Aspects of Arthropodan Biology - the Arthropodan Adaptive Suite

Key feature is presence of cuticle (exoskeleton), super-imposed on metameric segmentation inherited from their annelidan ancestors , diagram of insect cuticle
plus development of jointed limbs for locomotion and other uses.

The cuticle is mainly composed of a protein (sclerotin) and a polysaccharide (chitin).

Generally, the two thickest layers are the endocuticle and exocuticle. The endocuticle is the innermost layer,
outside of which is the exocuticle where hardening takes place to give the rigidity of the cuticle.
These two layers are sometimes referred to jointly as the procuticle.

The very thin outermost layer is the epicuticle, which is important as it provides a tough protective covering
and in terrestrial species has a waterproof layer of wax.

Consequences of having a cuticle:-

discrete muscles operating across particular joints (areas of flexible cuticle);

re-organised nervous system;

enabling more complex behaviour;

involving wider range of sense organs, and more complex nervous system;

many of these sense organs are cuticular adaptations;

the blood system can develop as an open system (the haemoceole [at same time coelom is reduced), because the cuticle is not leaky,

conferring advantages associated with large blood volume - greater tolerance of loss, better buffering powers (pH, toxins, ionic strength), storage medium, use as hydraulic system;

development of waterproof layer (wax) in arachnids and insects their success on land,

along with special features of their respiratory systems (book lungs and tracheae) with narrow spiracles cutting down water loss,

as well as use of Malpighian tubules to produce solid nitrogenous excreta (uric acid in insects, guanine in arachnids);

no ciliated epithelia;

wings of insects developed from cuticle;

very many uses of modified limbs - locomotion, sensory antennae, prey capture, filter feeding, chewing mouthparts, sucking mouthparts, respiratory gills (in Crustacea), reproduction (transfer of sperm, brooding of eggs, holding of young), spinnerets producing silk in spiders;

cuticle needs to be moulted to allow for growth - ecdysial cycle, with different instars between moults; these instars (stages) can be very different at different stages in life cycle - exploitive and distributive phases.

The tremendous advantages conferred by these adaptations led to tremendous success of arthropods, both in terms of population sizes (numbers of individuals) and diversity (number of species).

The arthropods as a polyphyletic group - Convergent Evolution

The three main arthropodan phyla - Crustacea, Chelicerata, Uniramia - had separate evolutionary origins and are not related to each other but show an amazing number of analogous features as a result of convergent evolution - the result of having a cuticle.

Crustacea - cuticle hardened by deposition of calcium salts; evolved in the sea and still predominantly marine organisms.

Chelicerata - cuticle hardened by formation of disulphide links between polypeptide chains; evolved in the sea (Merostomata) but now almost entirely terrestrial (Arachnida, e.g. spiders).

Uniramia - cuticle hardened by quinone bonds betwen polypeptide chains; evolved on land and now extremely successful terrestrial animals, with a few secondarily adapted to aquatic life.

Adaptation and Evolution
These concepts for the arthropods can be well considered alongside the general theme of "adaptation", with its three types (universal, physiological and evolutionary) and the concept of key adaptations in the evolution of major groups.

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