Antenna Zoology

Antennae are paired, jointed appendages found on the heads of most arthropods and serve as major sensory organs. Commonly known as feelers, they enable animals such as insects, crustaceans and myriapods to detect a wide range of environmental cues. Although their form and function vary greatly among arthropod groups, antennae consistently play central roles in navigation, communication, foraging and reproduction.

Evolutionary Background

The common ancestor of modern arthropods is believed to have possessed a single pair of uniramous antennal appendages, followed by additional biramous limb-like structures, as evidenced in living crustaceans and fossil trilobites. Today:

• Crustaceans possess two pairs of antennae.
• Insects and other non-crustacean arthropods possess one pair.
• Chelicerates (spiders, scorpions, mites) and proturan hexapods have no antennae.

Antennae in larval stages often differ substantially from adult forms, reflecting the distinct ecological roles of larvae. For example, many crustacean larvae (nauplii) use antennae for locomotion, whereas adults use them primarily for sensing.

Crustaceans

Crustaceans have a first pair of antennules (primary antennae) and a second pair of antennae (secondary antennae). These structures may be:

• Uniramous or biramous, depending on species and life stage.
• Segmented (each annulus with its own musculature) or flagellate (muscle only at the base, with a flexible flagellum).

Secondary antennae may be reduced or absent in certain highly modified groups such as barnacles. Crustaceans also use antennae for non-sensory functions: naupliar swimming, attachment to surfaces (as in barnacles), or even forming snorkel-like structures to channel water over the gills in burrowing species.

Insects

Insects possess one pair of antennae, which represent the appendages of the second head segment. These are their primary olfactory organs and are richly endowed with sensilla capable of detecting odours, humidity, vibration, temperature and chemical stimuli.
Antennal anatomy in insects includes:

• Scape: the basal segment, anchored in a socket called the torulus.
• Pedicel: the second segment, containing Johnston’s organ, a mechanosensory structure crucial for detecting motion and sound vibrations.
• Flagellum: a series of flagellomeres (true segments) that lack intrinsic muscles; they move through the actions of muscles in the scape and pedicel.

Some insect groups possess annuli—ring-like divisions that resemble segments but do not have the functional independence of true flagellomeres. In others, such as beetles and certain parasitic wasps, the apical flagellomeres may form a clava or club. The intermediate region, the funicle, varies in complexity across taxa.
Antennae in many insects exhibit specialised adaptations:

• Geniculate (elbowed) antennae in ants, bees and weevils, allowing folding at a sharp angle.
• Lamellate antennae in scarab beetles, which can be fanned open for sensing or closed for protection.
• Aristae in many flies—bristle-like structures used for air-speed detection and odour sensing.

Functions

Antennae perform a variety of sensory and behavioural roles across arthropods:

• Olfaction and gustation: Insects rely heavily on antennal receptors to detect pheromones, host plant chemicals and environmental odours. The action potentials generated by antennal neurons can be measured using electroantennograms.
• Mechanoreception: Antennae detect air movement, vibrations and sound. For example, male mosquitoes use specialised fibrillae on their antennae to detect the wingbeat frequency of females.
• Navigation: Monarch butterflies require functional antennae for time-compensated solar compass orientation during migration.
• Courtship and reproduction: Moths such as Spodoptera littoralis require antennal input for males to respond to female mating signals.
• Host plant identification: Species such as the diamondback moth and Papilio cresphontes butterflies use antennal cues to locate suitable plants for oviposition.
• Flight stabilisation: In crepuscular hawk moths like Manduca sexta, antennae provide mechanosensory feedback crucial for maintaining stability in flight.

Structural and Functional Diversity

Antennae may be modified for specific non-sensory purposes:

• Swimming in crustacean larvae
• Attachment to substrates (e.g., barnacles)
• Brooding and mating behaviours
• Anchoring or locomotion in specialised lineages