Mygalomorphae (Trapdoor Spiders and Tarantulas)

Mygalomorphae is a major infraorder within the order Araneae (arthropod subphylum Chelicerata) comprising more than 3,400 described species across 30 families, including tarantulas (Theraphosidae), trapdoor spiders (Euctenizidae, Halonoproctidae, and Idiopidae), funnel-web spiders (Atracidae), and purseweb spiders (Atypidae). Diverging from the sister infraorder Araneomorphae ("true" web-weaving spiders) during the Middle to Late Triassic (~240–250 Ma), mygalomorphs retain ancestral arachnid anatomical features alongside specialized adaptations for fossorial predation. Unlike araneomorphs that rely on aerial orb or cobwebs, most mygalomorph lineages construct silk-lined subterranean burrows equipped with camouflaged hinged doors, trip-lines, or funnel entrances that buffer microclimatic fluctuations and facilitate ambush foraging.

At the organismal and molecular scales, Mygalomorphae exhibits an unusual combination of life-history conservatism and genomic plasticity. Females of several Australian and North American trapdoor lineages display iterative post-maturation molting and lifespans reaching 20 to 43 years in the wild, paired with dispersal distances often limited to a few meters from the maternal burrow. Recent chromosome-scale genome assemblies—most notably the 4.6-gigabase assembly of the California coastal dune trapdoor spider (Aptostichus stephencolberti)—have further demonstrated that mygalomorphs possess large, transposable-element-rich karyotypes that have lost the canonical insect and arachnid (TTAGG)_n telomeres motif, relying instead on non-canonical chromosomal end-maintenance pathways.

Orthognathous Chelicerae and Strike Biomechanics

The primary synapomorphy separating Mygalomorphae from Araneomorphae is the orientation and articulation of the chelicerae (jaws) and venom fangs. Mygalomorphs possess paraxial (orthognathous) chelicerae, in which the basal segments (paturon) project forward parallel to the longitudinal body axis and the fangs articulate in a vertical, dorsoventral plane. During a predatory strike, the spider elevates its prosoma (cephalothorax) and drives both fangs downward into prey like parallel daggers, maximizing puncture force against heavily sclerotized ground beetles, millipedes, and small vertebrates. By contrast, araneomorph spiders possess diaxial (labidognathous) chelicerae that oppose one another transversely like pincers, enabling manipulation of prey suspended in silk threads without requiring a substrate counter-force. Mygalomorph venom glands are housed entirely within the basal cheliceral segment rather than extending into the prosomal cavity, and are innervated to inject complex neurotoxic peptide cocktails—such as the δ-hexatoxins of Australian funnel-web spiders (Atrax robustus)—that delay voltage-gated sodium channel inactivation.

Dual Book Lungs and Spinneret Reduction

Respiration in Mygalomorphae is mediated by two pairs of book lungs situated on the ventral surface of the second and third opisthosomal (abdominal) segments, visible externally as four pale sclerotized opercula. Each book lung consists of 100 to 300 hollow hemolymph-filled lamellae separated by cuticular struts that maintain a constant 1–5 μm air gap, facilitating passive diffusive gas exchange of O₂ and CO₂ between the atmosphere and the copper-based respiratory pigment hemocyanin. Whereas most araneomorphs replaced their posterior book lungs with branched tubular tracheae that deliver O₂ directly to tissues at higher metabolic rates, the retention of four book lungs in mygalomorphs constrains aerobic scope while minimizing evaporative H₂O loss inside humid burrows. Mygalomorphs also exhibit a reduction of the ancestral four pairs of abdominal spinnerets to two or three functional pairs (typically the posterior median and segmented posterior lateral spinnerets), which secrete structural silk used for burrow lining, egg sacs, and male sperm webs rather than adhesive viscid capture spirals.

Burrow Microclimates and Extreme Lifespans

Fossorial mygalomorphs exhibit some of the longest lifespans documented among terrestrial invertebrates. Long-term demographic monitoring initiated by Barbara York Main in Western Australia recorded a female armoured trapdoor spider (Gaius villosus, family Idiopidae) surviving 43 years in the wild. This longevity is driven by low resting metabolic rates (often 30% to 50% below predicted allometric values for poikilotherms of equivalent body mass), sit-and-wait foraging kinematics at the burrow rim, and the thermal buffering of burrows 15–60 cm below the soil surface. Unlike araneomorph females, which cease molting upon sexual maturity, adult female mygalomorphs undergo annual or biennial post-maturation ecdysis. During each molt, the spider sheds not only its external exoskeleton and sensory setae, but also the cuticular lining of its foregut, book lungs, and internal spermathecae (requiring remating to fertilize subsequent clutches). Juvenile maturation requires 5 to 8 years, creating demographic vulnerability to habitat fragmentation and harvesting.

