Tarsomere

A tarsomere (from the Ancient Greek tarsos, meaning "flat of the foot," and meros, meaning "part" or "segment") is one of the individual articulated subsegments that compose the tarsus, the fifth major podomere of the thoracic walking leg in insects and related hexapods, located distally to the tibia and proximally to the terminal pretarsus, studied in entomology and comparative arthropod biomechanics (Snodgrass, 1935; Beutel & Gorb, 2001). Unlike true leg segments (coxa, trochanter, femur, tibia, and pretarsus), which possess intrinsic skeletal musculature originating and inserting within adjacent podomeres, individual tarsomeres lack intrinsic muscle fibers altogether (Snodgrass, 1935; Chapman et al., 2013). Instead, the chain of tarsomeres functions as a passive, compliant multi-link suspension system spanned internally by the long cuticular apodeme of the pretarsal retractor unguis muscle (which inserts on the unguitractor plate) and moved as a unit at the tibio-tarsal joint by the extrinsic levator tarsi and depressor tarsi muscles housed inside the distal tibia (Radnikow & Bässler, 1991; Frantsevich & Wang, 2009). Across class Insecta, the number, relative length, ventral adhesive pad architecture (euplantulae), and setal modifications of the tarsomeres—recorded taxonomically as the tarsal formula—provide diagnostic family-level characters in Coleoptera, Hymenoptera, Orthoptera, and Diptera while housing dense arrays of contact chemoreceptive sensilla that taste substrates upon foot touchdown (Gorb, 2001; Chapman et al., 2013).

Extrinsic Tendon Actuation and Lack of Intrinsic Muscles

Why are tarsomeres classified morphologically as secondary subdivisions (subsegments) of a single ancestral podomere rather than true limb segments? Robert E. Snodgrass established the fundamental morphological criterion in comparative arthropod anatomy: a primary limb segment (podomere) is defined by the presence of intrinsic muscle attachments, whereas secondary subdivisions (meres or annuli, such as antennal flagellomeres and pedal tarsomeres) are ring-like sclerotized cylinders separated by flexible resilin-rich arthrodial membranes without inter-subsegmental muscles (Snodgrass, 1935):

1. Tibio-Tarsal Extrinsic Musculature: The proximal face of the first tarsomere (basitarsus) articulates with the distal margin of the tibia via a dicondylic or monocondylic hinge joint. Two antagonistic muscles originating on the internal walls of the tibia—the dorsal levator tarsi (musculus levator tarsi) and the ventral depressor tarsi (musculus depressor tarsi)—insert via short cuticular tendons onto the basal rim of the basitarsus, raising or depressing the entire tarsal chain relative to the tibia (Snodgrass, 1956; Radnikow & Bässler, 1991).
2. Trans-Tarsal Passage of the Pretarsal Retractor Apodeme: Running longitudinally through the hemolymph-filled lumen of all consecutive tarsomeres is the slender, heavily sclerotized cuticular tendon (apodeme) of the musculus retractor unguis (claw retractor muscle), whose contractile fiber bundles originate far upstream in the distal femur and proximal tibia. This tendon bypasses every inter-tarsomeric joint without inserting on the intermediate tarsomeres and attaches distally onto the basal unguitractor plate of the pretarsus (Federle et al., 2001; Gladun et al., 2009).
3. Passive Chain Flexion and Compliant Substrate Moulding: Because the retractor apodeme runs along the ventral floor of the tarsal lumen beneath the dorsal monocondylic articulations linking consecutive tarsomeres, tension on the retractor tendon during stance pulls the unguitractor plate proximally, flexing the pretarsal claws (ungues) while simultaneously causing the chain of tarsomeres to curve ventrally like a multi-jointed finger. When the retractor muscle relaxes at the end of stance, internal hemolymph pressure combined with elastic recoil stored in the dorsal inter-tarsomeric resilin ligaments straightens the tarsomere chain back to its resting posture (Frantsevich & Wang, 2009; Labonte & Federle, 2015).

