BIOSCI 208 - JACQUELINE

What are the Chelicerata?

A subphylum of Arthropoda consisting mostly of terrestrial animals with:
• A body divided into two segments: a prosoma/cephalothorax and an opisthosoma/abdomen).
• Four pairs of walking legs and a pair of chelicerae - pointed appendages used to grasp food.
• No mandibles or antennae.

• Contains the class Arachnida, of which there are four NZ orders: Acari (mites, ticks), Chelonethi (pseudoscorpions), Opiliones (harvestmen), and Araneae (spiders).

What are the features of Order Acari (Acarina)?

The most diverse and abundant Order of Arachnids (~1 million species), containing mites and ticks.

• Usually small size (<1mm length) - e.g. human follicle mites are ~0.1mm and are small enough for a hair follicle to support an entire family.

• Mites are among the oldest of all terrestrial animals (fossils from early Devonian ~400 mya).

• E.g. House dust mites consume minute particles of organic matter (e.g. skin cells) - possess a rudimentary gut (no stomach) and digestion occurs outside the body through the secretion of enzymes and deposition of fungus Aspergillus repens to pre-digest dust particles.

• Ticks are blood feeding ectoparasites found on mammals, birds and occasionally reptiles and amphibians and are important vectors of diseases (e.g. Lyme disease).

What are the features of Order Chelonethi (Pseudoscorpiones)?

Pseudoscorpions are small (2-8mm), flat-bodied, carnivorous animals with a short, usually oval, segmented abdomen and a pair of pedipalps - large pincerlike claws.

• Possess well-developed venom glands and ducts in the movable part of the pincer.

• Adult males and females perform elaborate courtship rituals and mating dances.

• Fertilization occurs externally - the male deposits a spermatophore on substrate. Fertilized eggs are kept in a brood pouch attached to the abdomen of the female.

What are the features of Order Opiliones?

Colloquially referred to as harvestmen (~3500 known species), distinguished from closely related groups by a globular body resulting from the fusion of the cephalothorax and the abdomen.

• Lack silk organs, fangs, and poison glands.

• Some species are predominantly carnivorous (small invertebrates), while others are omnivorous (dead plant and animal matter as well as small invertebrates and fungi).

• Of the many species in NZ, only a few have been named - biology and ecology poorly known.

What are the features of Order Araneae?

Colloquially referred to as spiders, consisting of mostly terrestrial, carnivorous animals, of which there are ~35,000 species.

• Body plan consists of a cephalothorax/prosoma and abdomen/opisthosoma joined by a thin, flexible pedicel which allows the spider to move its abdomen in all directions.
- The prosoma has an outer carapace which provides protection (e.g. against desiccation).

• The chelicera can be arranged paraxially (where the basal segment and fangs are parallel to each other and strike downwards) or diaxially (where the basal segment and fangs are directed inwards and strike towards each other).

• Possess book lungs consisting of an atrium or hollow pouch with a series of internal leaves or elongates which lie above one another like a book or open by a spiracle (slit).

What are the features of suborders of Araneae?

• Suborder Mesothelae: representative of earliest fossil spiders with a mid-ventral abdominal spinneret, paraxial chelicerae, 2 pairs of book lungs, and a clearly segmented abdomen.

• Suborder Mygalomorphae: more "primitive" with paraxial fangs and 2 pairs of book lungs.

• Suborder Araneomorphae: more "advanced" with diaxial fangs and 1 pair of book lungs.
E.g. Gradungulidae - NZ's largest native spider (15cm leg span) - cave-dwelling and preys on cave weta. "Living ancestor" - intermediate form between Mygalomorphae and Araneomorphae as 2 pairs of book lungs but diaxial fangs.

How do spiders reproduce?

Males construct a web using their spinnerets, adds a silken pad, and deposits a drop of sperm from the gonopore (underneath the abdomen). The tip of each embolus (palpal bulbs) are dipped into the sperm and stored in the internal spermaphor of the palp.

Mating systems can either be haplogyene (simplest) where the deposition of sperm and egg laying occurs through the same opening, or entelegyne, whereby insemination and oviposition involve different openings.

How do spiders feed?

Spiders are carnivores and often feed on insects but also other spiders (cannibalisitc).
• Mostly nocturnal (avoid predators e.g. birds).

• Prey is first physically pierced or crushed by the fangs and maxillae. Digestive enzymes from the mouth are ejected onto the food (pre-digested externally) and food is filtered through the maxillae and palate plate.

• Indigestible particles are spat out and liquified food passes along the oesophagus into a strong muscular sucking stomach.

How do spiders produce and use silk?

The abdomen bears appendages that have been modified into spinnerets (silk-spinning organs) that extrude silk from up to 9 types of secretory glands.
• These structures consist of many spigots, each of which exudes a drop of viscous fluid which realigns into a silk thread when pulled.

• Silk has many functions e.g. draglines, nests and egg sacs (protection), catching or wrapping up prey.

What are the insects?

The largest group within the phylum Arthropoda.
• Subphylum Uniramia, Superclass Hexapoda, Class Insecta.

