Another e-mailed question. Besides their eyes that give them a visual representaton of their environment, insects have a wide array of sensory hairs, sensillae, that give them information about temperature, humidity, chemicals, body position, and orientation. Sensillae can be individually distributed, or they can be grouped into sensory fields or even into brand new organs. In this post, I will simply give a generalised rundown of the common sensillae, nothing detailed. Check the reading list at the end for papers with much more information.
The most common are trichoid sensillae. These are typical the typical hairs that stick out of the cuticle. They’re linked directly to the nervous system, and provide the majority of the sensory stimuli, from mechanical sensations to chemical and temperature readings. They vary incredibly in size, structure, and shape, each category having its own special name. You can find them everywhere on an insect’s body, where they mostly serve as mechanosensory. They are also significant components of the antenna, and you can find many small ones in dense concentrations there, serving mostly as chemoreceptors, temperature receptors, humidity receptors. The picture above shows a variety of trichoid sensillae from the base of a moth’s antenna. Wings can also have them.
At the leg and wing bases, and the genitals (mostly), you can find another type of sensilla: the campaniform sensillae. These are not hairs that stick out, but bits of flexible, domed or indented cuticle with one sensilla inside them. They keep a constant record of the positions of the legs and wings, acting as proprioceptors that get activated when they are pressed on. The most impressive use of campaniform sensillae can be seen in flies. The main defining characteristic of flies is that their hind wings are modified to halteres, little stubs that aid in balancing during flight. Critical to their functioning is a field of densely-packed campaniform sensillae at their base, which deform as the haltere is moved in their direction. So if it moves to the left, the sensillae on the left will be “crushed”. Upon deformation, the sensilla inside is triggered, sending a message to the nervous system telling it that the haltere moved, so the fly can then gauge the response opf the flight pattern and react accordingly.
Finally, there are sensory units called scolopidia. These are never found alone, but congregate in fields ranging from a couple to a couple thousand to form the scolopophorous organs, also called the chordotonal organs, for example the vibration-detecting subgenual organ at bottom of the legs, or the hearing-enabling tympanal organs that some insects have convergently evolved. Much like the campaniform sensillae, scolopidia are proprioceptive, but they act as stretch receptors, getting triggered when they are stretched (e.g. by vibrations) rather than pressed on.
Further Technical Reading:
Ai H, Yoshida A & Yokohari F. 2010. Vibration receptive sensilla on the wing margins of the silkworm moth Bombyx mori. Journal of Insect Physiology 56, 236-246.
Akay T. 2004. Signals From Load Sensors Underlie Interjoint Coordination During Stepping Movements of the Stick Insect Leg. Journal of Neurophysiology 92, 42-51.
Akent’eva NA. 2012. Multimodal sensory organs in larvae of some insect species. Entomological Review 92, 379-389.
Anderson P, Hallberg E & Subchev M. 2000. Morphology of antennal sensilla auricillica and their detection of plant volatiles in the Herald moth, Scoliopteryx libatrix L. (Lepidoptera: Noctuidae). Arthropod Structure & Development 29, 33-41.
Benton R, Sachse S, Michnick SW, Visshall LB. 2006. Atypical Membrane Topology and Heteromeric Function of Drosophila Odorant Receptors In Vivo. PloS Biology 4, e20.
Budick SA, Reiser MB & Dickinson MH. 2007. The role of visual and mechanosensory cues in structuring forward flight in Drosophila melanogaster. Journal of Experimental Biology 210, 4092-4103.
Callahan PS. 1975. Insect antennae with special reference to the mechanism of scent detection and the evolution of the sensilla. International Journal of Insect Morphology and Embryology 4, 381-430.
Casas J & Dangles O. 2010. Physical Ecology of Fluid Flow Sensing in Arthropods. Annual Review of Entomology 55, 505–20.
Castrejón-Gómez VR & Rojas JC. 2009. Antennal Sensilla of Anastrepha serpentina (Diptera: Tephritidae). Annals of the Entomological Society of America 102, 310–316.
Castrejón-Gómez VR, Nieto G, Valdes J, Castrejón F & Rojas JC. 2003. The Antennal Sensilla of Zamagiria dixolophella Dyar (Lepidoptera: Pyralidae). Annals of the Entomological Society of America 96, 672–8.
Chapman RF. 2002. Development of phenotypic differences in sensillum populations on the antennae of a grasshopper, Schistocerca americana. Journal of Morphology 254, 186-194.
Chapman RF & Greenwood M. 1986. Changes in distribution and abundance of antennal sensilla during growth of Locusta migratoria L. (Orthoptera: Acrididae). International Journal of Insect Morphology and Embryology 15, 83-96.
Cocroft RB, Tieu TF, Hoy RR & Miles RN. 2000. Directionality in the mechanical response to substrate vibration in a treehopper (Hemiptera: Membracidae: Umbonia crassicornis). Journal of Comparative Physiology A 186, 695-705.
Couto A, Alenius M & Dickson BJ. 2005. Molecular, Anatomical, and Functional Organization of the Drosophila Olfactory System. Current Biology 15, 1535-1547.
Crespo JG. 2011. A Review of Chemosensation and Related Behavior in Aquatic Insects. Journal of Insect Science 11, 1-39.
Eberhard MJB, Lang D, Metscher B, Pass G, Picker MD & Wolf H. 2010. Structure and sensory physiology of the leg scolopidial organs in Mantophasmatodea and their role in vibrational communication. Arthropod Structure & Development 39, 230-241.
Gómez VRC & Carrasco JV. 2008. Morphological characteristics of antennal sensilla in Talponia batesi (Lepidoptera: Tortricidae). Annals of the Entomological Society of America 101, 181–188.
Hill SR, Hansson BS & Ignell R. 2008. Characterization of Antennal Trichoid Sensilla from Female Southern House Mosquito, Culex quinquefasciatus Say. Chemical Senses 34, 231-252.
Ljunberg H, Anderson P & Hansson BS. 1993. Physiology and morphology of pheromone-specific sensilla on the antennae of male and female Spodoptera littoralis (Lepidoptera: Noctuidae). Journal of Insect Physiology 39, 253-260.
Spinola SM & Chapman KM. 1975. Proprioceptive indentation of the campaniform sensilla of cockroach legs. Journal of Comparative Physiology A 96, 257-272.
Stupner A & von Helversen D. 2001. Evolution and function of auditory systems in insects. Naturwissenschaften 88, 159-170.
Yack JE. 2004. The structure and function of auditory chordotonal organs in insects. Microscopy Research and Technique 63, 315-337.
Zill SN, Büschges A & Schmitz J. 2011. Encoding of force increases and decreases by tibial campaniform sensilla in the stick insect, Carausius morosus. Journal of Comparative Physiology A 197, 851-867.