries in the substrate or within the leaves to protect the deposited eggs.

Larva

The larvae develop and hatch from the fertile eggs. Beetles in the larval stage can go from 3 to 5 stages, reaching up to 30 stages depending on the species.

Coleoptera larvae. Source: flickr.com

In this phase, the trained individual has the function of feeding himself and keeping reserves for his next stage. The larvae eat, grow and molt until they reach the pupal stage, in which they will complete the complete metamorphosis towards the adult or imago.

In this phase, a well-developed head is distinguished with a chewing-type oral appliance and spiracles along the body. The appearance of the larvae is subject to the type of feeding, development substrate and the family of beetles.

In this regard, the larvae can be flattened, cylindrical or ovoid, with the head sclerotized and darkened. They have a distinguishable thorax with three pairs of very small legs, and eight to nine abdominal segments.

Some species do not show molts, they simply increase in size as they feed, other individuals have more drastic changes. Parasitic species have a very active first stage, until they find the host, then they remain immobile, feeding inside the prey.

Feeding is restricted to the foliage of plants, while some species feed within the food source. The larval stage is subject to environmental conditions and food availability, lasting several years.

Pupa

Once the larva is ready to begin the metamorphosis process, it builds a protective structure called a cocoon where it remains immobile. Despite its inactive appearance, great changes occur within the pupa where the adult develops that will continue the life cycle.

Coleoptera pupae. Source: aquasnail.com

Adult

The adult emerges from the pupa after having fully matured inside the pupa, ready to start a new reproductive process. Adults can live for a season or eventually several months, depending on the species and ecological conditions.

Coleoptera adult. Source: flickr.com

Importance

The order Coleoptera comprises diverse organisms of biological, ecological, economic, medical and cultural importance. These beetles are bioindicators of the anthropic characteristics of an ecosystem, biodiversity and environmental conditions, as well as the degree of conservation of a habitat.

Different species include beneficial insects such as pollinators, weed controllers, and natural enemies of pest insects. However, there are beetles that in turn represent pests for food crops, stored flour and grains, and forestry operations.

At the agricultural level, phytophagous beetles cause great economic losses in commercial crops, their control requires large investments. However, there are predatory species that, used in biological control, allow excellent results to be obtained by including them in a comprehensive pest control.

At the ecological level, beetles participate dynamically in the functioning of agricultural and natural ecosystems. They act in the recycling process of decomposing organic matter, since they feed on litter, wood and organic waste.

On a medical level, certain species cause irritation to the skin of humans, by secreting toxic substances as a means of defense. On a cultural level, some species are edible for animals and man, especially in the larval stage, due to their high protein content.

References

1. Alonso-Zarazaga, MA (2015). Order Coleoptera. Magazine, (55) 1–18. Ibero Entomological Diversity. ISSN 2386-7183.
2. Bar, ME (2010). Order Coleoptera. Arthropod Biology 2010, 10.
3. Coronado Ricardo and Márquez Antonio (1986) Introduction to Entomology: Morphology and Taxonomy of Insects. Editorial Limusa. ISBN 968-18-0066-4.
4. Beetles (Order: Coleoptera) (2018) Mexican Biodiversity. National Commission for the Knowledge and Use of Biodiversity. Available at: biodiversity.gob.mx
5. Beetles (2018) Anticimex. Available at: anticimex.com
6. Zumbado, MA and Azofeifa, D. 2018. Insects of Agricultural Importance. Basic Guide to Entomology. Heredia, Costa Rica. National Program for Organic Agriculture (PNAO). 204 pp.