Phasmatodea

Phasmatodea (also known as Phasmida, stick insects, walking sticks, and leaf insects, from the Ancient Greek phasma, "phantom / apparition") is an order of hemimetabolous, specifically herbivorous insects within the superorder Polyneoptera, comprising approximately 3,400 described extant species distributed predominantly across tropical and subtropical forests worldwide. Together with their enigmatic web-spinning sister order Embioptera (webspinners), Phasmatodea forms the clade Eukinolabia, with stem-group fossils (Susumanioidea, and early leaf-mimicking Eophyllium messelense from the 47-million-year-old Eocene Messel Pit) extending back to the Middle Jurassic and Early Cretaceous.

Phasmatodeans are the undisputed masters of whole-body plant masquerade in the animal kingdom, exhibiting five extraordinary evolutionary specializations:

1. Acute Twig, Bark, Lichen, and Leaf Masquerade: Morphological elongation of the mesothorax and metathorax (in stick insects) or lateral expansion of the abdomen and femora with false leaf veins and necrotic "chewed" brown margins (in leaf insects, family Phylliidae), reinforced by cataleptic wind-swaying and adaptive neural body-posture memory.
2. The Longest Extant Insects on Earth: The Chinese giant stick insect (Phryganistria chinensis) and Bornean Phobaeticus chani reach total body lengths (with outstretched legs) of 56.7 to 64.0 centimeters (over two feet long!).
3. Facultative and Obligate Parthenogenesis (Including Hybrid Androgenesis and Hybridogenesis): Widespread asexual reproduction in which unfertilized females produce all-female clonal broods (Carausius morosus, Extatosoma, Bacillus).
4. Defensive Autotomy and Complete Nymphal Limb Regeneration: Self-amputation of grasped legs at a pre-formed trochanter–femur suture followed by regeneration of a functional four-segmented-tarsus leg over three molts.
5. Seed-Mimicking Eggs with Ant-Attracting Capitula (Convergent Myrmecochory): Hard-shelled eggs equipped with a lipid- and protein-rich cap (the capitulum) that chemically mimics plant elaiosomes, deceiving foraging ants into carrying stick-insect eggs into underground ant nests for protection!

Stick and leaf insects do not merely look like twigs and leaves; their nervous system is wired to make them behave like passive plant organs:

1. Thanatosis / Adaptive Catalepsy (Flexor–Extensor Co-Contraction): During daylight hours, stick insects (Carausius morosus) fold their forelegs straight forward in a groove alongside the long antennae—concealing the head and creating a continuous linear "twig" profile—and enter a state of adaptive catalepsy (flexibilitas cerea / waxy flexibility). If a branch or researcher bends the insect's leg to an arbitrary angle, central interneuronal circuits in the thoracic ganglia coupled to femoral chordotonal organ proprioceptors hold the limb motionless at that new angle for hours without muscle fatigue!
2. Anemotactic Wind-Swaying: When a breeze rustles the surrounding foliage (or when a walking stick steps across an open gap), mechanoreceptive hair plates on the antennae and legs trigger a low-frequency (1–3 Hz) lateral rocking oscillation of the body that matches the damped harmonic resonance of real wind-blown twigs and leaves, preventing motion-detecting avian predators from separating the insect from its background via optical flow parallax.
3. Circadian Epidermal Color Change in Carausius morosus: In addition to developmental green-versus-brown polyphenism (regulated across nymphal molts by juvenile hormone and background reflectance), adult Carausius morosus change color reversibly every single evening: at dusk, neurosecretory factors from the brain/corpora cardiaca stimulate epidermal cells to migrate dark ommochrome (ommin and xanthommatin) pigment granules upward toward the apical cuticle while spreading orange-red carotenoids laterally—turning a pale tan daytime insect dark mahogany brown within 60 minutes for nocturnal foraging!

Phasmatodea exhibits the widest spectrum of non-Mendelian reproductive modes in the class Insecta:

