Most insects have two compound eyes with multiple facets, varying from one to thousands in number. Many will also have 1 to 3 dorsal ocelli on the top of the head. Larval insects have lateral ocelli called stemmata, which form into compound eyes after metamorphosis. All types of eyes serve the same purpose – to direct light to light-sensitive receptor cells.
The compound eye is so named because it is composed of a collection of facets or lenses called ommatidia (Fig. 2), each having a lens or cornea (1), below which lies a crystalline cone (2). Below the cone are seven or eight retinular cells (4) which form around light-sensitive optic rod called the rhabdom (5) . At the base of the rhabdom are receptor axons (6) which are nerve fibres that lead to the protocerebrum of the insect brain. Each ommatidium is isolated from the other ommatidia by a pigmented layer (3) which serves to retain light. The eye is surrounded by the ocular suture (8) which is edged by the carina (7). The ocular suture provides additional strength to the head capsule and supports the form of the compound eye.
More details on eyes will be provided in the future sections on insect diversity.
On the head and near the eyes, are two segmented sensory organs commonly known as ‘feelers’. In most Hexapoda, each antenna consists of three parts, the basal scape which is connected to the head by flexible cuticle, followed by the second segment, the pedicel. The remaining sub-segments (annuli or flagellomeres), are together called the flagellum. A count of flagellomeres and the form of the antenna can assist in identification in many species.
The scape, pedicel and flagellum segments are articulated and movement is made possible by two pairs of muscles within the scape and pedicel. Only members of the Hexapod orders Collembola (springtails) and Diplura (two-pronged bristletails) have muscles in each segment of the flagellum. In some insects, movement in the flagellum, such as the fanning of the lamellae in some beetles, is controlled by changes of hemolymph (blood) pressure. This is made possible by the pumping action of an antennal heart near the base of the antenna in the head.
The flagellum takes on a variety of shapes or combinations of shapes (Fig. 4) that can be diagnostic characters as well indicative of sexual dimorphism. More details on antennas will be provided in the future sections on insect diversity.
Antenna are sensory organs with nerves that connect to the brain. They have two main functions, serving to detect both physical and chemical factors in the environment. Chemosensory function is mostly achieved through microscopic hairs called sensilla. The sensing of airborne chemical volatiles provides information for finding mates, receiving warning of predators, locating egg laying sites and finding food. Physical stimulation of the sensilla by air flow can help control insects in flight and physical contact can define obstacles when moving with the legs.
Antennae are sometimes also involved in non-sensing functions such as courting and dominance displays, clasping mates and sometimes even for holding prey (as in Chaoborus larva).
Gordh G. and D.H. Headrick. A Dictionary of Entomology. Cabi 2001.
Romoser, William S. The Science of Entomology, pp. 38/39. Collier-MacMillan 1973.
Resh, Vincent H. and R. T. Cardé, Eds. Encyclopedia of Insects, pp. 26-28. Elsevier 2003.