The Coleoptera Order belongs to the group of Holometabolic Insects (with
a metamorphosis divided into three distinct stages: larva, pupa and imago);
among them, they are a primitive Order, compared to Lepidoptera, Dyptera,
Hymenoptera, etc., but they are also the larger group of Animals we know,
with more than 300.000 species listed in the World. They are surely many
more, because a lot of them, mostly small species, are not yet identified,
especially in tropical regions. Their true number could reach 1 million
species or more! Every year many hundreds of new species are described,
also in well known countries (Europe, N. America, Japan). Several small
families are quite disregarded and unknown.
Anatomy of a Beetle
The most distinctive character of this Order is the first pair of wings
(elitrae). They are strong and coriaceous, useless to fly, and form a good
protective structure for the second pair of wings, usually well developed.
This peculiarity gave them their name Coleoptera (from two Greek words:
koléos = case, sheath, and ptéron = wing).
There are also many other distinctive features (in the structure of
antennae, buccal apparatus, wing nervatures, etc.) but their elitrae are
surely the most evident mark.
However, their wings, both the first and the second pair, are very
diversified in different Families and Tribes, and also in the same Genus.
Sometimes, they are extremely different in males and females of the same
species!
Why evolved their first pair of wings in these new structures? Surely
to protect their other wings! But we don't know much about when and how
they did. And, if this structure is so useful, why have so few other Insects
got it? (essentially Strepsiptera, a small Order of parasitic species,
closely related to Coleoptera, and the Order Heteroptera, but in a different
way). Probably, the first species of Beetles spent most of their time under
barks or among the rotting leaves and steams of the litter. So, a protection
for their second pair of wings could be very useful.
The most ancient fossil Beetles come from the Permic Period, when there
were no modern plants; especially, there weren't flowering plants (Angiosperms).
During that period there was no nectar to suck, so they maintained two
well developed mandibles, without evolving a sucking apparatus; effectively,
several groups of Coleoptera (e.g..: many Meloidae, some Buprestidae,
etc.) got it during following periods, but it is less perfect and specialized
than those of other Insects, especially Hymenoptera and Lepidoptera.
The anatomy of Coleoptera can vary very much.
Their basic scheme is quite simple, but their variations are virtually
numberless. These variations are used to separate this Order in:
Suborders, Series, Superfamilies, Families, Subfamilies, Tribes, Genera
and Species.
Any part of their body could be used, but the most important characters
are:
nervatures of the second pair of wings,
legs and their tarsal segments,
antennae, eyes, mandibulae, labium, palps, etc.,
shape, surface and hair of their head, prothorax, mesothorax, metathorax,
elitrae and abdomen,
and especially the male reproductive apparatus: aedeagus and endophallus.
