|Life Cycle & Structure of Beetles|
Within the order Coleoptera there are four suborders: Archostemata,
Myxophaga, Adephaga, and Polyphaga, which between
them contain 166 families. With about 350,000 species
described, Coleoptera is the largest order in the animal
kingdom. It is believed that there are millions more species as
yet undiscovered and undescribed. The earliest beetles are
found in the fossil record during the Lower Permian, about
265 million years ago.
The common name “beetle“ comes from an archaic English
word meaning “little biter“. The name Coleptera is attributed to
Aristotle; coming from the Greek „koleos,“—sheath or shield, and
“ptera,“—wings. The “shields” are the beetles’ eleytra, its wing
sheaths. Most insects have two pairs of flight wings; in beetles,
the forward pair has evolved into hard wing cases which protect
the wings and body. These eleytra are raised during flight, and
when closed over the wings and body they meet at the midline
of the beetle’s back.
Beetles go through complete metamorphosis. Their life
cycle consists of four phases: egg, larva, pupa, and adult. Eggs
are smooth, soft, and semi-permeable, allowing water and
oxygen to penetrate. Their size, shape, and color vary widely
among species. The egg hatches into a larva. Typically there
are 3-5 larval stages called instars, although some beetles may
have as many as 30 instars. The larva’s sole function is to eat
and as it develops, it outgrows and molts its exoskeleton, increasing
in size with each successive instar. The final larval
instar is followed by the pupal stage. Despite appearing to be
dormant, the pupal beetle is undergoing a dramatic transformation
from an organism optimized for eating to one whose
function is reproduction. The adult beetle will emerge to find a
mate, and give rise to another generation. Depending upon the
species, the lifespan of the adult beetle can vary from a single
season to several years.
Beetle bodies are made up of three sections: the head,
the thorax, and the abdomen, all encased in a hard exoskeleton.
The exoskeleton is made up of plates called sclerites, separated
by thin elastic sutures. This design combines the protective
qualities of armor with flexibility of movement.
The legs have five segments with a claw at the end of the
last tarsal segment. Diversity of leg lengths and forms reflects
the varied habitats and lifestyles. Leg forms are specialized for
digging, swimming, running, or leaping.
The beetle’s antennae function as organs of smell and
contain delicate sensory mechanisms that detect vibration,
changes in temperature and humidity, and pheremones. Antennae
vary in length, usually shorter than the beetle’s body length,
but in some long-horned beetles, up to three times greater than
body length. Also extremely varied in form, antennae may be
threadlike, sawtoothed, comblike, feather-like, or clubbed.
Beetles have hemispheric compound eyes with multiple
facets gathering light to form a single image. In species active
during daylight hours, the facets are smaller and flatter, while
nocturnal species have larger more convex facets. Beetles
are capable of perceiving light in the ultraviolet and infrared
ranges. Some pond dwelling species have split eyes, somewhat
like bifocals, which allow for visual acuity above and below the
The beetle possesses two pairs of “jaws,” one pair of maxillae
and one pair of mandibles. The mandibles are the most
visible structures, and function in a horizontal scissorlike motion
for grasping, cutting, or crushing food or enemies. Two pairs of
finger-like appendages, the maxillary and labial palpi, function
to move food into the mouth.
The basic structures that all beetles share exhibit enormous
diversity of form reflective of the diversity of environments
they occupy. Evolving to exploit every possible habitat and food
source, beetles have become the most successful creatures
on earth. Their ability to survive and ultimately capitalize on
environmental change virtually guarantees that they will
survive on this planet long after we are gone.