Hexapoda - insects page 1

Insect general information is spread over a number of pages. Click on the topic list below to be taken to the page you want, or the list of insect orders on the left, or use the search box on the top right corner of every page.

Smith, L. (2014). Characteristics of the insect orders. Amazon. Characteristics of the insect orders with drawings and photographs to help you understand the differences between the different types of insect, and identify which order an insect is in, as well as fast facts about each insect order, and links to web pages with more detailed information. Many orders have separate sections about the life cycle of the insect as well as its habitat requirements, and fossil history.

"If all mankind were to disappear, the world would regenerate back to the rich state of equilibrium that existed 10, 000 years ago. If insects were to vanish, the environment would collapse into chaos." E. O. Wilson

Overview of the insects

There are over a million described species and many more waiting to be described and even more waiting to be discovered; some authorities say as many as 4 million insect species await discovery. In fact more than 50% of all described species (animal and plant) on earth are insects. In the UK there are around 24,000 species. They are around 30 Orders, depending on which book you read. They range in size from less than 1 mm up to 20 cm in length.

The earliest fossil insects were found in Devonian rocks; were wingless and resembled modern springtails. Rhyniella precursor, a springtail found near Rhynie in N E Scotland dates from 380 MYA. The insects are thought to have started their colonisation of the land around 320 million years ago during the Carboniferous.

The secret of the success of the insects can be linked to the following features:

  • Tagmosis allowing specialisation in each body area.
  • Jointed appendages and striated muscles giving efficient locomotion.
  • Exoskeleton which provides protection and prevents dehydration while still being flexible and light enough to allow movement. The main component of the exoskeleton is a carbohydrate called chitin. The hardening of the cuticle is a result of sclerotization, i.e. the bonding of protein molecules and their cross-links within and between the lamellae of the procuticle. This forms sclerotin, a resistant and insoluble protein.
  • Complex musculature allowing rapid movement.
  • Branched tracheae and tracheoles allowing direct passage of oxygen to the cells, so enabling a relatively high metabolic rate. The trachea end in spiracles which run down the sides of the abdomen and thorax.
  • Complex sensory organs of vision, chemoreception, hearing and touch; all enhancing awareness of surroundings.
  • Complex behaviour patterns leading to social organisation in some.
  • Metamorphisim reducing competition between individuals of different stages in the life cycle, e.g. caterpillar and butterfly occupy different niches.

Insect body plan

There are a huge amount of body shapes, but the "basic insect" body plan is:-

  • Head. The head has the mouthparts in 3 parts (mandibles, maxillae and a labium) and most of the sensory organs including the eyes (three simple eyes as well as two compound eyes) and one pair of antennae.
  • Thorax. The thorax is subdivided into prothorax, mesothorax and metathorax; each bears a pair of legs - never more than 3 pairs in adult insects, and 2, 4 or no wings. The thorax is simply a box of muscles linked to the legs and wings. Adult insects have six legs, but larvae, e.g., caterpillars, maggots may have fewer or more than six legs (caterpillars have six true legs and up to ten prolegs).
  • Abdomen. There are eleven or fewer segments in the abdomen and they contain the organs for digestion, excretion and reproduction. The internal organs are bathed in haemolymph which transports waste and nutrients and is moved around the body by a tube shaped heart and the movement of the insect.

Flight

Insects were the first animals to fly. They had functional wings over 100 million years before flying reptiles or birds. The wings are outgrowths on the 2nd and 3rd segment of the thorax. Unlike the wings of bats and birds, insect wings are not modified limbs. In fact all 3 segments of the thorax bear a pair of legs in most adult insects.

dragonfly wing wasp wing, comparison of the number of veins

Wings are formed of the same material as the rest of the exoskeleton - chitin. The wings are a double cuticular membrane containing veins which provide structural stability.

The major veins in the wings may contain tracheae, blood and nerve fibres. The pattern of veins in the wings is sometimes used to identify different groups of insect, but the terminology used can vary between different orders of insects, and even within an order it can vary according to which book you use.

Insects that do not have two pairs of wings include those that are secondarily wingless, and the Diptera - flies which have one pair of wings and a pair of balancing organs (halteres), and the Strepsiptera which have a reduced forewing.

Rear wing of a bumblebee queen showing the hooks (hammuli) which join it to the frint wing

Generally the wings of species that evolved earlier e.g., dragonflies have more complex venation than those that evolved later e.g., wasps and bees.The drawing above right shows the very simplified venation of a parasitic wasp on the left, and the more complex venation of a dragonfly on the right, and the photograph on the right shows the rear wing of a bumblebee queen.

Wings can be naked or covered in scales (butterflies and moths) or hairs (caddis flies).

