Molecular & Biochemistry

1. Chitin Biosynthesis

Chitin is a linear β-1,4 polymer of N-acetylglucosamine (GlcNAc), the second most abundant biopolymer on Earth (after cellulose). Insect chitin synthesis runs through:

\[ \mathrm{glucose} \rightarrow \mathrm{glucose\text{-}6\text{-}P} \rightarrow \mathrm{fructose\text{-}6\text{-}P} \rightarrow \mathrm{glucosamine\text{-}6\text{-}P} \rightarrow \mathrm{GlcNAc\text{-}6\text{-}P} \rightarrow \mathrm{UDP\text{-}GlcNAc} \]

The final polymerisation:

\[ n\,\mathrm{UDP\text{-}GlcNAc} \;\xrightarrow{\;\mathrm{chitin\;synthase\;CHS1/2}\;}\; (\mathrm{GlcNAc})_n + n\,\mathrm{UDP} \]

Chitin synthase is membrane-bound; the polymer extrudes through the apical plasma membrane and crystallises immediately on contact with the extracellular space. Chitinases (GH18 family glycoside hydrolases) cleave chitin during ecdysis. The chitin–chitinase pair is the target of major insecticides (lufenuron, novaluron) that block CHS1.

2. Sclerotization — Quinone Tanning of the Cuticle

The hard cuticle is chitin filaments cross-linked by sclerotin proteins, themselves cross-linked by ortho-quinones derived from N-acetyldopamine (NADA) and N-β-alanyldopamine (NBAD). The chemistry is one of the most widespread quinone-tanning systems in biology:

\[ \mathrm{NADA} \;\xrightarrow{\;\mathrm{laccase}\;}\; \mathrm{NADA\text{-}quinone} \;\xrightarrow{\;\text{Lys}\;\text{or}\;\text{His}\;\text{nucleophile}\;}\; \text{cross-linked sclerotin} \]

Tyrosine→DOPA→dopamine→NADA is the upstream chain. The dehydration after cross-linking produces the dark colour of mature cuticle. The same chemistry underlies cephalopod ink, mammalian eumelanin, and squid beak hardness — a deep convergence on the catechol-quinone cross-linking motif.

3. The Ecdysone–Juvenile Hormone Axis

Insect metamorphosis is regulated by two steroid/sesquiterpenoid hormones:

• 20-hydroxyecdysone (20E): the molting steroid, derived from cholesterol via 20E synthesis (the Halloween-gene CYP enzymes phantom, disembodied, shadow, shade, spook). Pulse of 20E triggers ecdysis at each instar.
• Juvenile hormone (JH): a sesquiterpenoid methyl ester (JH I–III); biosynthesised in the corpora allata via the mevalonate pathway. High JH suppresses metamorphosis — molts produce another larva. Low JH at the final instar permits 20E to drive pupation.

The molecular mechanism of the “status quo” effect of JH was clarified by Charles & Iwema (2011): JH binds the bHLH-PAS receptor Methoprene-tolerant (Met), which dimerises with Taiman (Tai) and induces the transcription factor Krüppel-homolog 1 (Kr-h1). Kr-h1 then represses the metamorphic genes E93 and BR-C. Pesticidal JH analogues (methoprene, pyriproxyfen) exploit exactly this pathway: trapping the insect in a permanent juvenile state and preventing reproduction.

4. Hemolymph & the Trehalose Sugar Pool

Insects circulate trehalose (1,1-α,α-glucose disaccharide) at concentrations up to 50 mM in hemolymph — about 10× the glucose level of mammalian blood. Two reasons:

• Reducing-end protection: trehalose has no anomeric hydroxyl to enter the Maillard reaction, so it doesn’t glycate proteins. Insects can carry high circulating sugar without glycation damage.
• Anhydrobiosis: trehalose forms vitreous glasses that stabilise proteins and membranes during desiccation — the basis of insect resistance to drought (and the chemistry that lets tardigrades survive vacuum).

Trehalase enzymes hydrolyse trehalose to glucose at the cell surface for uptake. Flight muscle expresses high trehalase activity to sustain β-oxidation + glycolysis bursts during sustained flight.

5. UV Tetrachromacy: Insect Vision Chemistry

Most insects have UV (~360 nm), blue (~440 nm), and green (~530 nm) opsins; many also have a red opsin for tetrachromacy. The chromophore is 11-cis-retinal (same as mammals) but also, in dim-light conditions or some Diptera, 11-cis-3-hydroxyretinal — a hydroxylated polyene with a slightly red-shifted absorption.

The phototransduction cascade in insects differs from vertebrate vision: rather than cGMP/CNG-channel depolarisation, insect rhabdomeric photoreceptors use:

\[ \text{rhodopsin}^* \to G_q\text{-PLC} \to \mathrm{IP_3} + \mathrm{DAG} \to \text{TRP / TRPL channel opening} \]

The TRP channels (Hardie 1991, the source of the entire mammalian TRP family naming) gate Ca²⁺ and Na⁺ influx, depolarising the photoreceptor. Single-photon detection (single-bump events) is achievable inDrosophila at low intensities — comparable to vertebrate rod sensitivity but in a different molecular architecture.

6. Pheromone Chemistry: Why It Smells Like It Does

Insect pheromones are dominated by long-chain unsaturated alcohols, acetates, aldehydes, and ketones — for example silk-moth (Bombyx mori) bombykol ((E,Z)-10,12-hexadecadien-1-ol):

\[ \mathrm{CH_3{-}(CH_2)_2{-}CH=CH{-}CH=CH{-}(CH_2)_8{-}CH_2OH} \]

The reason it works as a sex attractant at attomolar concentrations: bombykol is volatile but heavy (M_w = 238) so its diffusion plume is concentrated near the female and slow to disperse in still air. The male moth’s olfactory receptor BmOR1 binds bombykol with K_d ~10⁻¹³ M. A single bombykol molecule per second on the male antenna is enough to elicit upwind flight (Schneider 1969).

Insect olfactory transduction also differs from mammalian: insect ORs are ligand-gated cation channels, NOT GPCRs (Sato 2008). The complex of a tuning OR + the universal co-receptor Orco forms the channel directly — ultra-fast (<1 ms) detection without the GPCR-cAMP cascade. This molecular speed is why insects can localise pheromone plumes turbulently rather than chemo-tactically as mammals do.

7. Cuticular Hydrocarbons (CHCs): Species & Sex ID

Insect surface lipids are species-specific blends of long-chain alkanes (C₂₃–C₃₃), methyl-branched alkanes, and unsaturated alkenes. They serve as desiccation barriers and short-range chemical signals. Each Drosophila species has a CHC fingerprint detectable by GC-MS; the speciation literature (Coyne, Kyriacou) uses CHCs as one of the primary reproductive isolation phenotypes. The biosynthetic enzymes (elongases, desaturases, OXSC reductases) are encoded in clustered loci that evolve rapidly under sexual selection.