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Noninsect Hexapods

Insects belong to a group of animals called hexapods (see pp. 10-11). This group also includes three classes - collectively known as noninsect hexapods - that are generally regarded as being distinct from insects: the Diplura (diplurans), Protura (proturans), and Collembola (springtails).

Most noninsect hexapods live in soil or leaf litter. None have wings, and some even lack eyes and antennae. The major difference between these hexapods and insects, however, concerns their mouthparts. Unlike insects, the mouthparts of the noninsect hexapods are enclosed within a pouch, which is located on the underside of the head. When in use, the mouthparts are pushed out of the pouch to scrape, bite, or pierce the food.

hemolymph: transports nutrients -and waste products pharynx: connects month to esophagi/.

salivary gland: secretes saliva to start digestive process ganglion: nerve INTERNAL OkiiANS Ol renter with,n A I I()M Y HI I body segment j Malpighian tubule: extracts waste products from hemolymph midgut: produces digestive enzymes and abso / bs n u trient s hindgut: where food absorption is completed ventral nerve cord: conducts impulses between brain and ganglia (nerve masses)

Springtails

These arthropods are the most abundant and widespread o f the no ninsect hexapod classes. They are either elongate or rounded - like the springtail shown here (See pp.201-1 /).

brain:

dorsally located in insects esophagus: carries food to crop heart: tubular and located at the bad' of the abdomen: pumps hemolymph around body r

TYPES OF INSECT Insects can be divided into three groups, depending on the way that they develop during their lifetime. Primitive, wingless insects, such as silverfish, develop to adulthood by molting periodically throughout their life. Winged insects change either by a gradual process, called incomplete metamorphosis, or undergo a more sudden transformation, called complete metamorphosis, which involves a pupal stage (see pp.20-23).

The very first winged insects developed by gradual metamorphosis. A pupal stage did not evolve until the Permian period (290-245 million years ago), perhaps in response to climatic conditions (the pupa made it possible for insects to survive a period of cold). An increasing degree of tissue reorganization within the pupa also meant that larval stages were no longer just miniature versions of adults. Larvae became "eating machines" and adults "breeding machines." The success of the pupal stage can be seen clearly today. Kighty-five percent of all living insect species develop in this way, and the majority of those that do belong to one of four large, successful orders: the (loleoptera, Diptera, Lcpidoptera and I lymenoptera.

A Most Primitiyi

The first inserts were wing/ess scavengers that appeared more than 400 million years ago. The most primitive insects alive today, the hristlet ails (above) and silverfish, are similar in function and appearance.

A Most Advanced

Insects o f the order Hymenopte/a (see pp. / 7S-206), such as bees (above), are considered to be the most advanced. Many live in colonies, often with t astes that perform separate tasks.

Smallest

Some parasitic wasps (above) are among the tiniest insects on Earth, with a body length less than 'A.'in (I mm).

Biggest

Insects in prehistoric times were much larger than they are today. l.arge species, however, still survive, and the spider-hunting wasp, shown here, which grows up to ¿Vdti (1cm), is among the biggest species alive today. The smallest species could sit on its foot.

elytra beginning to open large/ • hin divings twisted, prodtn ing vortices that help to generate lift hind legs trailing elytra fully open and raised hind wings unfolding

Wings and Flight

One of the key factors in the success of insects as terrestrial species was the evolution of flight. Insects were the first animals to take to the air, and this ability enabled them to evade enemies and find food and mates efficiently. Inserts had evolved wings before the Carboniferous period (350-290 million years ago), but these early fliers were not able to fold their wings back along their bodies. By the middle of this period, some insects had evolved this ability, and it allowed them to use a far greater range of microhabitats, such as cracks and crevices in ^^ dead wood, inside leaf ¿a^»^

litter, or under stones. It also meant that they could hide from predators. The descendants of these species were highly successful, and today it is only dragonflies and mayflies that cannot fold their wings in this way.

elytra beginning to open large/ • hin divings twisted, prodtn ing vortices that help to generate lift hind legs trailing elytra fully open and raised hind wings unfolding

Flight Sequence

These steps show the cockchafer beetle preparing to fly and then in full flight. I '//like birds, insects need to warn/ up their flight muscles before they can take off. '¡'hey do this by basking in the sun or vibrating their wings.

The Secrets of Success

1 hroughout their evolution, several factors have combined to make insects the most successful of all species on this planet. Today, they make up over half of the species alive. There are several reasons for their success, mainly their ability to fly and reproduce quickly, their generally small size, and their protective cuticle (external exoskeleton) and their insulated central nervous system.

Factor

Effect

Cuticle (ex ternal exoskeleton)

Tough and waterproof the cuticle helps to pr otect insects from predators and also from dehydration.

Flight

This allows insects to escape from enemies, to find new habitats and food sources rapidly, and to establish new colonies.

Fast reproduction

Insects evolve at a high rate and adapt quickly to changing environmental conditions.

Insulated central nervous system

Insulation of the centr al nervous system allows nerves to work efficiently and also allows survival in hot or dry places.

Size

Small size allows utilization of a wide range of microhabitats -a tree, for example, may support hundreds of insect species.

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