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The maxillary and labial appendages of the ants (Formicidae) are reduced, and instead the mandibles are used for numerous functions that include defensive, manipulatory, predatory functions and for lifting and carrying objects. The mandibles of male stag beetles (family Lucanidae) are greatly enlarged and used in male combat. The leaffeeding scarab beetle Popillia japonica has an enlarged molar process and reduced incisor process on the mandible. The galea enditic lobe of the maxilla has become dentate and functions to cut leaves instead of the mandible whilst the mandible molar surface has evolved into a masticatory organ. In some leaf-mining lepidopteran larvae the molar process has evolved to become saw-like as opposed to a crushing/grinding tooth surface. Outward facing mandibles are present in the parasitic elephant louse Haematomyzus and the leaf mining saw fly Phyllotoma aceris (Tenthredinidae). The adult dung beetle Onthophagus feeds on soft, fluid portion of dung and no longer requires mandibles to chew and the mandibles of this species have evolved to function Two of the most diverse insect orders, the Hymenoptera and Coleoptera, possess biting/chewing mandibles.
in sifting and filter feeding. Other beetle species, for example the tiger beetles (Cicindelidae) that digest food extra-orally have lost the molar process (Snodgrass, 1950; Holldobler and Wilson, 1994; Hughes and Kaufman, 2000; Angelini and Kaufman, 2005; Grimaldi and Engel, 2005; Simonnet and Moczek, 2011).
The mouthparts of several insect orders have been modified even further from typical mandibulate mouthparts into piercing or sucking mouthparts. These include numerous members of the superorder Paraneoptera such as Hemiptera (a diverse order including aphids, cicadas, leafhoppers, planthoppers, and shield bugs), Thysanoptera (thrips) and Phthiraptera (lice), as well as the Lepidoptera (butterflies and moths), Siphonaptera (fleas) and numerous dipteran families (Grimaldi and Engel, 2005). The mandibles in these lineages have undergone more drastic modifications, or reduction or loss of the entire appendage itself. The mandibles are lost and the maxillae are highly reduced in cyclorrhaphous dipterans like Drosophila. Instead, the proboscis is derived from the labial appendage which forms the sponge-like labella.
Profound modifications have occurred in other lineages, for example, mandibles are modified in some blood-sucking dipterans (like mosquitos from the family Culicidae) to form accessory lancets that aid in puncturing skin. The stylet of Hemipterans such as Oncopeltus is derived from the mandibular and maxillary appendages and modified into a tube.
The primitive mandible has one dorsal attachment point called a condyle or articulation. The mandible is suspended like a pendant from this articulation. This pendant mandible with one articulation, known as a monocondylic mandible, is plesiomophic and is present in numerous crustaceans and archaeognathan hexapods (see fig.1.4B). The maxilla, is also a monocodylic appendage (fig. 1.4D). There are variations in mandibular structure, for example in the numbers of articulations or attachment points of the mandibles.
Fig.1.4. Examples of mandibular articulations and musculature compared to an insect maxilla and leg.
Figure is adapted from Snodgrass (1935, 1950). Condyles/articulations are labelled as blue dots. The axis of rotation is indicated by a dotted line between these two articulations. The telopodite is highlighted blue, the protopodite is highlighted in yellow. (A) Generalised protopodite musculature of an arthropod leg appendage. The dorsal premotor (A) and dorsal remotor (P) muscles of the leg are homologous to the anterior rotator (A) and the posterior rotator (P) muscles of the mandible. Muscles are attached to the dorsal body cuticle (the tergum). The ventral promotor (vpm) and ventral remotor (vrm) are homologous to the ventral muscles (V) of the mandible. These muscles are attached to the ventral body cuticle (the sternum). (B) Generalized pendant monocondylic mandible present in Archaeognatha and numerous crustacean taxa. The ventral adductor is the main muscle used to shut the mandible jaws in front of the mouth (Mth), the muscles are attached to each other with a ligament. The A and P muscles are attached to the dorsal head capsule. (C) A generalized decapod mandible with two articulations and a gnathal edge in line with the axis of rotation present in the majority of Malacostraca and Chilopoda.
(D) Generalized insect maxilla. The maxilla is monocondylic, with notable similarities of the A, P, and V muscles to both the leg and mandible. The lacinia is attached by a muscle to the stipes segment. (E) Generalized diplopod/symphylan mandible with an independently movable gnathal lobe (gnL) connected to the mandible base (mdB) by a muscle (I). (F) Generalized dicondylic mandible with perpendicular orientated gnathobase present in Isopoda, Amphipoda, Lepismatidae and Pterygota. The posterior rotator is greatly enlarged and has taken over the function of closing the mandible.
Some arthropods possess mandibles with two articulation points with the gnathal edge orientated in line with the axis of rotation (fig.1.4C) or perpendicular to the axis of rotation (fig.1.4F). This structural arrangement is particularly well adapted to the function of biting and chewing as the axis of rotation enables the gnathal edges of the mandible to open and close more forcefully and is present in the some crustaceans (Isopoda and Amphipoda). It is also present in the majority of insects.
These insects form a clade called the Dicondylia which includes the Ephemeroptera and Zygentoma (Engel and Grimaldi, 2004). The mandible of Dicondylia is doubly articulated with two attachment points (condyles) and a perpendicularly orientated gnathobase. These doubly articulated mandibles of some crustaceans and insects are convergent structures.
