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).
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).
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).
The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects).[3][4] The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs (six legs). Most other arthropods have more than three pairs of legs.[5] Most recent studies have recovered Hexapoda as a subgroup of Crustacea.[6]
Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax, and posterior abdomen.[7][8] The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura),[9] followed by six segments, all closely fused together, with the following appendages:
The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip).[10] In true insects (class Insecta) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. Similar appendages are found on the heads of Myriapoda and Crustacea, although the crustaceans have secondary antennae.[11]
Collembolans and diplurans have segmented antenna; each segment has its own set of muscles. The antennea of insects consist of just three segments; the scape, the pedicel and the flagellum. Muscles occurs only in the first two segments. The third segment, the flagellum, don't have any muscles and is composed of a various number of annuli. This type of antenna is therefore called annulated antenna. Johnston's organ, which is found on the pedicel, is absent in the Entognatha.[12][13]
The thorax is composed of three segments, each of which bears a single pair of legs.[14] As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on the abdominal segments of some immature aquatic insects.[15] In most insects the second and third thoracic segments also support wings.[16] It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.[17]
The abdomen follow epimorphic development, where all segments are already present at the end of embryonic development in all the hexapod groups except for Protura, which has an anamorphic development where the hatched juveniles has an incomplete complement of segments, and goes through a post-embryonic segment addition with each molting before the final adult number of segments is reached. All true insects have eleven segments (often reduced in number in many insect species), but in Protura there are twelve, and in Collembola only six (sometimes reduced to only four).[18][19] The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.[20][21][22]
The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity.[23] These were then considered subclasses of a subphylum called Uniramia or Atelocerata.[24] In the first decade of the 21st century, however, this was called into question, and it appears the hexapods' closest relatives may be the crustaceans.[25][26][27][28]
The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha,[29] or as several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (springtails)[30] or the Protura. There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere.[31]
Molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period 440 million years ago, coinciding with the appearance of vascular plants on land.[32]
The cladogram below follows the work of Bernhard Misof et al. (2014)[33] and shows the relationships between the extant orders of Hexapoda:
Hexapoda Elliplura Cercophora Insecta Dicondylia Pterygota Palaeoptera Neoptera Polyneoptera Haplocercata Notoptera Eukinolabia Dictyoptera Eumetabola Condylognatha Holometabola Aparaglossata Neuropteriformia Neuropterida Coleopterida Panorpida Amphiesmenoptera AntliophoraThe following cladogram is given by Kjer et al. (2016):[34]
HexapodaCollembola (springtails)
Protura (coneheads)
Diplura (two-pronged bristletails)
EctognathaArchaeognatha (jumping bristletails)
Zygentoma (silverfish)
Pterygota (winged insects)
An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago,[35][36] although some researchers oppose this view and suggest that the fossil may instead represent a decomposed crustacean or other non-insect.[37]
The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs (six legs). Most other arthropods have more than three pairs of legs. Most recent studies have recovered Hexapoda as a subgroup of Crustacea.
