Cardo (Entomology)

The cardo (plural: cardines; from the Latin cardo, meaning "hinge of a door" or "pivot") is the proximal basal sclerite of the paired maxillae (first maxillae) in insects and related hexapods, suspending the main maxillary shaft (stipes) and its distal lobes (galea, lacinia, and maxillary palp) from the posteroventral margin of the head capsule (cranium), studied in entomology and comparative arthropod functional morphology (Snodgrass, 1935; Matsuda, 1965; Chapman et al., 2013). In serial limb homology across Arthropoda, the cardo and stipes together represent the subdivided coxopodite (basal limb segment) of the maxillary head segment—either corresponding to a precoxal subcoxa (cardo) and coxa (stipes) or a bipartite subdivision of the primary coxa itself (Snodgrass, 1935; Machida, 2000). Whereas the insect mandible in winged insects (Dicondylia) is locked to the cranium by two ball-and-socket joints that restrict its movement to a single transverse biting plane, the cardo articulates with the cranial margin via a single basal condyle (cranial–cardinal joint) and joins the stipes distally along a flexible elbow-like hinge (the cardo–stipital suture; Snodgrass, 1935). This two-joint lever system gives the maxilla three rotational degrees of freedom—abduction/adduction, protraction/retraction, and axial supination—enabling chewing insects to manipulate irregular food items and allowing long-tongued bees (Hymenoptera), butterflies (Lepidoptera), and blood-feeding vectors (Diptera) to extend and retract elongated suctorial or piercing proboscises (Snodgrass, 1956; Krenn, 2010).

The Monocondylic Cranial Joint and Internal Cardinal Process

In a generalized mandibulate insect—such as the locust (Schistocerca gregaria), American cockroach (Periplaneta americana), or carabid ground beetle (Carabus)—the cardo is seated in the membranous maxillary fossa on the ventral/posterior side of the head capsule (postgena and hypostoma; Snodgrass, 1935):

1. Articular Condyle on the Hypostomal Margin: The proximal apex of the cardo bears a single rounded or cupped articular condyle that fits into a socket (the maxillary acetabulum) on the hypostomal ridge of the cranium. Because this joint is monocondylic, the cardo can both swing inward and outward in the transverse plane and pivot forward and backward in the sagittal plane.
2. Basicardo and Disticardo Subdivision: The external surface of the cardo is frequently crossed by an internal strengthening ridge (expressed externally as the cardinal suture) that divides the sclerite into two triangular or subquadrate plates:
   • Basicardo (Subcardo or Proximal Cardo): The basal plate adjoining the cranial articulation, whose inner surface projects a strong cuticular apodeme (the cardinal process) into the head cavity to receive the cranial rotator/promoter muscle.
   • Disticardo (Alacardo or Distal Cardo): The broader distal plate that articulates along a transverse hinge with the base of the stipes and receives the tentorial adductor muscles on its inner surface (Snodgrass, 1935; Lawrence & Britton, 1994).
3. The Cardo–Stipital Elbow Hinge: At rest in chewing insects, the longitudinal axis of the cardo is oriented nearly perpendicular (at ~70°–110°) to the dorsoventral axis of the stipes. Because the cardo and stipes form an inward-pointing elbow, downward and forward rotation of the distal end of the cardo pushes the entire stipes—along with the lacinia and galea—ventrally (protraction) and laterally (abduction), opening the maxillae wide to grasp food.

Cranial Rotator and Tentorial Adductor Muscles

Movement of the cardo is driven by two primary extrinsic muscle groups innervated by the maxillary nerve of the subesophageal ganglion (Snodgrass, 1935; Chapman et al., 2013):

1. Cranial Rotator / Promoter of the Cardo (musculus craniocardinalis): Originating on the posterior dorsal or postgenal wall of the head capsule and inserting onto the lever-like cardinal process on the basicardo, immediately lateral to the cranial condyle. Because its insertion lies on the outer side of the pivot point, contraction of the craniocardinalis pulls the basal process upward, rocking the distal end of the cardo downward and outward. Through the cardo–stipital hinge, this action simultaneously protracts and abducts the entire maxilla (Snodgrass, 1935).
2. Tentorial Adductors of the Cardo (musculi tentoriocardinales): One or two broad flat muscle bundles originating on the anterior or posterior arms of the tentorium (the internal cuticular endoskeleton of the head) and inserting across the inner surface of the disticardo. Contraction pulls the distal end of the cardo inward toward the midline and upward toward the tentorium, adducting and retracting the maxilla during the power stroke of chewing or proboscis folding.