The mandible gnathal lobe in myriapods
The myriapod mandible has the gnathal edge present on either a flexible or movable gnathal lobe that is independently musculated (see fig.1.4E). This movable lobe is held against the mandible base. Chilopod (centipede) mandibles possess a flexible gnathal lobe, whilst the Diplopoda (millipedes) possess mandibles with a gnathal lobe that is clearly separated (see fig.1.4E, fig.1.7D,E and fig.1.9C). The diplopod mandible consists of two segments, a cardo and stipes (see fig.1.9C).
One of the most obvious indications that the mandible has evolved from the base of the leg is the presence of a palp (corresponding to the long articulated leg) on some crustacean mandibles (see fig.1.1F and fig.1.4B,C). The mandibles of ostracod crustaceans are the most leg-like, and have also lost the characteristic gnathal edge (Snodgrass, 1950; Boxshall, 2004; Richter and Kornicker, 2006). The primitive mandible would have possessed two palps, an exopodite derived palp and a telopodite derived palp as is present in Cambrian arthropods (see fig.1.7B,C). Loss of these two palps has occurred frequently throughout Mandibulata.
The two rami of biramous mandibles, which represents the ancestral condition, have been lost in almost all mandibulate lineages. Biramous mandibular appendages are present in crustacean nauplius larvae, and in ostracod mandibles. Mandibles with a single telopodite-derived palp are the most common form of mandible with a palp and are exclusively present in crustacean taxa such as branchiopods, cephalocarids and malacostracans (Snodgrass, 1950).
The telopodite derived palp has been lost independently from the mandibular appendage in several lineages of mandibulates (see fig.1.3D). The palp has been lost in all hexapods and myriapods. Loss of the palp therefore occurred in the stem lineage to each of these groups. Numerous crustacean taxa have also lost the telopodite derived palp such as isopod crustaceans (woodlice). The loss of the mandibular palp in terrestial taxa is a possible example of convergent evolution to a terrestial habitat adaptation.
Homology of the arthropod mandible
In spite of all the diversity of mandibular structures outlined above, the mandible of insects, crustaceans and myriapods is considered to be a homologous structure that had a single origin in the ancestor to the mandibulate arthropods.
Comparison of the musculature of the mandible to the base of a leg (the coxa) reveals similarities in structure that suggest that the mandible has evolved from a leg (see fig.1.4). According to Snodgrass, there are obvious homologies between different muscles of more primitive mandibles with those of typical trunk appendages (Snodgrass, 1950). In the coxa of generalized legs, there are two pairs of muscles attached to the coxa, a ventral pair and a dorsal pair (fig.1.4A). In the generalized primitive mandible, there are homologous muscles (shown in fig.1.4B). Snodgrass hypothesized that the mandibulate arthropods constituted a monophyletic group called the Mandibulata.
Manton hypothesized contra Snodgrass that the mandible of myriapods and hexapods was telognathic, that is to say, the gnathal edge was present on the telopodite (rather than protopodite) of the mandible. In place of Mandibulata, Manton constructed the clade Uniramia that included myriapods, hexapods and onychophorans (Manton, 1964; Manton, 1977). This view has been thoroughly disproved by evidence from molecular development that the mandible of hexapods and myriapods is in fact made from the base of the limb (gnathobasic).
The mandibular gnathal edge is a homologous structure
The defining character of the mandible that distinguishes it from other appendages is the gnathal edge (see fig.1.1A,B,D,E). The gnathal edge of the mandibular appendage of myriapods, crustaceans and hexapods is considered homologous by Edgecombe et al. (Edgecombe et al., 2003). This structure originated once in the ancestor to mandibulate arthropods and is evidence of the monophyly of this group (Snodgrass, 1950; Kraus, 2001; Edgecombe et al., 2003). The gnathal edge is often made of three parts, the incisor and molar processes and a small moveable dentate plate structure called the lacinia mobilis. The authors conclude from comparative morphology of mandibles across Mandibulata that the incisor and molar processes may be homologous. The incisor process is the distal-most structure of the gnathal edge and is dentate, made up of tooth-like structures. The hypothesized primitive structure of the molar process is made up of a series of rows of spines. In many pancrustacean mandibles, this basal configuration of rows of spines is covered with scaly transverse ridges, which is interpreted to be a derived state.
Morphological characters in support of Mandibulata
Mandibulata3 as a group was originally hypothesized on morphological grounds and there are numerous morphological characters in support of Mandibulata, including the structure of the mandibular gnathal edge, the particular mandibulate arrangement of segments in the gnathocephalon (see fig.1.1C), along with a whole suite of neurological and morphological characters (Snodgrass, 1938; Snodgrass, 1950;
Edgecombe et al., 2003; Edgecombe, 2010).
The original Mandibulata hypothesis included the Atelocerata plus Crustacea rather than Pancrustacea/Tetraconata plus Myriapoda.
1.4 Arthropod Phylogeny One method to determine the evolutionary history of a morphological character, such as the mandible, which displays both significant diversity and elements of similarity that could represent homology or convergent evolution, is to map developmental and morphological characters onto a robust phylogenetic tree. By evaluating the most parsimonious explanation of the distribution of character states, they can be described as either novel or ancestral characters (Akam, 2000; Telford and Budd, 2003; Jenner, 2006). In order to do this, it is necessary to have a robust phylogeny on which to place these characters.
Despite the attention given to the phylum and the years of research devoted to understanding their evolutionary history, the origin of the arthropods and the phylogenetic relationships between the different subphyla are still uncertain.
Articulata and Atelocerata: two early victims of the new molecular phylogeny.
Molecular phylogenies have overturned numerous phylogenetic hypotheses based on more traditional cladistic analyses of morphology in the last two decades.