Dispersal Limitation and Allopatric Speciation

Because most trapdoor spiders and tarantulas do not engage in aerial silk "ballooning" (or restrict low-altitude ballooning to a few genera such as Ummidia and Sphodros), juvenile dispersal occurs primarily by terrestrial walking. Emergent spiderlings frequently establish their permanent burrows within 0.5 to 5 meters of the maternal chamber, forming dense, genetically structured aggregations. While adult females remain within a single burrow throughout their lives, adult males abandon their burrows after their terminal maturation molt (which equips them with tibial apophyses and palpal bulbs on the pedipalps) to wander in search of receptive females before dying within months. This stark sex-biased dispersal asymmetry produces discordant phylogenetics signals between maternally inherited mitochondrial DNA and biparentally inherited nuclear loci, driving fine-scale allopatric evolution across isolated coastal dunes, sky islands, and riverine barriers.

Repeat-Rich Karyotypes and Chromosome Evolution

Cytogenetic and sequencing surveys reveal that mygalomorphs maintain high diploid chromosome numbers (2n = 40 to 110) dominated by telocentric and acrocentric chromosomes alongside complex multiple sex chromosome systems (X₁X₂0 or X₁X₂X₃0 in males). The chromosome-level genome assembly of Aptostichus stephencolberti spans 4.6 Gb across 52 chromosomes—approximately 1.5 times the size of the human genome—with repetitive sequences accounting for more than 65% of total sequence length. Rather than arising from recent lineage-specific whole-genome duplication beyond the shared ancestral arachnopulmonate paleopolyploidy event, this genome expansion reflects continuous proliferation of DNA transposons, long interspersed nuclear elements (LINEs), and long terminal repeat (LTR) retroelements that inflate intronic and intergenic intervals while keeping protein-coding gene counts near 22,000–26,000.

Absence of Canonical Arthropod Telomere Repeats

In most insects and basal chelicerates, chromosome ends are capped by tandem arrays of the pentanucleotide repeat (TTAGG)_n synthesized by the reverse transcriptase telomerase. High-coverage long-read sequencing and fluorescence in situ hybridization (FISH) across mygalomorphs (Aptostichus stephencolberti, Acanthoscurria geniculata) and derived araneomorphs indicate a complete absence of (TTAGG)_n arrays at chromosomal termini, accompanied by sequence degradation or transcriptional silencing of the catalytic telomerase reverse transcriptase (TERT) subunit. Instead of short microsatellites, mygalomorph chromosome ends are enriched in subtelomeric satellite blocks and site-specific non-LTR retrotransposons or maintained through recombination-mediated pathways analogous to Alternative Lengthening of Telomeres. Coupled with continual stem-cell proliferation during decades of adult ecdysis and appendage regeneration, this alternative telomere architecture provides an evolutionary model for studying chromosomal stability in long-lived post-mitotic and regenerative somatic tissues.

How do you distinguish a mygalomorph spider from a true araneomorph spider?

Mygalomorphs are distinguished externally by two primary anatomical characters: their chelicerae and their book lungs. Mygalomorph jaws project straight forward and their fangs swing vertically downward parallel to one another (orthognathous), whereas araneomorph fangs swing inward horizontally like pincers (labidognathous). In addition, examining the underside of the abdomen reveals four pale book-lung covers (two pairs) in mygalomorphs, compared to a single anterior pair of book lungs and a posterior tracheal spiracle in most araneomorphs.

Why do female tarantulas and trapdoor spiders live for decades while males die quickly?

Female mygalomorphs remain inside thermally buffered burrows, maintain low resting metabolic rates, and continue molting every 1 to 2 years after reaching sexual maturity, allowing them to replace damaged cuticle, regenerate lost limbs, and survive 15 to 43 years. Conversely, once a male completes his final maturation molt, he ceases feeding or molting, develops specialized pedipalpal copulatory organs, and expends his metabolic reserves wandering above ground to locate females, typically dying from predation or exhaustion within 3 to 18 months.

How does the genome of Aptostichus stephencolberti maintain its chromosome ends without TTAGG telomeres?

Chromosome-scale sequencing of the 4.6 Gb Aptostichus stephencolberti genome demonstrated that canonical (TTAGG)_n arthropod telomeric repeats are absent from all 52 chromosomes. Like several other spider lineages that have lost canonical telomerase activity, its chromosomal termini are buffered by complex subtelomeric satellite repeats, retrotransposon arrays, and homologous recombination mechanisms that counteract replication-associated end-shortening across the spider's multi-decade lifespan.

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