• Automictic and Apomictic Thelytokous Parthenogenesis: Hundreds of stick-insect species (Carausius morosus, Medauroidea extradentata, Sipyloidea sipylus) are facultatively or obligately parthenogenetic. In the Indian laboratory stick insect (Carausius morosus), populations consist of >99.9% triploid/tetraploid females that lay fertile eggs without mating via apomictic or endoreduplicative parthenogenesis. Even in sexual species such as the Australian spiny leaf insect (Extatosoma tiaratum), unmated females lay viable parthenogenetic eggs (which take 2–3 months longer to hatch than fertilized eggs and yield 100% daughters).
• Hybridogenesis and Natural Clonal Androgenesis (Bacillus and Leptynia): In Mediterranean stick insects of the genus Bacillus (B. rossius, B. grandii, B. whitei, and B. lynceorum, studied by Valerio Scali and Barbara Mantovani), interspecific hybridization gave rise to two extraordinary reproductive modes:
  1. Hybridogenesis (Bacillus rossius–grandii): Hemiclonal females transmit their maternal rossius chromosome set clonally without recombination to every egg while discarding the paternal grandii genome at each oogenesis, mating anew in every generation to "borrow" a fresh paternal genome for somatic vigor!
  2. Natural Androgenesis ("Father-Only Clones"): Occasionally, when an egg fails to eliminate both sperm nuclei during polyspermic fertilization, two haploid sperm nuclei fuse inside the egg cytoplasm while the maternal nucleus degenerates fully—producing viable offspring whose nuclear genome is 100% paternal (derived fully from the father, inside the mother's mitochondrial cytoplasm!).

1. Prothoracic Monoterpene Chemical Weapons (Anisomorpha buprestoides)

Whereas winged stick insects flash startling bright red or blue hindwings (deimatic display) or re-evolve functional wings from wingless ancestors (Whiting et al., 2003), wingless New World striped walking sticks (Anisomorpha buprestoides and Peruvian Peruphasma schultei) possess paired prothoracic defensive glands opening promptly behind the head that aim and fire a atomized dual-jet spray up to 30–40 centimeters directly into the eyes and mouth of attacking birds, mice, dogs, or humans!

• The spray contains concentrated cyclopentanoid monoterpene dialdehydes—anisomorphal, dolichodial, or peruphasmal (plus glucose, which powers an energy-neutral glucose-transporter recycling loop in the gland epithelium!)—that potently activate vertebrate TRPA1 nociceptive pain channels, causing intense corneal pain and temporary reversible conjunctivitis.

2. Convergent Myrmecochory: How Stick-Insect Eggs Trick Ants into Planting Them

Female stick insects do not glue their eggs to leaves (except Timema and a few bark-inserters); instead, they flick or drop 100 to 1,000+ eggs individually from the forest canopy onto the leaf litter below. Every euphasmatodean egg is encased in a thick, calcium-oxalate- and calcium-carbonate-mineralized capsule equipped with a hinged anterior lid (the operculum).

In hundreds of species (Extatosoma tiaratum, Diapheromera, Clitarchus), the outer surface of the operculum is crowned by a fleshy, yellow-or-orange knob called the capitulum:

• Visually, the mineralized egg capsule + capitulum is a dead-ringer mimic of a plant seed bearing an elaiosome (such as Ricinus castor seeds or Acacia seeds dispersed by ants via myrmecochory).
• Chemically, the stick-insect capitulum is packed with triacylglycerols and free 1,2-diolein (oleic acid diglyceride)**—the precise chemical phagostimulant that triggers corpse- and seed-carrying behavior in foraging ants (Myrmica, Leptomyrmex, Camponotus)!
• Deceived ants pick up the stick-insect egg by its capitulum handle, carry it underground into their climate-controlled ant nest, chew off and eat the nutritious capitulum (which is fully non-essential to the embryo inside!), and discard the intact, hard-shelled egg into the nest's subterranean refuse chamber—where the embryo develops for 6 to 24 months fully shielded from avian predators, parasitoid wasps (Amiseginae), desiccation, and forest fires before the first-instar nymph pops open the operculum (looking and walking like a red-headed Leptomyrmex ant mimic!) and climbs safely out of the ant nest into the trees!

1. Bradler, S., & Buckley, T. R. (2018). Stick and leaf insects (Phasmatodea). In Insect Biodiversity: Science and Society (Vol. 2, pp. 281–313). Wiley-Blackwell. Google Scholar
2. Whiting, M. F., Bradler, S., & Maxwell, T. (2003). Loss and recovery of wings in stick insects. Nature, 421(6920), 264–267. Google Scholar
3. Hughes, L., & Westoby, M. (1992). Capitula on stick insect eggs and elaiosomes on seeds: convergent adaptations for burial by ants. Functional Ecology, 6(6), 642–648. Google Scholar
4. Dossey, A. T., Walse, S. S., & Edison, A. S. (2008). Developmental and geographical variation in the chemical defense of the walkingstick insect Anisomorpha buprestoides. Journal of Chemical Ecology, 34(5), 584–590. Google Scholar

• Insecta
• Orthoptera
• Mantodea
• Hymenoptera
• Natural Selection