ARTHROPODA Subphylum Hexapoda Protura, Springtails, Diplura, and Insects
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NINE Phylum ARTHROPODA SUBPHYLUM HEXAPODA Protura, springtails, Diplura, and insects ROD P. MACFARLANE, PETER A. MADDISON, IAN G. ANDREW, JOCELYN A. BERRY, PETER M. JOHNS, ROBERT J. B. HOARE, MARIE-CLAUDE LARIVIÈRE, PENELOPE GREENSLADE, ROSA C. HENDERSON, COURTenaY N. SMITHERS, RicarDO L. PALMA, JOHN B. WARD, ROBERT L. C. PILGRIM, DaVID R. TOWNS, IAN McLELLAN, DAVID A. J. TEULON, TERRY R. HITCHINGS, VICTOR F. EASTOP, NICHOLAS A. MARTIN, MURRAY J. FLETCHER, MARLON A. W. STUFKENS, PAMELA J. DALE, Daniel BURCKHARDT, THOMAS R. BUCKLEY, STEVEN A. TREWICK defining feature of the Hexapoda, as the name suggests, is six legs. Also, the body comprises a head, thorax, and abdomen. The number A of abdominal segments varies, however; there are only six in the Collembola (springtails), 9–12 in the Protura, and 10 in the Diplura, whereas in all other hexapods there are strictly 11. Insects are now regarded as comprising only those hexapods with 11 abdominal segments. Whereas crustaceans are the dominant group of arthropods in the sea, hexapods prevail on land, in numbers and biomass. Altogether, the Hexapoda constitutes the most diverse group of animals – the estimated number of described species worldwide is just over 900,000, with the beetles (order Coleoptera) comprising more than a third of these. Today, the Hexapoda is considered to contain four classes – the Insecta, and the Protura, Collembola, and Diplura. The latter three classes were formerly allied with the insect orders Archaeognatha (jumping bristletails) and Thysanura (silverfish) as the insect subclass Apterygota (‘wingless’). The Apterygota is now regarded as an artificial assemblage (Bitsch & Bitsch 2000). Though fewer in numbers of species than the beetles, Collembola (springtails) are perhaps the most abundant arthropods on earth, especially in soil litter. Found in the same environment, Protura are very small pale arthropods that are rarely encountered; Diplura include a few Celatoblatta vulgaris. families of larger pale arthropods. The vast majority of hexapod species are Alastair Robertson and Maria Minor, Massey University insects, classified among 26–30 orders (some gene-sequencing studies suggest the amalgamation of some orders). The earliest known hexapods in the fossil record are a Collembolon and an insect, both terrestrial, from the Early Devonian Rhynie Chert in Scotland (Engel & Grimaldi 2004). Both have relatively advanced features, showing that insects must have originated earlier, in the Silurian. �Traditionally, springtails, Protura, and Diplura were united in a group called Entognatha, so named because members of these classes all have the base of the mouthparts internalised, so that the 233 NEW ZEALAND INVENTORY OF BIODIVERSITY Summary of New Zealand hexapod taxonomic diversity Taxon Described Known Estimated Adventive Endemic Endemic living undescribed/ unknown species+ species+ genera species+ unrecorded species subspecies subspecies named subspecies species+ +new subspecies Protura 17 1 0–10 2 10 0 Collembola 346+34 0 650 44+3 266+29 20 Diplura 8+1 2 0–5 4? 6 0 Archaeognatha 2 0 0–2 0 2 0 Thysanura 3 8 >8 2 8 0+2 Ephemeroptera 48 3 10 0 51 20 Odonata 17 0 0 3 10 3 Plecoptera 99 21 20 0 120 19 Blattodea 35 9 2–5 13 31 5+1 Isoptera 9 0 0 5 3 0 Mantodea 2 0 0 1 1 0 Dermaptera 21 1 0–3 9? 13? 0 Orthoptera 116 69 <5 7? 173 27+1 Phasmatodea 22 7 7 1? 29 10 Hemiptera 1,079+15 78 235–440 280? 826?+14 119+1 Thysanoptera 122 1 >10 55 65? 11 Psocoptera 62 7 30 5 31 3 Phthiraptera 340+8 10 20* 90 29 1 Megaloptera 1 0 0 0 1 0 Neuroptera 14 0 0 7 7 0 Coleoptera 5,062+29 417–420+1 3,000 418+29 5,002–5,005+30 533+2 Mecoptera 1 0 0 0 1 0 Siphonaptera 28+6 0 0–5 11 17 0 Diptera 2,483+9 736–785 130–1,640 195 3,030–3,026+9 192+17 Strepsiptera 2 1 2? 0 4 0 Hymenoptera 721 820–934 >500 259? 740–742 55+21 Trichoptera 228 26 10–50 0 241 33+2 Lepidoptera 1,686+8 14 >100 139+1 1,389+9 140 Totals 12,573+110 2,231–2,397 4,735–6,525 ~1,585+33 12,105–12,091+91 1,188+48 * Not including possible species on vagrant birds mandible and maxilla are partly contained within the head capsule. In addition to this similarity in mouth structure, these three classes share reduced Mal pighian tubules. The precise relationships of these groups to one another and to Insecta are still uncertain, however; analyses of morphological and developmental characters and gene sequencing give conflicting results (Cook et al. 2001; Giribet et al. 2004; Regier et al. 2005; Carapelli et al. 2007; Dell’Ampio et al. 2008; Timmermans et al. 2008). The majority of studies support the monophyly of Hexapoda. All hexapods undergo several moults as they develop and grow in size, necessitated by the fact that the exoskeleton, particularly in insects, is