Box 9.2. Non-insect hexapods (Collembola, Protura, and Diplura)
The Collembola, Protura, and Diplura have been united as the “Entognatha”, based on a similar mouthpart morphology in which mandibles and maxillae are enclosed in folds of the head, except when everted for feeding.
Although the entognathy of Diplura is thought not to be homologous with that of Collembola and Protura (section 7.2), it is convenient to treat these classes together here. All have indirect fertilization — males deposit sperm bundles or stalked spermatophores, which are picked up from the substrate by unattended females. For phylogenetic considerations concerning these three classes, see sections 7.2 and 7.3.1.
The proturans are non-insect hexapods, with over 600 species in eight families. They are small (<2 mm long) to very small, delicate, elongate, pale to white, with a fusiform body and conically shaped head. The thorax is poorly differentiated from the abdomen. Eyes and antennae are lacking, and the mouthparts are entognathous, consisting of slender mandibles and maxillae, slightly protruding from a pleural fold cavity; maxillary and labial palps are present. The thorax is weakly developed, and bears legs each comprising five segments; the anterior legs are held forward (as shown here for Acerentulus, after Nosek 1973), fulfilling an antennal sensory function. The adult abdomen is 12-segmented with the gonopore between segments 11 and 12, and a terminal anus; cerci are absent. Immature development is anamorphic (with segments added posteriorly during development). Proturans are cryptic, found exclusively in soil, moss, and leaf litter. Their biology is little known, but some species are known to feed on mycorrhizal fungi.
The springtails are treated as non-insect hexapods, but intriguing evidence suggests an alternative, independent origin from Crustacea (see section 7.3). There are about 9000 described species in some 27 families, but the true species diversity may be much higher. Small (usually 2–3 mm, but up to 12 mm) and soft-bodied, their body varies in shape from globular to elongate (as illustrated here for Isotoma and Sminthurinus, after Fjellberg 1980), and is pale or often characteristically pigmented grey, blue, or black. The eyes and/or ocelli are often poorly developed or absent; the antennae have four to six segments. Behind the antennae usually there is a pair of postantennal organs, which are specialized sensory structures (believed by some to be the remnant apex of the second antenna of crustaceans). The entognathous mouthparts comprise elongate maxillae and mandibles enclosed by pleural folds of the head; maxillary and labial palps are absent. The legs each comprise four segments. The six-segmented abdomen has a sucker-like ventral tube (the collophore), a retaining hook (the retinaculum), and a furca (sometimes called furcula; forked jumping organ, usually three-segmented) on segments 1, 3, and 4, respectively, with the gonopore on segment 5 and the anus on segment 6; cerci are absent. The ventral tube is the main site of water and salt exchange and thus is important to fluid balance, but also can be used as an adhesive organ. The springing organ or furca, formed by fusion of a pair of appendages, is longer in surface-dwelling species than those living within the soil. In general, jump length is correlated with furca length, and some species can spring up to 10 cm. Amongst hexapods, collembolan eggs uniquely are microlecithal (lacking large yolk reserves) and holoblastic (with complete cleavage). The immature instars are similar to the adults, developing epimorphically (with a constant segment number); maturity is attained after five molts, but molting continues for life. Springtails are most abundant in moist soil and litter, where they are major consumers of decaying vegetation, but also they occur in caves, in fungi, as commensals with ants and termites, on still water surfaces, and in the intertidal zone. Most species feed on fungal hyphae or dead plant material, some species eat other small invertebrates, and only a very few species are injurious to living plants. For example, the “lucerne flea” Sminthurus viridis (Sminthuridae) damages the tissues of crops such as lucerne and clover and can cause economic injury. Springtails can reach extremely high densities (e.g. 10,000–100,000 individuals m—2) and are ecologically important in adding nutrients to the soil via their feces and in facilitating decomposition processes, for example by stimulating and inhibiting the activities of different microorganisms.
The diplurans are non-insect hexapods, with some 1000 species in eight or nine families. They are small to medium sized (2–5 mm, exceptionally up to 50 mm), mostly unpigmented, and weakly sclerotized. They lack eyes, and their antennae are long, moniliform, and multi- segmented. The mouthparts are entognathous, and the mandibles and maxillae are well developed, with their tips visible protruding from the pleural fold cavity; the maxillary and labial palps are reduced. The thorax is little differentiated from the abdomen, and bears legs each comprising five segments. The abdomen is 10-segmented, with some segments having small styles and protrusible vesicles; the gonopore is between segments 8 and 9, and the anus is terminal; the cerci are filiform (as illustrated here for Campodea, after Lubbock 1873) to forceps-like (as in Parajapyx shown here, after Womersley 1939). Development of the immature forms is epimorphic, with molting continuing through life. Some species are gregarious, and females of certain species tend the eggs and young. Diplurans are generally omnivorous, some feed on live and decayed vegetation, and japygid diplurans are predators.