Beetles
Beetles are a group of insects that form the order Coleoptera. The word
"coleoptera" is from the Greek κολεός, koleos, meaning "sheath"; and πτερόν, pteron, meaning "wing", thus
"sheathed wing", because most beetles have two pairs of wings, the
front pair, the "elytra", being hardened and thickened into a
shell-like protection for the rear pair and the beetle's abdomen. The order
contains more species than any other order, constituting almost 25% of all
known animal life-forms. About 40% of all described insect species are beetles
(about 400,000 species), and new species are discovered frequently. The largest
taxonomic family, the Curculionidae (the weevils or snout beetles), also
belongs to this order.
The diversity of beetles is very wide-ranging. They are found in almost
all types of habitats, but are not known to occur in the sea or in the polar
regions. They interact with their ecosystems in several ways. They often feed
on plants and fungi, break down animal and plant debris, and eat other
invertebrates. Some species are prey of various animals including birds and
mammals. Certain species are agricultural pests, such as the Colorado potato
beetle Leptinotarsa decemlineata, the boll weevil Anthonomus grandis, the red
flour beetle Tribolium castaneum, and the mungbean or cowpea beetle
Callosobruchus maculatus, while other species of beetles are important controls
of agricultural pests. For example, beetles in the family Coccinellidae
("ladybirds" or "ladybugs") consume aphids, scale insects,
thrips, and other plant-sucking insects that damage crops.
Species in the order Coleoptera are generally characterized by a
particularly hard exoskeleton and hard forewings (elytra, singular elytron).
These elytra distinguish beetles from most other insect species, except for a
few species of Hemiptera. The beetle's exoskeleton is made up of numerous
plates called sclerites, separated by thin sutures. This design creates the
armored defenses of the beetle while maintaining flexibility. The general
anatomy of a beetle is quite uniform, although specific organs and appendages
may vary greatly in appearance and function between the many families in the
order. Like all insects, beetles' bodies are divided into three sections: the
head, the thorax, and the abdomen. Coleopteran internal morphology is similar
to other insects, although there are several examples of novelty. Such examples
include species of water beetle which use air bubbles in order to dive under
the water, and can remain submerged thanks to passive diffusion as oxygen moves
from the water into the bubble.
Beetles are endopterygotes, which means that they undergo complete
metamorphosis, a biological process by which an animal physically develops
after birth or hatching, undergoing a series of conspicuous and relatively
abrupt change in their body structure. Coleopteran species have an extremely
intricate behavior when mating, using such methods as pheromones for
communication to locate potential mates. Males may fight for females using very
elongated mandibles, causing a strong divergence between males and females in
sexual dimorphism.
External morphology:
The morphology of a beetle, with a fiddler beetle as an example species
Beetles are generally characterized by
a particularly hard exoskeleton and hard forewings (elytra). The beetle's
exoskeleton is made up of numerous plates, called sclerites, separated by thin
sutures. This design provides armored defenses while maintaining flexibility.
The general anatomy of a beetle is quite uniform, although specific organs and
appendages may vary greatly in appearance and function between the many
families in the order. Like all insects, beetles' bodies are divided into three
sections: the head, the thorax, and the abdomen.
Head:
The head, having mouthparts projecting forward or sometimes downturned,
is usually heavily sclerotized and varies in size. The eyes are compound and
may display remarkable adaptability, as in the case of whirligig beetles
(family Gyrinidae), where they are split to allow a view both above and below
the waterline. Other species also have divided eyes – some longhorn beetles
(family Cerambycidae) and weevils – while many have eyes that are notched to
some degree. A few beetle genera also possess ocelli, which are small, simple
eyes usually situated farther back on the head (on the vertex).
Beetles' antennae are primarily organs
of smell, but may also be used to feel a beetle's environment physically. They
may also be used in some families during mating, or among a few beetle species
for defence. Antennae vary greatly in form within the Coleoptera, but are often
similar within any given family. Males and females sometimes have different
antennal forms. Antennae may be clavate (flabellate and lamellate are subforms
of clavate, or clubbed antennae), filiform, geniculate, moniliform, pectinate,
or serrate.
Beetles have mouthparts similar to
those of grasshoppers. Of these parts, the most commonly known are probably the
mandibles, which appear as large pincers on the front of some beetles. The
mandibles are a pair of hard, often tooth-like structures that move
horizontally to grasp, crush, or cut food or enemies (see defence, below). Two
pairs of finger-like appendages, the maxillary and labial palpi, are found
around the mouth in most beetles, serving to move food into the mouth. In many
species, the mandibles are sexually dimorphic, with the males enlarged
enormously compared with those of females of the same species.
Thorax:
The thorax is segmented into the two discernible parts, the pro- and
pterathorax. The pterathorax is the fused meso- and metathorax, which are
commonly separated in other insect species, although flexibly articulate from
the prothorax. When viewed from below, the thorax is that part from which all
three pairs of legs and both pairs of wings arise. The abdomen is everything
posterior to the thorax. When viewed from above, most beetles appear to have
three clear sections, but this is deceptive: on the beetle's upper surface, the
middle "section" is a hard plate called the pronotum, which is only
the front part of the thorax; the back part of the thorax is concealed by the
beetle's wings. This further segmentation is usually best seen on the abdomen.
Extremities:
The multisegmented legs end in two to
five small segments called tarsi. Like many other insect orders, beetles bear
claws, usually one pair, on the end of the last tarsal segment of each leg.
While most beetles use their legs for walking, legs may be variously modified
and adapted for other uses. Among aquatic families – Dytiscidae, Haliplidae,
many species of Hydrophilidae and others – the legs, most notably the last
pair, are modified for swimming and often bear rows of long hairs to aid this
purpose. Other beetles have fossorial legs that are widened and often spined
for digging. Species with such adaptations are found among the scarabs, ground
beetles, and clown beetles (family Histeridae). The hind legs of some beetles,
such as flea beetles (within Chrysomelidae) and flea weevils (within Curculionidae),
are enlarged and designed for jumping.
Wings:
The elytra are connected to the
pterathorax, so named because it is where the wings are connected (pteron
meaning "wing" in Greek). The elytra are not used for flight, but
tend to cover the hind part of the body and protect the second pair of wings
(alae). They must be raised to move the hind flight wings. A beetle's flight
wings are crossed with veins and are folded after landing, often along these
veins, and stored below the elytra. A fold (jugum) of the membrane at the base
of each wing is a characteristic feature. In some beetles, the ability to fly
has been lost. These include some ground beetles (family Carabidae) and some
"true weevils" (family Curculionidae), but also desert- and
cave-dwelling species of other families. Many have the two elytra fused
together, forming a solid shield over the abdomen. In a few families, both the
ability to fly and the elytra have been lost, with the best known example being
the glow-worms of the family Phengodidae, in which the females are larviform
throughout their lives.
Abdomen:
The abdomen is the section behind the
metathorax, made up of a series of rings, each with a hole for breathing and
respiration, called a spiracle, composing three different segmented sclerites:
the tergum, pleura, and the sternum. The tergum in almost all species is
membranous, or usually soft and concealed by the wings and elytra when not in
flight. The pleura are usually small or hidden in some species, with each
pleuron having a single spiracle. The sternum is the most widely visible part
of the abdomen, being a more or less sclerotized segment. The abdomen itself
does not have any appendages, but some (for example, Mordellidae) have
articulating sternal lobes
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