The Rod-Like Swinging Cardines of Bees (Hymenoptera: Apoidea)

In bees (Hymenoptera: Apoidea, such as the western honeybee Apis mellifera), the cardo has undergone a remarkable geometric transformation into a long, slender, cylindrical suspension rod that powers the deployment and Z-folding of the nectar-lapping proboscis (Snodgrass, 1956):

1. Suspension in the Proboscidial Fossa: Beneath the bee's head capsule lies a deep membranous cavity (the proboscidial fossa). High on the lateral walls of this fossa, each rod-like cardo articulates with a cranial condyle and hangs downward to join the proximal end of the boat-shaped stipes.
2. The Lorum Linkage Coupling Maxillae and Labium: Connecting the distal tips of the left and right cardines across the midline is a flexible, V-shaped sclerotized band called the lorum (submentum accessory sclerite), whose apex cradles the triangular submentum of the labium (Snodgrass, 1956). Because the lorum mechanically bridges the two maxillary cardines to the base of the labium, when the cranial protractor muscles of the cardines contract, both cardines swing forward through an arc of ~90°–120°, simultaneously projecting the paired maxillae (stipites and galeae) and the central labium (prementum and glossa) out of the head fossa as a single coordinated unit. When feeding finishes, the tentorial retractor muscles swing the cardines backward, folding the entire labiomaxillary complex into a compact Z-shape tucked safely beneath the head capsule during insect flight.

Internalization and Stylet Actuation in Diptera and Hemiptera

In piercing–sucking insects where the outer maxillary lobes are reduced to slender stylets, the cardo is drawn inward from the surface of the head capsule to act as an internalized muscular lever (Snodgrass, 1935; Clements, 1992; Krenn & Aspöck, 2012):

• Mosquitoes and Nematoceran Diptera: In female mosquitoes (Aedes, Anopheles, Culex) and black flies, the cardo and stipes are fused and invaginated into the head floor as a sclerotized rod (the maxillary apodeme or stipital rod) attached to the base of the saw-toothed lacinia stylet. Antagonistic protractor and retractor muscles inserting on this cardinal–stipital rod drive the rapid reciprocating sawing strokes that pull the proboscis fascicle into vertebrate skin.
• True Bugs and Cicadas (Hemiptera): In Hemiptera, the embryonic cardinal and stipital territories fuse with the lateral wall of the head capsule to form the external maxillary plate (lorum of Hemiptera), while an internal lever arm connects the base of the needle-like lacinial stylet to the protractor and retractor muscles inside the head pouch.

How does the cardo differ from the stipes in the insect maxilla?

The cardo is the proximal basal hinge sclerite that attaches the maxilla to the head capsule (cranium) via a single articular condyle, whereas the stipes is the main central body sclerite articulated distally to the cardo that directly bears the terminal lobes (lacinia and galea) and the segmented maxillary palp.

Why do bees have long rod-shaped cardines?

In bees (Apis mellifera and other Apoidea), the paired cardines are elongated into slender swinging rods joined across the midline by a V-shaped sclerite called the lorum. Swinging these two cardinal rods forward projects both the maxillae and the central labial tongue out of the head capsule together so the bee can probe deep flowers, and swinging them backward folds the entire proboscis into a Z-shape beneath the head during flight.

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. Snodgrass, R. E. (1956). Anatomy of the Honey Bee. Comstock Publishing Associates, Cornell University Press. https://doi.org/10.7591/9781501717925
3. Matsuda, R. (1965). Morphology and Evolution of the Insect Head. Memoirs of the American Entomological Institute, No. 4. https://doi.org/10.1093/aesa/59.1.224
4. Machida, R. (2000). Serialiski homology of the mandible and maxilla in the jumping bristletail Pedetontus unimaculatus Machida, based on embryological observations (Hexapoda: Archaeognatha, Meinertellidae). Journal of Morphology, 245(1), 19–28. https://doi.org/10.1002/1097-4687(200007)245:1<19::AID-JMOR2>3.0.CO;2-H245:1<19::AID-JMOR2>3.0.CO;2-H)
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