chapter 2 structure and systematics

Niels P. Kristensen† and Jens Böcher

Discussions about the composition and interrelationships lineages. The most anterior tagma bears the mouth open- of the principal arthropod taxa have been as vigorous in ing and a series of adjacent mouth appendages which the 21st century as ever in zoological history, but while because of their role in food uptake are more or less pro- these subjects are a significant scientific challenge in their foundly modified relative to the following locomotory own right, they are of little consequence for the purpose limbs. This anterior tagma similarly bears the visual of an identification manual. Suffice it here to note that organs, and in myriapods and hexapods moreover a pair while there has long been near-universal agreement that of antennae, on which numerous sensory organs (chemo- the Arachnida are a ‘natural group’, in as much as they are receptors in particular) are located. The opening of the considered to be monophyletic in the strict modern sense gut is morphologically located on the hind end of the ani- (i.e., the group comprises all known descendants of its mal, but can topographically appear shifted more or less members’ last common ancestor), the monophyly of the extensively forward. The location of the genital opening is Myriapoda has been continuously debated, and the also diagnostic of major arthropod lineages. monophyly of the Hexapoda has recently become so too. Small protuberances of the exoskeleton come in many A comprehensive account of arthropod structure, func- forms and may contribute significantly to the appearance tion and phylogeny is evidently outside the scope of the of more or less extensive body regions. Of special note are present work; reference must be made to a text of inverte- the ʻsensillaʼ, comprising, together with the visual organs, brate zoology such as Ruppert et al. (2004) or Brusca & the principal receptors of external stimuli. A basic sensil- Brusca (2003), as well as to the recent information-rich lum type is the socketed hair or ʻsetaʼ, which arises from a treatise edited by Minelli et al. (2013). flexible socket. Setae may be innervated and respond to mechanical or chemical stimuli, but chemoreceptors can also have other shapes (cones, plates etc.), and they may Arthropod body design be elevated or sunk in depressions; their surfaces have single (larger) or numerous (minute) pores permitting Few zoologists will contest the notion that the extant arthro- access for molecules of external substances. Additional pod groups descended from an ancestor with many overall sensillum types exist, but it is mostly chemoreceptors on similar segments, most if not all of which bore one pair of seg- antennae and (particularly) the larger mechanoreceptive mented limbs. The arthropod body wall comprises a single cell setae on certain trunk and limb regions that have taxo- layer (ʻepidermisʼ) which secretes an outer covering cuticle nomic importance (with respect to number, size, position mainly composed of chitin in a protein matrix. It is the shape etc.) in many groups. Broad and flat (ʻlamellarʼ) setae are and pigmentation of the cuticle that primarily determines the referred to as ʻscalesʼ. They are particularly well known external appearance of an arthropod. The cuticle forms an from the wings of moths and butterflies, but also form outer skeleton (ʻexoskeletonʼ) on which most of the animalʼs dense covers on some springtails, and occur in scattered muscles are attached (other muscles surround inner organs members of other groups as well (e.g., along wing veins of such as the gut and genital ducts). Most of the muscle attach- mosquitoes). Most scales are not innervated and this also ments are located on little-flexible parts of the body wall applies to some ʻordinaryʼ (hair-like) setae, particularly in (ʻscleritesʼ), where the cuticle is hardened and usually dark- cases where they form dense coverings. ened; the intervening flexible (and often unpigmented) body The ground plan configuration of the paired segmental wall areas are referred to as ‘membranesʼ. A generalized arthro- appendages has been much debated (for a recent review pod segment is equipped with a dorsal and ventral sclerite see Boxhall 2013). A subunit in an appendage is referred (ʻtergumʼ and ʻsternumʼ, respectively) with the segmental to as a ʻpodomere1ʼ, or as a ʻsegmentʼ (like the units in limbs arising from the lateral soft-bodied ʻpleuralʼ areas, which may bear sclerotizations between the limbs and the tergum. 1 The termination -mere (from Greek méros, part, portion) is also The patterns of ʻtagmosisʼ (or ʻtagmatizationʼ), i.e., encountered in terms such as 'antennomeres' (=subunits of the the integration of adjacent segments into functional antenna), 'flagellomeres' (=subunits of the antennal flagellum of units, ʻtagmataʼ are diagnostic of the principal arthropod Insecta s.str.) and 'tarsomeres' (=subunits of the tarsus of the leg). 