Key features include:
• 3 pairs of legs.
• The possession of a labium (fused pair of second maxillae), 3 thoracic segments (prothorax, mesothorax, and metathorax), and 11 abdominal segments.
• The loss of mandible articulations and abdominal appendages.

What is some of the evidence of the success of the insects?

Numbers
• ~1,000,000 species have been named and this is thought to only be a small portion of the total species (thought to be ~8 million).
• Insects represent 5/6ths of all known animal species - by far the most diversified class of animals.
• In terms of biomass, insects are estimated at up to 224kg/ha (in the tropics) compared to 16kg/ha of humans.

Longevity as a taxon
• First appeared ~400 mya and have therefore had hundreds of millions of years to evolve and diversify.

Ecological diversity
• Insects are found in all environments (terrestrial, fresh water, marine) with the only exception being the polar ice caps - even at latitudinal/altitudinal extremes (e.g. butterflies at 80° North - within the Arctic Circle).
• While the majority of insects are stenothermic (capable of surviving only within a narrow temperature range), there are exceptions to this (e.g. mosquito larvae remain active at 0°c, midge larvae are active at 49-51°c in Yellowstone hot springs).

Why are insects so diverse?

Their relative age (emerged ~400 mya) and low extinction rates when compared to other taxon has meant they have had a long time for diversification.

How has the size of insects contributed to their evolutionary success?

Insects are small (size ranges from <0.01 to 0.1m in length and <0.001 to 70g in mass) which is advantageous as it means that:
• Insects require less food to reach maturity (e.g. a honey bee only requires 300 mg of food to go from an egg to an adult in 21 days).
• Insects can utilize small retreats to escape/avoid predators.
• Insects can utilize "small" food sources often overlooked by other organisms (e.g. eggs of other insects).

How has the cuticle of insects contributed to their evolutionary success?

Insects possess a protective cuticle that offers a mixture of flexibility and strength.
• Provides a strong exoskeleton of the body and the limbs, apodemes (internal support and rigid attachment for muscles), and wings, and protects against desiccation and environmental stresses.

• Outward appearance is highly variable, e.g. Coleoptera (beetles) possess modified hardened wing cases to protect the wings.

How has the nervous system of insects contributed to their evolutionary success?

Insects possess an efficient nervous system consisting of a dorsally located brain, ventral nerve cord, and thoracic and abdominal ganglia.
• The fluid of the CNS is isolated from the rest of the haemolymph (fluid in the haemocoel - equivalent to blood) and the nervous system is therefore specialized and able to function efficiently at all times (crucial for flight).

How has the ability of insects to fly contributed to their evolutionary success?

Insects were the first animals to fly (as long as ~330 mya - 100 million years before birds and mammals).
• Primitive groups of insects (e.g. silverfish) have never evolved wings (apterous), although most are winged and capable of flight (pterygota) although some have secondarily lost their wings (e.g. fleas).

• Fully-developed wings occur only in adult insects and usually consist of 2 pairs of wings (a forewing on the 2nd thoracic segment, and a hindwing on the 3rd thoracic segment).

The evolution of flight was only possible due to the rigid cuticle, a well-developed nervous system, acute sense of vision, etc.

Flight is advantageous as it aids in:
• Food capture
• Escape from predators
• Dispersal to new environments (gives access to new resources while avoiding inhospitable environments)
• Sex (locate mates more easily and reproduce safely)

How has the holometabolous development of insects contributed to their evolutionary success?

Insects undergo a complete metamorphosis at each of three stages: larva, pupa, and adult.
• Compared to ametabolous: simply involves a gradual increase in size from young to adult, and hemimetabolous: young resembles adult form and gradually develops adult features (e.g. wing pads).

• This method of development is advantageous as it involves a complete change in morphology from larvae to adult and means that juveniles and adults are not competing for the same resources and occupy different niches.
- E.g. Caterpillars eat leaves while butterflies eat nectar.

• The most diverse orders of insects all have holometabolous development (e.g. Coleoptera/beetles, Diptera/flies, Lepidoptera/butterflies).

How was the high reproductive rate of insects contributed to their evolutionary success?

Insects typically have high fecundity (produce large numbers of offspring) and a short generation time.
• The average rate of reproduction for insects is 100-150 fertilized eggs in one lifetime.
- This can be extremely high in some species, e.g. Queen honey bee can produce up to 200,000 eggs/year, Queen driver ants can lay broods with up to 3-4 million eggs every 25 days.

• This group of organisms that are more responsive to short-term environmental change (selective pressures) has therefore been able to experience pronounced evolutionary success.

How do insects grow?

Growth in insects occurs through ecdysis (moulting), which involves the periodic shedding of the cuticle (as in other arthropods).
• The stages of insect life-cycle between ecdyses are called instars.

How does gas exchange occur in insects?

All insects are aerobic organisms. Due to the possession of a hardened exoskeletons, respiration cannot occur solely through diffusion in larger, active insects, which must forcibly ventilate their tracheae (although gas exchange does occur by simple diffusion in small or less active insects).
• The respiratory system is specialized and is separate from the circulatory system.
• Air-filled tubes, or trachea, open out on each side of most segments of the thorax and abdomen. The openings are called spiracles and can be closed to reduce water loss.

