Molecular & Biochemistry

1. Cuticle Chitin & the Quinone-Tanning Crosslink

Spider exoskeleton, like that of all arthropods, is built on chitin (β-1,4-poly-N-acetylglucosamine) cross-linked by sclerotin proteins. Sclerotization runs through the same quinone-tanning chemistry as insects: dopa → dopamine → N-acetyldopamine → quinone-mediated cross-link to Lys/His residues of the matrix protein. The colour darkens as the cross-link density accumulates — a chemical clock readable in age-dating field-collected spiders.

Spider cuticle is unusual in carrying a thick endocuticle layer with reduced cross-linking, leaving it more elastic than the cuticle of most insects. The gradient hard-outside, flexible-inside composite is the structural complement to the hydraulic locomotion described in Section 3.

2. Haemocyanin: Copper-Based Oxygen Transport

Spider blood (hemolymph) contains haemocyanin — a copper-based oxygen carrier, not iron-based. Each haemocyanin subunit binds one O₂molecule between two Cu⁺ ions:

\[ 2\,\mathrm{Cu^{+}} + \mathrm{O_2} \;\rightleftharpoons\; \mathrm{Cu^{2+}{-}O_2^{2-}{-}Cu^{2+}} \quad (\text{side-on peroxo bridge}) \]

The deoxy state is colourless; the oxy state is the source of spider blood’s characteristic blue colour at higher concentrations. The peroxo Cu-O₂-Cu charge transfer band absorbs at ~340 nm (UV-near), with a shoulder around 550 nm that gives the visual blue. The geometry is conserved with horseshoe-crab and cephalopod haemocyanins.

Each haemocyanin assembly is a 24- or 48-subunit cooperative oligomer in spiders; cooperativity is high (Hill coefficient up to 9) for efficient O₂loading at the book-lung surface and unloading in active tissues. The circulating-protein concentration runs at ~50–100 mg/mL — comparable to human haemoglobin in red cells but spider haemocyanin is dissolved in plasma, not compartmented.

3. Hydraulic Locomotion & Hemolymph Pressure Chemistry

Spiders extend their legs by hydraulic pressure generated in the prosoma, not by extensor muscles (Anderson & Prestwich 1975). The pressure is generated by contracting the dorsal and ventral diaphragms of the prosoma, squeezing hemolymph into the leg lumen. Internal pressures reach ~50 kPa during normal walking and up to 600 kPa during prey capture jumps.

Hemolymph composition supports this hydraulic role:

• High concentrations of free amino acids (alanine, proline, taurine) maintain osmotic pressure.
• Trehalose, glycine, and small organic phosphates buffer rapid pH transients during tissue metabolism bursts.
• Haemocyanin contributes to colloid osmotic pressure as well as O₂transport.

The locomotion chemistry is also why spiders curl up on death: without metabolic pressure, the legs collapse via flexor muscle action with no opposing extensor force.

4. Jumping-Spider Vision & Tetrachromatic Chemistry

Salticid jumping spiders carry one of the most sophisticated invertebrate visual systems: four pairs of eyes, with the anterior medial pair (AME) achieving acuity of ~0.04° visual angle — comparable to small primates. The molecular basis:

• Four opsin classes covering UV (~360 nm), blue, green, and red. The red-shifted opsin uses 11-cis-3-hydroxyretinal instead of 11-cis-retinal — the hydroxylated polyene shifts λ_maxinto the long-wavelength visible.
• A four-tier retina behind each AME with each tier optimised for a different wavelength via lens chromatic-aberration positioning — effectively a mechanical-spectral-filtering eye.
• Phototransduction via Gq–PLC–TRP/TRPL cation-channel cascade (rhabdomeric, like all arthropods).

Recent psychophysics (Zurek 2014; Glenszczyk 2022) shows salticids discriminate prey colour and even respond to learned 2D images of conspecifics — the behavioural complexity unique among arachnids hangs on the molecular chemistry of these tetrachromatic photoreceptors.

5. Pheromones: Cuticular Hydrocarbons & Web Silk Markers

Spider chemical communication uses two channels: cuticular hydrocarbons for short-range identification, and silk-incorporated volatiles for trail/web signaling. Female-produced contact pheromones in Pholcus phalangioides are blends of methyl-branched alkanes (C₂₇–C₃₁) sampled by male tarsal sensilla on first contact. Web-marking volatiles (varied across families) are mostly small carboxylic acids and lactones from the silk-gland secretions, transported through the silk fibroin matrix and slowly released over days — long-acting compared with insect alarm pheromones, exploiting the same volatility/persistence trade-off examined in the ant module.

6. Silk Spinning: pH and Salt Gradients (Cross-Link to Module 1)

Module 1 develops the silk-fibroin nanostructure; the molecular trigger that turns soluble fibroin into solid fibre is a gradient. Silk dope in the ampullate-gland lumen is at pH ~7.2 with high NaCl/sodium phosphate, keeping fibroin in soluble micelles. As the dope progresses through the spinning duct, pH drops to ~6.3 and the solute environment becomes potassium-rich. The protonation of fibroin N-termini (charge neutralisation) plus the Hofmeister-series ion exchange destabilises the soluble micelles; shear stress at the spigot then aligns the proteins into β-sheet crystallites embedded in an amorphous matrix — the molecular event of silk solidification.