Structural Subdivisions: Basitarsus, MediMeres, and Distitarsus

In a generalized pentamerous (five-segmented) insect tarsus, the individual tarsomeres exhibit distinct structural and biomechanical specializations along the proximal–distal axis:

• Basitarsus (First Tarsomere, T1): Often the longest and most robust subsegment, the basitarsus (metatarsus on the metathoracic leg) receives the full mechanical torque of the tibial levator and depressor muscles and frequently bears specialized grooming combs, pollen-collecting brushes, or acoustic/vibratory subgenual-adjacent chordotonal linkages.
• Intermediate Tarsomeres (T2–T4): Shorter cylindrical or bilobed subsegments that distribute normal and shear loads across irregular terrain and house ventral adhesive cushions (euplantulae) or dense tenent setal fields.
• Distitarsus (Terminal Tarsomere, Typically T5): The most distal tarsomere (unguitarsus), whose hollow distal apex houses the invaginated unguitractor plate, the dorsal articular process (unguifer) on which the paired pretarsal claws pivot, and the hydraulic valve regulating hemolymph flow into the arolium, pulvilli, or empodium (Snodgrass, 1935; Beutel & Gorb, 2001).

Evolutionary Reduction and Pseudotetramerous Tarsi

Ancestral winged insects (Pterygota) possess five tarsomeres per leg, which remains the plesiomorphic state across Orthoptera (katydids and crickets), Blattodea (cockroaches), Mantodea, Lepidoptera, Diptera, Hymenoptera, and most families of Coleoptera. In systematic entomology, the number of tarsomeres on the prothoracic, mesothoracic, and metathoracic legs is written as a three-number tarsal formula (for example, 5-5-5 in carabid ground beetles and honeybees; 3-3-3 in short-horned grasshoppers, Acrididae; 2-2-2 in aphids, Aphididae; and 1-1-1 in sucking lice, Anoplura).

Within the order Coleoptera, variations in the tarsal formula define major superfamily lineages:

1. Pentamerous (5-5-5): Characteristic of Adephaga (ground beetles, diving beetles) and polyphagan superfamilies Scarabaeoidea, Elateroidea, and Staphylinoidea.
2. Heteromerous (5-5-4): Diagnostic of the superfamily Tenebrionoidea (darkling beetles, blister beetles, and allies), in which the forelegs and midlegs retain five tarsomeres while the hindlegs possess only four.
3. Cryptopentamerous / Pseudotetramerous (Apparently 4-4-4, Anatomically 5-5-5): Characteristic of the hyperdiverse phytophagous clade Phytophaga—comprising leaf beetles (Chrysomelidae), longhorn beetles (Cerambycidae), and weevils (Curculionoidea). In these beetles, the third tarsomere (T3) is deeply bilobed and cushioned ventrally with dense adhesive tenent setae, while the true fourth tarsomere (T4) is reduced to a minute, ring-like nodule hidden at the base of the elongated fifth tarsomere (T5) inside the bilobed cleft of T3 (Lawrence & Britton, 1994; Beutel & Gorb, 2001).

Ventral Adhesive Cushions (Euplantulae) vs. Pretarsal Pads

In addition to the terminal pretarsal adhesive organs (arolium, pulvilli, and empodium), many orthopterans, stick insects (Phasmatodea), cockroaches, and mantises possess paired, cushion-like pads on the ventral surface of individual tarsomeres called euplantulae (or plantulae; Gorb, 2001; Labonte & Federle, 2013). While the terminal arolium generates strong pulling adhesion when the leg drags inward toward the body on overhangs or ceilings, the tarsomeric euplantulae are specialized for pushing friction and normal-load damping when the insect walks upright or climbs vertically upward (Endlein & Federle, 2012; Labonte & Federle, 2013). Internally, each euplantula contains a spongy network of branched procuticular rods bathed in hemolymph beneath a flexible epicuticle sculpted into hexagonal or transverse microridges that squeeze out water films on wet foliage.

