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Antennae (singular: antenna), sometimes referred to as "feelers," are paired appendages used for sensing in arthropods.

Antennae are connected to the first one or two segments of the arthropod head. They vary widely in form, but are always made of one or more jointed segments. While they are are typically sensory organs, the exact nature of what they sense and how they sense it is not the same in all groups. Functions may variously include sensing touch, air motion, heat, vibration (sound), and especially smell or taste. Antennae are sometimes modified for other purposes, such as mating, swimming, and even attachment to substrate.

The common ancestor of all arthropods likely had two pairs of uniramous (unbranched) antenna-like structures, as seen in some modern crustaceans. Except the chelicerates and proturans, which have none, all non-crustacean arthropods have a single pair of antennae.

Insects

Antennae are the primary olfactory sensors of insects and are accordingly well-equipped with a wide variety of sensilla (singular: sensillum). Paired, mobile, and segmented, they are located between the eyes on the forehead. Embryologically, they represent the appendages of the second head segment.

All insects have antenna, however they may be greatly reduced in the larval forms. Amongst the non-insect classes of the Hexapoda, both Collembola and Diplura have antenna, but Protura do not.

Structure

The three basic segments of the typical insect antenna are the scape or scapus (base), the pedicel or pedicellus (stem), and finally the flagellum, which often comprises many units known as flagellomeres. The pedicel (the second segment) contains the Johnston's organ which is a collection of sensory cells.

The scape is mounted in a socket in a more or less ring-shaped sclerotised region called the torulus, often a raised portion of the insect's head capsule. The socket is closed off by the membrane into which the base of the scape is set. However, the antenna does not hang free on the membrane, but pivots on a rigidly sprung projection from the rim of the torulus. That projection on which the antenna pivots is called the antennifer. The whole structure enables the insect to move the antenna as a whole by applying internal muscles connected to the scape. The pedicel is flexibly connected to the distal end of the scape and its movements in turn can be controlled by muscular connections between the scape and pedicel. The number of flagellomeres can vary greatly between insect species, and often is of diagnostic importance.

True flagellomeres are connected by membranous linkage that permits movement, though the flagellum of "true" insects does not have any intrinsic muscles. Some other Arthropoda do however have intrinsic muscles throughout the flagellum. Such groups include the Symphyla, Collembola and Diplura. In many true insects, especially the more primitive groups such as Thysanura and Blattodea, the flagellum partly or entirely consists of a flexibly connected string of small ring-shaped annuli. The annuli are not true flagellomeres, and in a given insect species the number of annuli generally is not as consistent as the number of flagellomeres in most species.

In many beetles and in the chalcidoid wasps, the apical flagellomeres form a club shape, and the collective term for the segments between the club and the antennal base is the funicle; traditionally in describing beetle anatomy, the term "funicle" refers to the segments between the club and the scape. However, traditionally in working on wasps the funicle is taken to comprise the segments between the club and the pedicel.

Quite commonly the funicle beyond the pedicel is quite complex in Endopterygota such as beetles, moths and Hymenoptera, and one common adaptation is the ability to fold the antenna in the middle, at the joint between the pedicel and the flagellum. This gives an effect like a "knee bend", and such an antenna is said to be geniculate. Geniculate antennae are common in the Coleoptera and Hymenoptera. They are important for insects like ants that follow scent trails, for bees and wasps that need to "sniff" the flowers that they visit, and for beetles such as Scarabaeidae and Curculionidae that need to fold their antennae away when they self-protectively fold up all their limbs in defensive attitudes.

Because the funicle is without intrinsic muscles, it generally must move as a unit, in spite of being articulated. However, some funicles are complex and very mobile. For example, the Scarabaeidae have lamellate antennae that can be folded tightly for safety or spread openly for detecting odours or pheromones. The insect manages such actions by changes in blood pressure, by which it exploits elasticity in walls and membranes in the funicles, which are in effect erectile.

In the groups with more uniform antennae (for example: millipedes), all segments are called antennomeres. Some groups have a simple or variously modified apical or subapical bristle called an arista (this may be especially well-developed in various Diptera).

Functions

Olfactory receptors on the antennae bind to free-floating molecules, such as water vapour, and odours including pheromones. The neurons that possess these receptors signal this binding by sending action potentials down their axons to the antennal lobe in the brain. From there, neurons in the antennal lobes connect to mushroom bodies that identify the odour. The sum of the electrical potentials of the antennae to a given odour can be measured using an electroantennogram.

In the case of the monarch butterfly, antennae are necessary for proper time-compensated solar compass orientation during migration, antennal clocks exist in monarchs, and they are likely to provide the primary timing mechanism for sun compass orientation.

Crustaceans

Crustaceans bear two pairs of antennae. The first pair are uniramous and are often referred to as antennules, while the second pair are biramous, meaning that each antenna is composed of two parts, joined at their base . In most adults, the antenna are sensory organs, but they are used by the nauplius larva for swimming. In some groups of crustaceans, such as the spiny lobsters and slipper lobsters, the second antennae are enlarged, while, in others, such as crabs, the antennae are reduced in size.

References

1. Chapman, R.F. (1998). The Insects: Structure and Function (PDF) (4th ed.). Cambridge, UK: Cambridge University Press. pp. 8–11. ISBN 0521570484.
2. ^ Boxshall, Geoff; Jaume, D. (2013). Functional Morphology and Diversity: Antennules and Antennae in in the Crustacea. Oxford University Press. pp. 199–236.
3. Fortey, Richard A.; Thomas, Richard H. (1998). Arthropod Relationships: Phylogenetic Analysis (1st ed.). The Systematics Association. p. 117. ISBN 978-94-011-4904-4.
4. Darby, Gene (1958). What is a butterfly?. Chicago: Benefic Press. p. 8. OCLC 1391997.
5. Gullan, Penny J.; Cranston, Peter S. (2005). The Insects: an Outline of Entomology (3rd ed.). Oxford, UK: Blackwell Publishing. p. 38. ISBN 1-4051-1113-5.
6. Chapman, Reginald Frederick (1998). The Insects: Structure and Function (4th ed.). New York: Cambridge University Press. p. 8. ISBN 0-521-57890-6.
7. ^ Thomas A. Keil (1999). "Morphology and development of the peripheral olfactory organs". In Hansson, Bill S. (ed.). Insect Olfaction (1st ed.). Springer. pp. 5–48. ISBN 978-3-540-65034-8.
8. Pass, Guenther. The Anatomy and Ultrastructure of the Antennal Circulatory Organs in the Cockchafer Beetle Melolontha melolontha L. (Coleoptera, Scarabaeidae) Zoomorphology 01/1980; 96(1):77-89. DOI: 10.1007/BF00310078
9. Lawrence, Eleanor, ed. (2005). Henderson's Dictionary of Biological Terms (13th ed.). Pearson Education. p. 51. ISBN 978-0-13-127384-9.
10. "Electroantennography (EAG)". Georg-August-Universität Göttingen. Retrieved March 27, 2010.
11. Merlin, Christine; Gegear, Robert J.; Reppert, Steven M. (September 2009). "Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies". Science. 325 (5948): 1700–1704. doi:10.1126/science.1176221. PMC 2754321. PMID 19779201.
12. "Superphylum Arthropoda". University of Arizona.

• Arthropod anatomy