Introduction to Arthropods

Arthropods

Arthropoda: a highly diverse phylum (around 1,170,000 species) of jointed-limbed animals, which includes the crustaceans, arachnids, and insects as the major components
- Arthropod means 'articulated leg' or 'jointed feet'
- made up of Chelicerata, Myriapoda, Crustacea, and Hexapoda
- related to other Ecdysozoans, including Kinorhyncha, Priapulida, and Loricifera

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Tagmosis: in metamerically segmented animals, functional specialisation that leads to differentiation among the segments and the formation of tagmata
- there are two tagma in Arthropods: the cephalothorax and the abdomen
have an exoskeleton secreted by the underlying epidermis
- the exoskeleton is a jointed skeletal covering made of chitin and bound by a protein
growth is mediated by ecdysone
compound and simplex eyes

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the abdomen is the hind tagma of the body
- the abdomen of Crustacea is not homologous with the abdomen of arachnids and insects
- usually shows at least some trace of segmentation
hypodermis secretes the exoskeleton and the exoskeleton encloses the body
- Schmidt layer is located immediately above the epidermis, and above that is the procuticle
- in terrestrial arthropods, there is a thin epicuticle covered in wax or grease
- cuticle is relatively impermeable to water and has a high strength-to-weight ratio
- apodemes: one of the internal ridges or ingrowths from the exoskeleton that supports the internal organs, provides points of attachment for muscles, and constitutes the endoskeleton of the animal
- the only way to grow with an exoskeleton is to shed

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aquatic arthropods possess gills
- gills are outgrowths of the integument and are covered by the exoskeleton
terrestrial arthropods possess tracheae and book lungs
- tracheal tubes (but not tracheoles) are mounted along with the rest of the exoskeleton
- most spiders possess tracheae and book lungs, but many large spiders and scorpions only have book lungs

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arthropods have compound eyes
ommatidium: one of the individual, light-sensitive units that together form a compound eye
- consists of a cuticular lens below which is the crystalline cone, below this are six or seven retinula cells which are surrounded by pigment cells

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arthropods possess an open circulatory system consisting of a dorsal heart and a system of arteries that may be very limited or extensive
- arteries deliver blood into hemocoels, from which it eventually drains back into a large pericardial sinus surrounding the heart
varying number of ostia are located along the length of the heart and permit blood flow in when the valves are open

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have a central nervous system that is ventrally locate and consists of a set of ganglia for each body segment, connected by paired nerve cords
brain is separated into three parts: the protocerebrum, the deutocerebrum, and the tritocerebrum
- protocerebrum: the first segment of the panarthropod brain, receiving the nerves of the eyes and other organs, and contains neuropils
- deutocerebrum: part of the brain containing the association centres for the first antennae of insects and crustaceans
- tritocerebrum: part of the brain containing association neuropils for the second antennae

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photoperiod is more important than temperature at determining lifecycle
stages of growth:
- underlying cells release enzymes that digest the base of the old exoskeleton
- underlying cells secrete the new exoskeleton beneath the old one
- new exoskeleton, which is soft and flexible is stretched by localised, elevated blood pressure augmented by the intake of water and air
some arthropods stop moulting when they reach sexual maturity, others continue to grow throughout their life
growth in terrestrial groups starts in the nervous system and continues with hormones, growth in wet environments is entirely hormonal

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in water, waste will be excreted in liquid form as ammonia
- excretion in water requires sophisticated nephridia
- nephridium: in many invertebrates, an organ probably concerned with excretion and the regulation of the water content of the body
myriapods, insects, and some arachnids possess Malphagian tubes, which open into the intestine
- this means excretory and digestive wastes exit from the anus
- will excrete solid uric acid via the anus
Blattabacterium inhabit all species of cockroach studied to date
- strain Bge codes for the complete urea cycle so that, in combination with urease, ammonia is produced as the product (unlike most insects which produce uric acid)
- use ammonia as a warning spray when they are threatened

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in many isopods, the respiratory structures on the endopods are internal, with a spiracle and pseudotrachaea
- spiracle: one of the pores on the body of some arthropods at which the tracheae open to the outside
- pseudotrachaea resemble lungs
aquatic arthropods tend to possess some form of gills in which gas exchange takes place by diffusing through the exoskeleton
- some may breathe atmospheric air when submerged
- may respire using a plastron

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arthropods are found in almost all habitats on Earth
arthropods are the only invertebrates capable of flight
small plankton crustaceans are major links in the food chain between photosynthetic phytoplankton and larger carnivores
important for pollination, with roughly 2/3 of flowering plants being pollinated by arthropods
arthropods are significant carriers of diseases such as malaria, yellow fever, dengue, elephantiasis, African sleeping sickness, typhus fever, bubonic plague, Rocky Mountain spotted fever, and Lyme disease

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