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Containing group: Arthropoda

Introduction

While crustaceans are the dominant group of arthropods in marine environments, hexapods, including insects, rule the land. Their dominance among animals is evident in their numbers of species, with over 750,000 described (Wilson, 1988), as well as biomass (e.g. Fittkau and Klinge, 1973).

Hexapods include three orders of wingless arthropods (Collembola, Protura, Diplura), as well as the insects. The former orders are soil or litter dwellers. Collembola (springtails) are perhaps the most abundant arthropods on earth. Proturans are very small, pale arthropods that are rarely encountered. Diplurans include a few families of larger, pale arthropods that are frequently found in moist soils. The majority of hexapod species are insects, many of which are winged as adults.

Characteristics

The most distinctive feature of the hexapods is the reduction in walking appendages to six, with three body segments consolidating to form the thorax, which provides much of the locomotory ability of the animals. (This is in contrast to other arthropods, most of which have more than three pairs of legs.)

Discussion of Phylogenetic Relationships

The position of Diplura in the hexapods is unclear. Traditionally, it has been placed with the Collembola and Protura in a group called "Entognatha",

                     === Collembola
                  ===|
     ==Entognatha=|  === Protura
=====|            |
     |            ====== Diplura
     |
     =================== Insecta

so named because members of these three orders all have the base of the mouthparts internalized, so that the mandible and maxilla are partly contained within the head capsule. In addition to this derived similarity in mouth structure, these three orders share reduced Malpighian tubules and compound eyes. However, there is some evidence that diplurans may instead be the sister group of insects:

        === Collembola
     ===|
     |  === Protura
=====|
     |  === Diplura
     ===|
        === Insecta

Derived characteristics linking diplurans with insects include the presence of filiform cerci, and an extra set of nine single tubules in the axoneme of the sperm. For a more detailed discussion of the evidence, with additional references, see Kristensen (1991).

Other Names for Hexapoda

• Hexapods
• Insects, springtails, diplurans, and proturans

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Information on the Internet

• Biodiversity Explorer: Hexapoda