Some insects have wings that are modified such as the Coleoptera (beetles) where the first pair of wings forms a hard case (elytra) covering and protecting the second pair. Other insects may have wings for only a short time in their adult life, e.g. the sexual stages of ants and termites.

There is no doubt that the evolution of wings greatly helped in the dispersal and radiation of insects.

different kinds and shapes of insect antennae

The antennae

Insect antennae (left) can vary greatly in size and shape even between sexes of the same species.

Insects detect smells mainly using their antennae. The antennae are also used to touch, taste and in some cases, to detect sound.

The drawing on the left shows just a few of the many different types of antennae.

There are a range of modifications to the insect body parts that partly account for their great taxonomic and ecological diversity.

Leg modifications (see below and right) include:

  1. The raptorial foreleg of preying mantises
  2. The hind legs of grasshoppers, etc. that are modified for jumping
  3. The hind legs of social bees that have modifications for storing and packing pollen.
  4. The paddle-shaped hind legs fringed with hair that are modified for swimming in aquatic beetles.

The leg of the cat flea, Ctenocephalides felis, (not illustrated) enables it to jump 30 cm. This is because it has a hugely elastic material called resilin which is usually held compressed by the muscles in the thorax. When these muscles relax the resilin springs back to its original shape releasing a burst of energy which is transmitted to the hind legs.

The legs often have hairs and bristles which are sensitive to touch and air movement. Below left is the typical insect leg. There are usually between 1 and 5 tarsal segments, usually ending in 2 claws. The number of segments is often important in identifying species. Tarsal hairs and pores are often sensitive to smell and taste. Crickets have their ears on their front legs at the top of each tibia.

insect legs showing the great variety of shapes and functionsTypical insect legs with named parts

Insect reproduction

Insect reproduction falls into 3 main phases: courtship, mating and parenting. Not all insects do all 3 phases.

Insect Courtship

This is when the male and female attract each other. In some this involves sexual dimorphisim and behaviour, i. e. the sexes look different - usually it is the male that tries to attract the female, e. g. stag beetles with their huge mandibles, "blue" butterflies, where only the male is brightly coloured, some male dragonflies defend a territory against all but females of the same species.

Scent or pheromones may be the attractant. When pheromones are involved in sexual attraction usually the sex receiving the pheromone will have much more elaborate antennae than the emitter of the pheromone. The insect orders using pheromones as sexual attractants include Lepidoptera (butterflies and moths), Mecoptera (scorpion flies), and Neuroptera (lacewings, etc.).

Sound. Sound as an attractant e. g. in cicadas, crickets and grasshoppers, is produced by stridulation, i. e. one part of the body being rubbed or scraped against another. Male deathwatch beetles bang their heads against the sides of their tunnels to signal a mate. In midges and mosquitoes the high frequency of the female wing beats produces a sound that can be detected by males.

Light. Light as an attractant is used most famously by the glow worm - actually a beetle.

Insect mating

Mating often involves the presentation of a gift by the male to distract the female, and allow the male to leave safely after mating. In scorpion flies at the end of mating the male exudes a sticky blob of protein-rich saliva which hardens in contact with the air, which he presents to the female. with Empid flies the males present an insect wrapped up in a silk parcel. Whereas springtail males produce spermatophores which they just leave in a position where they think the female is likely to find it.

Actual copulation can take from just a few minutes to many hours with the male clinging to the female's back, or joined end to end. Male dytiscid beetles have pads of suckers on their forelegs to enable them to grip on to the female securely. Dragonflies form a mating loop so that the female can reach the sperm capsule in the second segment of the male's abdomen.

Ovipositing (egg laying)

Some females have visible ovipositors, e. g. crickets, ichneumon wasps and sawflies for putting their eggs in plants, wood, soil, and even other insects.

Parthenogenesis

Not all insects reproduce solely by mating; some females can produce young without males. Indeed in insects adult males do tend to have a much shorter lifespan than females. Parthogenetically produced eggs are unfertilized, and certain bees, wasps, ants crickets, moths, stick insects, and aphids can reproduce parthenogenetically.

Insect parenting

Parenting in most insects consists simply of the female laying eggs and abandoning them, but some get more involved. The social insects - bees, ants, wasps and termites rear their young in a nest community, where the young are tended until they reach adulthood.

An earwig female tends to her batch of eggs for the 2 or 3 weeks it takes them to hatch, turning them and licking them to prevent fungal infection, and defending the nest cavity against intruders. Once the young are born she feeds them saliva and regurgitated food, and will carry them off to a safer site if the chamber is disturbed. Some Heteropteran bugs also behave in this way, and burying beetles (Silphidae) feed their larvae with liquid food from the animal carcass the parents have buried.

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