Arachnida and Spiders

• Study program is Veterinary Medicine, 1st Year, 2nd Semester, VF/AF/VM-U31
• Life Sciences II covers Zoology and Laboratory Animals

Phylum Arthropoda

• Subphylum Branchiata, which include Class Crustacea, and Subclasses Malacostraca and Branchiopoda
• Subphylum Tracheata, which include Class Insects (Insecta)
• Subphylum Chelicerates (Chelicerata), which include Class Arachnids

Class Arachnids (Arachnida)

• Arachnid tagmata includes a cephalothorax and abdomen
• The cephalothorax usually bears a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs
• Antennae and mandibles are lacking in arachnids
• Most arachnids are predaceous and may possess claws, fangs, poison glands, or stingers
• Arachnids use sucking mouth parts or a sucking pharynx to consume fluids and soft tissues from their prey
• Spiders have spinning glands as an adaptation
• Over 100,000 arachnid species have been described

Order Spiders (Aranei)

• Spiders are a large group of 35,000 species
• Spider body is divided into a compact cephalothorax and abdomen, joined by a slender pedicel
• Anterior appendages consist of chelicerae with terminal fangs connected to poison glands
• Spiders also have a pair of pedipalps
• Fourth pair of walking legs terminate in claws

Spider Digestive System

• Spiders digest food outside their bodies
• They expel digestive enzymes from their intestinal tract onto their prey
• Enzymes break down the prey's body tissues
• After a few seconds, the spider intakes the predigested, liquid tissues
• Sucking stomach is the main pump for drawing food into the body
• The gut is directly behind the stomach, with parts extending into the front portions of the spider's legs
• Branches of the midgut extends into the abdomen, surrounding other bodily structures

Spider Excretory System

• Excretory system consist of Malpighian tubules
• Works in conjunction with specialized rectal glands

Spider Circulatory System

• Open circulatory system, lacking true blood or veins
• Haemolymph is pumped through arteries by a heart into sinuses spaces surrounding internal organs

Spider Nervous System

• Basic arthropod central nervous system consist of a pair of nerve cords below the gut
• Paired ganglia serve as local control centers for different segments
• A brain is formed by the fusion of ganglia for head segments
• Spider brain encircles the esophagus
• Primarily four pairs of eyes on the top-front of the cephalothorax, in varying patterns
• Possess slit sensillae in the joints of their limbs to detect force and vibrations
• Web-building spiders rely more on mechanical and chemical sensors than eyes
• Hunting spiders primarily use their eyes

Spider Respiratory System

• Spiders breathe using book lungs
• Book lungs, unique to spiders, consist of many parallel air pockets extending into a blood-filled chamber
• Air enters the chamber through a slit in the body wall

Spider Reproductive System

• Separate sexes: male and female
• Sexual reproduction with internal fertilization is indirect as sperm is transferred via pedipalps
• Spider life cycle: egg, spiderling, and adult
• Spiders can lay up to 3,000 eggs, usually in one or more silk sacs

Order Ticks and Mites (Acari)

• Acarines differ from other arachnids due to complete fusion of the cephalothorax and abdomen
• No external division or segmentation
• Ticks are efficient carriers of disease
• Ticks attach firmly, feed slowly, and may go unnoticed for a considerable time
• Ticks take several days to complete feeding
• Tick life stages: egg, six-legged larva, eight-legged nymph, and adult

Tick Life Cycle

• Two-host ixodid ticks have a life cycle that usually spans over two years
• Gravid females drop off the second host after feeding to lay eggs, usually in the fall
• Eggs hatch into six-legged larvae, overwintering in this stage
• Following spring, the larvae attaches to the first host, typically a rodent or lagomorph
• The larvae molt into nymphs on the first host
• Engorged nymphs drop off the first host in late summer or fall, overwintering in the nymphal stage
• Nymphs molt into adults the following spring and seek out the second host, usually a larger herbivore
• Adults feed on the second host during mating season during the summer
• In the fall, females drop off the second host to continue the cycle
• Females can reattach and feed multiple times
• Humans can serve as first or second hosts for ticks
• The second host doesn't need to be a separate species or even a separate individual

Order Mites (Acari)

• Term commonly refers to a group of insect-like organisms
• Some bite, some cause irritation, some are scavengers, some feed on plants, and many feed on other arthropods
• Like ticks, mites pass through four stages of development: egg to larva to nymph to adult
• All stages have the 8 legs except the six-legged larva

Class Insects

• Largest group of organisms on earth, inhabiting nearly every conceivable habitat
• Occupy land, freshwater, plus a few have invaded the sea
• Three body sections: head, thorax, and abdomen
• Possess three pairs of legs
• May have one or two pairs of wings
• Breathe through trachea or branched tubes, commencing at lateral apertures called stigmata or spiracles
• Trachea ramifies, leading to every part of animal
• Open circulatory with a dorsal heart
• Liquid waste is excreted through Malpighian tubes
• Separate sexes and reproduce sexually though some insect groups reproduce without fertilization by sperm (parthenogenesis)

Insect Mouthparts

• Chewing mouthparts: beetles, grasshoppers, acrididae, dragonflies, wasps, cockroaches
• Chewing-sucking parts: bees, bumblebees
• Piercing and sucking mouthparts: female mosquitoes, lice, bugs
• Sucking mouthparts: butterflies
• Lapping mouthparts: typical for flies

Insect Life Cycles

• Almost all insects undergo metamorphosis during development
• Sexes are separate with internal fertilization
• Holometabolous or Complete Metamorphosis: larvae and aduls are different, pupae is formed - beetles, flies, bees, lacewings, butterflies, ants, and caddis flies
• Incomplete or simple Metamorphosis (hemimetabolous): nymph resembles the adult and there is not a pupal stage - plant bugs, crickets, true bugs, termites, grasshoppers, and cockroaches

Arachnids (Chelicerata) include spiders. Arachnids have tagmata which includes a cephalothorax and abdomen. Spiders have spinning glands as an adaptation. Over 100,000 arachnid species have been described with 35,000 species are spiders.