4 2. Arthropod structure and systematics – Kristensen and Böcher the trunk); it is typically moved by antagonistic muscles In most the prosoma is covered by a solid arising in the podomere proximad from it. The number dorsal carapace, while its ventral surface has one or more of immediately identifiable podomeres varies across the sternal plates or is covered by the coxae of the append- principal arthropod lineages. ages. Appendages are borne on the prosoma (and its For obvious reasons most internal organ systems play homologues). They comprise an anterior pair called little role for practical identification of arachnids and ʻcheliceraeʼ followed by the ʻpedipalpsʼ (or just ‘palps’) and hexapods. Suffice it to note that the external openings the locomotory legs. Most arachnids are carnivores that (ʻspiraclesʼ) of the invaginated, branching tracheal (respi- capture and kill the prey (usually small ) with ratory) systems of many taxa are often easily discernible the chelicerae. The pedipalps perform a variety of func- and their location, structure and/or the modification of tions related to food uptake and in some taxa (male spi- adjacent body regions is occasionally taxonomically infor- ders being an important example) to sperm transfer. mative. Similarly, the regions surrounding (or immedi- Seven podomeres are usually recognizable in ately inside) the openings of the genital systems in both locomotory legs; they are here referred to (from base to sexes are often characteristically modified. tip) as coxa, trochanter, femur, patella/genu (mite work- ers’ term), tibia, tarsus and pretarsus/metatarsus. While compound eyes as found in (and crusta- Systematization ceans) are entirely absent from arachnids, simple eyes may occur in variable numbers and arrangements. Mainstream zoological systematics endeavours to classify Invaginations to cuticle-lined respiratory cavities (book animals according to their phylogeny, i.e., into a hierarchy lungs and/or tracheae) are variably located. Some reflecting their relationships in terms of relative recency lack special respiratory organs, and gas exchange then of common ancestry. The ʻmaturityʼ of contemporary takes place through the general body wall. The genital ori- taxonomy varies considerably across the arthropods, and fice of both sexes is found on ventral side of the second it is practically certain that some of the current concepts abdominal segment except in some mites. Indirect sperm of higher taxa (e.g., families) adopted in the present work transmission is characteristic of most arachnids and in will eventually prove to refer to entities that are not mono- many cases the male produces a spermatophore; fre- phyletic in the strict sense, i.e., not including all descen- quently a more or less complex courtship behaviour pre- dants of their membersʼ last common ancestor. While cedes copulation. reference is here made, conservatively, to ʻordersʼ of Arachnid juveniles are overall similar to adults (leg arthropods, it must be emphasized that these like all other numbers in 1st instar mites excepted), but characters nec- supraspecific taxa are not in any sense ʻequivalentʼ evolu- essary for species identification are often undeveloped tionary entities, and no formal ranks are here ascribed to before sexual maturation. other higher taxa, which are above family-group rank and Currently about ten ‘orders’ are recognized in the hence not covered by naming conventions required in the Arachnida; just three of these are represented in International Code of Zoological Nomenclature. Greenland: Araneae (18.1), (18.2) and (18.3).

Arachnida The myriapods Arachnids are immediately distinguishable from other extant terrestrial/limnic arthropods by the absence of The myriapods are an assemblage (perhaps indeed antennae and by presence of four pairs of locomotory monophyletic) of arthropods which all have an elongate limbs (early-instar mite juveniles excepted, they have just trunk comprising numerous leg-bearing segments, hence three). Tagmosis is also characteristic, with the body they are ʻmany-leggedʼ, as the scientific name implies. typically divided into an anterior prosoma and a posterior Like the and hexapods they are ʻmandibulate abdomen, alternatively called opisthosoma (Fig. 18.1–1).2 arthropodsʼ in as much as their most anterior paired mouth Again the mites are aberrant in lacking this kind of body appendages are biting/chewing mandibles. They moreover division; their tagmosis patterns come in different forms resemble hexapods in having a single pair of antennae (Figs 18.3.1–1&2). and the segment between the latter and the mandibles being devoid of appendages. Most myriapod visual 2 Emboldened numerals refer to other chapters. organs are aggregations of simple eyes. The segmental