The Basitarsal Strigilis and Pollen-Harvesting Apparatus

In Hymenoptera, the first tarsomere (basitarsus) has evolved complex mechanical tools on both the prothoracic and metathoracic legs (Snodgrass, 1956; Schönitzer, 1986):

1. Prothoracic Antenna Cleaner (Strigilis): At the junction of the fore tibia and the fore basitarsus, a semi-circular, comb-lined notch (veilchen or antenna comb) excavated in the proximal ventral margin of the basitarsus opposes a movable, flattened tibial spur (velum). When a bee, wasp, or ant flexes the tibio-tarsal joint, the velum closes over the basitarsal notch to form a circular iris through which the insect draws its antenna, scraping dust and pollen off the olfactory sensilla.
2. Metathoracic Pollen Brush and Rastrum-Auricle Press: In worker honeybees (Apis mellifera), the flattened inner face of the hind basitarsus bears 10 to 12 transverse rows of stiff golden spines (the pollen brush or scopa). After combing pollen grains from its branched plumose body hairs onto these basitarsal rows, the bee rubs the two hindlegs together: the distal tibial rastrum (rake) scrapes pollen off the opposite basitarsal brush into a gap between the tibia and the spiny basal lip of the basitarsus (the auricle), which acts as a mechanical piston when the joint flexes to pack the moistened pollen pellet upward into the concave tibial pollen basket (corbicula; Snodgrass, 1956).

Tarsal Contact Chemoreception

In Diptera (blowflies Phormia regina, fruit flies Drosophila melanogaster, and mosquitoes Aedes aegypti), Lepidoptera (butterflies and moths), and Hymenoptera, the ventral surface of the distal tarsomeres is densely innervated by uniporous gustatory trichoid sensilla (Chapman et al., 2013). Each tarsal taste hair contains four to five bipolar gustatory receptor neurons tuned selectively to sugars (sucrose, trehalose), salts (NaCl), water, deterrent bitter alkaloids, or cuticular hydrocarbons/oviposition pheromones. When a foraging blowfly or butterfly steps into a droplet of floral nectar, firing of the tarsomeric sugar-sensitive neurons triggers the proboscis extension reflex (PER) within 50–100 milliseconds, uncoiling or extending the mouthparts before the head ever touches the food source.

How many tarsomeres do insects have?

Ancestral and generalized winged insects possess five tarsomeres per leg (a 5-5-5 tarsal formula), as seen in bees, ants, flies, butterflies, cockroaches, and ground beetles. However, evolutionary fusion or loss has reduced this count in many lineages to four, three (grasshoppers and webspinners), two (aphids and scale insects), or a single fused tarsomere (sucking lice and larval holometabolous insects).

Why do insects not have muscles inside their tarsomeres?

Individual tarsomeres are secondary subsegments of a single primary limb segment (the tarsus) rather than independent podomeres. Keeping contractile muscle mass concentrated proximally inside the femur and tibia dramatically lowers the rotational moment of inertia of the distal leg, allowing insects to swing their legs at high step frequencies (10–40 Hz) while actuating the tarsomere chain and terminal claws via a lightweight, high-tensile chitinous tendon (the retractor unguis apodeme).

1. Snodgrass, R. E. (1935). Principles of Insect Morphology. McGraw-Hill Book Company (reprinted 1993, Cornell University Press). https://doi.org/10.7591/9781501717918
2. Beutel, R. G., & Gorb, S. N. (2001). Ultrastructure of attachment specializations of hexapods (Arthropoda): Evolutionary patterns inferred from a revised ordinal system. Journal of Zoological Systematics and Evolutionary Research, 39(4), 177–207. https://doi.org/10.1046/j.1439-0469.2001.00155.x
3. Frantsevich, L., & Wang, W. Z. (2009). Findings on the flexibility of the tarsus in Coleoptera (Insecta). Journal of Experimental Biology, 212(7), 1029–1037. https://doi.org/10.1242/jeb.025213
4. Labonte, D., & Federle, W. (2013). Functionally different pads on the same foot allow control of attachment: Stick insects have load-sensitive "heel" pads for friction and shear-sensitive "toe" pads for adhesion. PLOS ONE, 8(12), e81943. https://doi.org/10.1371/journal.pone.0081943
5. Chapman, R. F., Simpson, S. J., & Douglas, A. E. (2013). The Insects: Structure and Function (5th ed.). Cambridge University Press. https://doi.org/10.1017/CBO9781139035460