Sensillum

Sensillum, plural sensilla, is a small sensory organ found in arthropods, especially insects and crustaceans. A sensillum consists of a cuticular projection that houses one or more sensory neurons together with supporting cells, often with an external pore or opening to the environment. The arrangement allows environmental stimuli to be transduced into neural signals that reach the central nervous system.

Based on morphology and function, sensilla are commonly categorized as mechanosensory or chemosensory. Classical insect classes include trichoid (hair-like), basiconic (peg-like with pores), and coeloconic (cone-shaped recessed) sensilla. In crustaceans, olfactory sensilla known as aesthetascs occur on antennules and are highly specialized for chemical detection. The pore-containing sensilla enable chemical detection, while others primarily respond to mechanical stimuli.

Distribution is broad but concentrated on sensory appendages such as antennae, mouthparts, legs, and wings, where environmental contact is frequent. Mechanosensory sensilla detect touch, airflow, and substrate vibration, whereas chemosensory sensilla detect odors and tastes, guiding behaviors such as foraging, mating, and host selection. Some sensilla also respond to humidity or temperature.

Developmentally, sensilla arise from epidermal cells that form cuticular outgrowths, with sensory neurons extending dendrites into the sensillum and projecting to the CNS via peripheral nerves. Sensillum structure and composition are diverse, enabling fine-tuned sensory discrimination across species and ecological niches.