Chapter 13: Arthropods

Cuticle

Consists of outer, relatively think epicuticle and an inner, thicker procuticle, found in most protostomes

Class Diplopoda

-Millipedes
-Cylindrical bodies with 25-100 segments
-Four thoracic segmented bear one pair of legs
-Abdominal segments bear 2 pairs of legs
-typically herbivorous

Exoskeleton

hardened cuticle that is secreted by underlying epidermis, must be molted (shed) in order to grow, contains chitin proteins

Appendages

uniramous (one tip legs)
Biramous (multiple tip legs)
Chelicerae (pinchers by mouth)
Mandibles (by mouth)

Subphylum Crustacea

-2 pairs of antennae
a pair of mandibles and -2 pairs of maxillae
one pair of appendages on each segment
-All appendages except perhaps first antennae care biramous
- Basal protopod, an outer branch called on exopod and an inner branch called endopod

Crustacean Segmentation

-Primitive Crustaceans may have up to 60 segments, derived crustaceans have fewer
-Tagmata are usually head, thorax and abdomen
-Not homologous across taxa
-In most, one or more thoracic segments are fused with the head as a cephalothorax
-Dorsal covering is the carapace
- May cover most of body or just cephalothorax

Crustacean Appendages

Have wide variety of walking legs, mouthparts, swimmerets from modification of the basic biramous appendages
-some have lost 1 branch and are uniramous
Crayfish
- 1st 3 pairs of thoracic appendages are celled maxillipeds
-5 pairs walking legs
- 1st pair of swimmerets in male called monopods used in copulation
-Uropods are last pairs of appendages used for backwards movements and protection for eggs or young on swimmerets

Suspension feeders

generate water currents in order to feed

Predators

hunt for their food, consume larvae, worms, crustaceans, snails, and fishes

Scavengers

eat dead animals and plant matter, many have a two-part stomach

crustacean life cycle

Most crustaceans have separate sexes
Most crustaceans brood eggs in brood chambers, in eggs sacs attached to abdomen, or attached to abdominal appendages
Crayfishes develop directly without a larval form
Most crustaceans have a larva unlike the adult in form and undergo metamorphosis
Nauplius is a common larval form with unsegmented body, frontal eye, 3 pair of appendages

Branchiopoda

3 orders are recognized
-Anostraca: includes fairy shrimp and brine shrimp
-Notostraca: includes tadpole shrimp
-Diplostraca: Includes water flea and clam shrimp

Brachiopod Anatomy

Flattened leaf-like legs serve as respiratory organs, assist in suspension feeding, and except for cladocerans, locomotion
Most are freshwater
Important component of freshwater zooplankton

Subclass Copepoda

roup of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic, some are benthic, a number of species have parasitic phases

Epicuticle

outer most layer of arthropods, made of protein and often lipids

Procuticle

Divided into exocuticle (secreted before a molt) and endocuticle (secreted after molting). Both layers contains chitin bound with proteins

Ecdysis

The process of molting ends in the shedding of skin

molting

necessary for an arthropod to increase in size, since exoskeleton does not grow

Segmentation

Typically each arthropod segment has a pair of jointed appendages
Arrangement is often modified with both segments and appendages specialized for a range of functions
Each section of an appendage functions as a hollow tube moved by muscles, which insert on the inside

Class Pycnogonida

-sea spiders
-4 pairs of walking legs
-suctorial proboscis
-much reduced abdomen
-male use ovigers (pair of legs) to carry egg masses
-common in the waters

Cephalothorax and abdomen are joined by?

Thin Pedicel

Class Arachnida

-over 100,000 species described
-spiders, scorpion, whip scorpions, ticks, mites, harvestmen, etc
-cephalothorax and abdomen

Order Araneae

-Spiders
-All predaceous
-chelicerae function as fangs
-web spinning to trap pray
-others are cryptic or stalk prey

Trilobite Fossil

-extinct for 250 million
-Abundant during Cambrian and Ordovician period
-Trilobed shaped body, pair of longitudinal grooves
Probably bottom dwellers and scavengers

Class merostomata

Horseshoe Crab- looks like a crab but it closely related to spiders
- ancient marine group
-carapace, abdomen, telson
-their blue blood is harvested and used to test medicine

Spiracles

opening on the outside of the arthropods that connects to the tracheae for breathing

Air piped directly to cells

Most land arthropods have a highly efficient tracheal system of air tubes, which deliver oxygen directly to tissues and cells

Tracheae are not present in aquatic arthropods, which breathe mainly by gills

Pros and Cons of Arthropods

Pros: Important source of products
Cons: Cause many diseases and economic losses

Order scorpionida

scorpions, females carry young on their back

Order Acari

ticks and mites that like to eat dead skin cells, common allergy to humans

Lyme disease

occurs when bitten by Ixodes pacificus (blacklegged tick) who are carriers of this disease, can be carried by any in the genus Ixodes
Opossums are vital in tick control

Class thecostraca

barnacles, very long penis that can inseminate females, slows ships

Order isopoda

terrestrial crustaceans, decomposers, rollie pollies, some are parasitic

Order euphausiacea

krill, major component of animals diets

Decapoda

crabs, have ten legs in total