What are the features of the head of insects?

The head consists of 6 segments compressed together.
• Almost all insects have a prominent pair of compound eyes, which usually consist of many facets.
• Each facet is the outer layer of a complex, cone-shaped structure called an ommatidium.
- The number of these ommatidia differs greatly between species (e.g., worker ants may have 100-600 ommatidia per eye, while adult dragonflies, which rely more on vision, have ~29,000 per eye).
• Most insects also have three ocelli (simple eyes). These are typically arranged in a triangle on the top of the head and function in detecting movement and contrast (do not form advanced images).
• Antennae come in many different forms. These may possess chemoreceptors (odour), mechanoreceptors (movement), thermoreceptors (temperature), and hydroreceptors (moisture). The antennae are inserted between the eyes and house the olfactory system. Specialized sensory hairs (sensilla) detect different compounds depending on the kind of receptors present.
- On the base of the antennae is the Johnston's organ (one of the defining characters of the class Insecta). It is a mechanoreceptive organ on the pedicel (2nd segment) of the antenna.

What are the features of insect mouthparts?

Insect mouthparts are complex and highly variable between different orders, and within orders.

Generally:
• The 2nd maxilla (pair of pincerlike structures) are fused as the labium, which forms the lower mouthpart of the insect.
• Display a loss of mandible articulation (i.e., a lack of joints).
• The basic plan, front to back, consists of the labrum (forms the roof of the mouth) -> mandibles (paired appendages used to grasp, crush, or cut the insect's food) -> maxillae -> labium.
• The mandibles, maxillae, and labial palps constitute the moveable aspects of the insect's mouth.

How do chewing insects feed?

Chewing insects possess the least specialized mouthparts (considered to be the 'basic type' and is the most common) whereby they are directed downwards and function in processing plant materials.
E.g., Coleoptera (beetles), Lepidoptera (butterfly) larvae.

How do piercing-sucking insects feed?

Most insects that feed using this method belong to Hemiptera ('true bugs' e.g., circadas, aphids). All Hemipterans have long, threadlike mouthparts and feed by spitting saliva onto the material and sucking the dissolved nutrients back up.
• These attack different parts of the plant than chewing insects, e.g., the parenchyma, phloem, xylem.

These insects are also commonly associated with disease transmission (e.g., malaria).

How do lapping insects feed?

Lapping occurs mostly in bees (order Hymenoptera, family Apidae) which possess mouthparts adapted for consuming nectar.
• The glossa (tongue), maxillary galeae, and labial palps form a tubular proboscis containing a food canal.
• Honey bees dip the glossa into the nectar and retract it so that the nectar is carried into the space between the galeae and labial palps.

How do siphoning insects feed?

The proboscis (galeae) is a modification of the maxillary galeae) and consists of paired galeae which fit together to form a straw-like tube used to suck up nectar.

How do sponging insects feed?

The labium is modified to form a sponge-like pad to absorb food in liquid form, which is dissolved by saliva.

What are the features of the thorax of an insect?

The thorax is divided into three segments: prothorax, mesothorax, and metathorax.
• Each of these segments usually bears a pair of legs (3 pairs total).
• The tarsus of each leg is usually segmented itself with 5 segments.
• The pretarsus bears a pair of claws.
• Legs are often modified, e.g., for digging (fossorial), jumping (saltorial), running (cursorial) or grasping prey or mates.
• The mesothorax and metathorax each bear a pair of wings in winged insects.

What are the features of the abdomen of an insect?

The abdomen is more flexible than the head and thorax due to the extendible conjunctival membrane between segments.
• The first 7 or 8 segments are similar and relatively simple, each with a pair of spiracles.
• Segments 8 or 9 and the remainder are modified to form the terminalia (terminal regions).
• Segment 11, if present, bears the filamentous cerci, which are highly modified between species, e.g., into a pair of 'forceps' in earwigs.

What are the features of insect reproduction?

Most insects reproduce sexually, although some species are parthenogenetic (females reproduce asexually without the need of a male - cloning).
• There is a huge diversity in genitalic structures: some insects display direct sperm transfer, others display indirect transfer, while others display traumatic insemination.
- Direct sperm transfer is the most common method of insemination. Most mate directly with the male and female genitalia coming together. Many insects use extensive courtship rituals. Some flying insects can mate midflight.
- Indirect sperm transfer is displayed in some insects such as dragonflies. Male dragonflies produce packets of sperm (spermatophores) and place them in a depression on the lower surface of the 2nd abdominal segment. The mle then flies out and grasp's a female's head with his clasping genitalia at the abdominal tip. The male often guards the female to fend of competing males.
- Traumatic insemination occurs in some insects (e.g., bedbugs) where males possess sharp genitalia that pierce the body wall of females. Sperm is transferred to the female body cavity, and travels in the haemolymph to storage structures, and then fertilization occurs in the ovaries. This avoids sperm competition between males.