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.
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.)
Hexapods are arthropods, i. e., animals with segmented bodies. Segments are organized in three distinct functional units, or tagmata: the head, thorax, and abdomen (Chapman 1998, Weidner 1982). The head is a capsule formed by the fusion of several segments (the exact number is controversial, Weidner 1982). It features mouthparts for feeding as well as a pair of antennae (absent in Protura) and other sensory organs.
The most distinctive feature of the hexapods is the reduction of walking appendages to six, with three body segments consolidating to form the thorax, which provides much of the locomotory ability of the animals (Kristensen 1981, 1991). This is in contrast to other arthropods, most of which have more than three pairs of legs. The hexapod abdomen, primitively with 11-segments plus a postsegmental telson, is specialized for digestion, excretion, and reproduction. It generally lacks legs, but many apterygote (wingless) hexapods and some pterygote insects feature a variety of abdominal appendages, including a pair of cerci on the terminal segment (absent in Collembola and Protura), which function as sense organs (Chapman 1998, Weidner 1982).
The hexapod central nervous system consists of the brain, which is located in the head and a nerve cord composed of a series of ganglia extending ventrally along the longitudinal axis of the body (ventral nerve cord, Chapman 1998, Niven et al. 2008). In the basic hexapod body plan, there was most likely one ganglion associated with each body segment, but modern hexapods display varying degrees of ganglionic fusion (Chapman 1998, Nation 2002). The central nervous system controls muscles, glands, and other organs, and it receives input from a diverse array of sensory systems.
Hexapods possess many different kinds of sensory receptors that monitor both the external and internal environment. A great variety of mechano- and chemosensory systems have been described across different groups of hexapods (Chapman 1998, Nation 2002, Weidner 1982); however, visual perception appears to be important only in insects, many of which feature highly specialized compound eyes (Horridge 1975); while the mostly soil- and litter-dwelling Collembola, Protura, and Diplura entirely lack eyes, although some Collembola have ocelli (Gillot 2005).
Life History and Behavior
The typical reproductive system of female hexapods features paired ovaries which release eggs into lateral oviducts (Chapman 1998, Stys & Bilinski 1990). The egg then travels through the median oviduct to the genital chamber (vagina). Fertilization usually occurs immediately before oviposition by sperm that are retrieved from one or more sperm storage organs (spermathecae). In most hexapods, there is a delay between sperm transfer from the male to the female and sperm usage by the female; so sperm are often stored in spermathecae for considerable periods of time (up to many years in some ants, e. g., Tschinkel 1987).
Sperm are usually received through the female genital opening (gonopore), which also serves as the exit for fertilized eggs. In most insects, eggs are laid through an ovipositor, a tube-like structure of varying length created by the fusion and modification of the abdominal body wall (Chapman 1998, Lawrence et al. 1991, Weidner 1982). Upon leaving the body, hexapod eggs are often accompanied by the excretions of female accessory glands. These substances may be used to attach the eggs to the substrate, or they may protect the eggs from predators or the elements (Chapman 1998).
In the male reproductive system, paired testes release sperm into the vasa deferentia which may feature a seminal vesicle where sperm are stored before leaving the body through the ejaculatory duct (Chapman 1998). The male accessory glands secrete seminal fluid, which supports sperm survival and fertilization success. Accessory gland secretions also form the spermatophore, a specialized structure that encapsulates sperm and seminal fluid and protects them during transfer to the female (Chapman 1998).
In apterygote hexapods (Collembola, Diplura, Archaeognatha, Zygentoma, nothing is know about the mating behavior of Protura) sperm transfer is indirect; i. e., males deposit (usually stalked) spermatophores in the environment, and females actively pick up sperm packets and absorb them into their reproductive tract (Proctor 1998, Schaller 1971). In pterygote insects, spermatophores or unencapsulated sperm are usually transferred directly from male to female through copulation, and males have highly specialized intromittent organs for this purpose (Chapman 1998, Eberhard 1985).
Evolution and Systematics
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).
Physiology and Cell Biology
Most insects breathe with the aid of a complex system of tubes (tracheae) which deliver oxygen directly to its sites of utilization (Chapman 1998, Weidner 1982). Tracheae are also present in non-insect hexapods, but their tracheal systems are simpler, with much less branching of tracheae and no anastomosis (reconnecting of branches, Gillot 2005). Most Collembola and Protura do not have any tracheae at all (Gillot 2005, Hopkin 1997), and gas exchange occurs entirely through the external body wall. Since non-insect hexapods are generally very small, their surface area to volume ratio is high, and oxygen easily diffuses from the atmosphere into all parts of the body cavity (Hopkin 1997).
The circulatory system of hexapods does not have a role in gas exchange. Its main function is the transport of nutrients, hormones, water, salts, wastes, etc. throughout the body (Chapman 1998). The hexapod circulatory system is open, i. e., the blood (hemolymph) fills the entiry body cavity (hemocoel), which is usually loosely subdivided into different compartments by muscular sheets or tissue membranes (diaphragms). Movement of the hemolymph is achieved by a contractile dorsal vessel and various accessory pulsatile organs supplying the appendages (Gereben-Krenn & Pass 1999, Jones 1977, Pass 2000).
The digestive system of hexapods is greatly modified in different groups to facilitate the exploitation of a great diversity of food sources (Chapman 1998, Weidner 1982). The alimentary canal is usually a continuous tube that extends from the mouth to the anus. Most digestion and absorption of nutrients occurs in the midgut, and food reserves are stored in the fat body, a large aggregation of cells suspended in the hemocoel (Chapman 1998, Weidner 1982). As the insect's principal metabolic organ, the fat body synthesizes and accumulates lipids, carbohydrates, amino acids, and proteins. Excretion and water regulation are achieved by the Malpighian tubules, a group of blindly ending tubes that are attached to the anterior end of the hindgut. They absorb water and solutes from the hemolymph and transfer waste products to the hindgut for transport out of the body via the anus (Chapman 1998, Weidner 1982). Malpighian tubules are absent in Collembola and aphids, and Diplura, Protura, and Strepsiptera feature excretory papillae rather than tubules at the junction of midgut and hindgut (Chapman 1998).
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