Sensillum

Definition

A cuticular sensory organ of arthropods, consisting of a hair-like, peg-like, or pit-like structure innervated by one or more bipolar sensory neurons. Sensilla serve as the peripheral interface for environmental detection, with morphological specialization determining modality: hair sensilla (trichoid) typically mediate mechanoreception and chemoreception; peg sensilla (basiconic) often detect olfactory cues; and coeloconic sensilla house thermo- and hygroreceptors. Each sensillum contains a cuticular apparatus (hair shaft, pore, or chamber) and associated dendritic processes of sensory neurons that transduce stimuli into neuronal signals.

Etymology

Latin diminutive of sensus 'sense, feeling'

Example

The antennae of a male silkmoth (Bombyx mori) bear approximately 17,000 sensilla trichodea, each containing two olfactory receptor neurons tuned to the female sex pheromone bombykol, enabling detection at concentrations below one molecule per cubic centimeter of air.

Synonyms

• sensory peg
• hair sensillum

Related Terms

• Chemoreceptor
• mechanoreceptor
• thermoreceptor
• hygroreceptor
• seta
• trichoid sensillum
• basiconic sensillum
• coeloconic sensillum
• campaniform sensillum

Usage Notes

Sensillum (plural sensilla) is the precise morphological term for arthropod sensory organs; 'seta' and 'hair' are often used loosely for visible external structures but lack the specific neural connotation. Sensilla are classified by external form (trichoid, basiconic, coeloconic, campaniform, scolopidial) and by modality, though single sensilla may be multimodal. The term is restricted to arthropods; analogous structures in other phyla (e.g., vertebrate hair cells, nematode sensilla) are not homologous. In electron microscopy and neuroethology, 'sensillum' specifically denotes the cuticular apparatus plus its receptor neurons, distinguishing it from isolated cuticular hairs or state that lack innervation.