inelastic and cannot expand. The newly moulted insect has a soft, flexible, lightly pig- mented cuticle; it swallows air or water and so increases its volume before the cuticle hardens and darkens and increases in thickness. Life-cycles vary. In some groups the young are called nymphs, which are similar in form to the adult except that the wings are not developed until the adult stage. This is called incomplete metamorphosis and insects showing this are termed hemi- metab olous. Representative orders include Ephemeroptera (mayflies), Odonata (damselflies and dragonflies), Plecoptera (stoneflies), Orthoptera (crickets and kin), Dermaptera (earwigs), Hemiptera (true bugs, cicadas, and kin), and Thysan- Dorsal and ventral views of Amphientulus optera (thrips) among others. Holometabolous insects pass through a complex zelandicus. metamorphosis always accompanied by a pupal stage. The wings develop R. Nielsen, from Tuxen 1986 internally and the larvae are usually specialised. Representative orders include 234 PHYLUM ARTHROPODA HEXAPODA the Coleoptera (beetles), Hymenoptera (ants, wasps, and kin), Diptera (true flies), Trichoptera (caddis), and Lepidoptera (moths and butterflies) among others. Living hexapods range in size from tiny hymenopteran parasites less than a fifth of a millimetre to a slender 56 cm-long stick insect that lives on the island of Borneo. While the overall largest insects today are Goliath beetles, the heaviest documented species is the Little Barrier giant weta (Deinacrida heteracantha), which used to live on the northern North Island mainland; one specimen weighed more than 70 grams. Although many hexapods are major pests, parasitising humans and livestock, causing damage to crops and stored products, or transmitting diseases, others are profoundly beneficial, as pollinators of economic crops or sources of products ranging from honey to silk. Scavenging hexapods help recycle dead animals and fallen trees; in fact insects of soil litter are responsible for much of the process by which topsoil is made. About 1200 species are used as human food. Geographically, hexapods range from polar zones to the equator and can be found even in hot springs and deserts. Not only soils, but fresh waters support their own extensive insect faunas, and a number of species are adapted to life in the sea shore and, as larvae, in the shallow subtidal. There is not a single species of seed plant that does not provide food for one or more species of insect. Hexapods are so ubiquitous and important that our terrestrial planetary ecosystems could not survive without them. Class Protura Sometimes referred to as coneheads, Protura are possibly the simplest of all living hexapods. The name means ‘first tail’, first in this context implying original, alluding to the primitive form of the abdomen lacking specialised structures at the rear end. These small white to transparent creatures, only 0.6–1.5 millimetres long in New Zealand, are quite distinct because they lack antennae, eyes, and wings. In consequence of lacking antennae, the front legs, which are somewhat hairy, are raised in front of the body to act as sensory detectors. The abdomen is tubular with up to 12 segments and the legs are simple with five segments. Protura are unique among hexapods in that eggs hatch into larvae with only a few abdominal segments, with the number increasing with subsequent moults to the full adult complement. Proturan sperm is unique too, differing from any other hexapod sperm and also very different in the two proturan orders. Protura live only in humid places, mainly in acid soils, sometimes in rotten wood, as part of the community of decomposers that help break down and recycle organic nutrients. They are frequently associated with leaf litter and moss under trees where they may feed on fungi. The first Protura were not discovered until 1907, in NewY ork State. About 500 species have been named so far, arranged in two orders – Eosentomoida and Acerentomoida – segregated not only on sperm characters but the presence or absence of spiracular openings to a tracheal respiratory system. Tuxen (1986) described seven new species out of the 18 species currently known from New Zealand. Eosentomon is the most diverse genus, with seven species. Protura cannot disperse widely owing to their vulnerability to desic- cation and saline water and their habitat restriction to soil or rotting wood. It is therefore not surprising that ~56% of the species are endemic and 30% are indigenous with Gondwanan distributions. Two species are accidental introductions from Europe. Within New Zealand, distributional records are very limited for 14 of the species, with only one to six locality records for each. Collection and identification involve extraction of specimens from soil or logs and careful preparation for mounting on microscope slides.