The mandibles of insects are often easy to see, especially in beetles and ants, but in true bugs and thrips the mandibles have been modified into piercing stylets. Similar modifications occur to the chelicerae of some mites.
The mouthparts of Collembola, Diplura, Symphyla, Protura, and some mites are difficult to see because they are enclosed within the head or gnathosomal capsule. On a cleared and slided specimen, the mouthparts can be seen to include a pair of opposed mandibles, sometimes with a molar grinding area, or stylets. Mites with a well developed gnathosomal capsule might seem not to have mouthparts. Maggots (larval flies, Insecta) have internal mouthparts, but they are often highly modified hooks.
Under their heads, centipedes have a pair of limbs modified as poison claws that clasp towards the midline of the body.
The true bugs (Insecta) are characterized by beak-like mouthparts. The beaks can be short and sharp as in many predatory bugs, more elongate as in plant-feeding forms such as aphids and whiteflies, or a long flexible tube as in scale insects.
Some springtails have short buccal cones. Snout mites have elongate capituli (=gnathosomas) that taper into a needle-like snout with a pair of small hook-like fixed digits at the tip.
Tardigrades have a complex internal pharyngial apparatus composed of a series of rods and struts. Stylets are also present, but these often fade or dissolve in mounts.
The first pair of limbs in arachnids are called chelicerae. Primitively these are pincer-like and composed of 3-segments: basal article, middle article (ending in a fixed digit) and the movable digit. In some groups the basal article is strongly reduced, modified or lost. In others, e.g., spiders, most whipscorpions, and many mites, the fixed digit is reduced and the movable digit is fang or stylet-like.
The fangs of spiders and amblypygids are distinctive modifications of the arachnid chelicerae and consist of a tubular basal segment (sometimes with teeth on its margin) and a distal fang. Some groups of mites have similar chelicerae.
Mandibles (and associated mouthparts) or chelicerae have been converted into needle-like stylets in a variety of plant-feeding and parasitic insects and mites.
Regression of the fixed digit is sometimes accompanied by hook-like or claw-like movable digits in a variety of groups of mites, mostly predators or parasites.
Functional mouthparts are absent in a few groups of insects and mites and from all hypopi, the dispersal stage of mites in the suborder Astigmata. Chelicerae are entirely absent and palps are represented by a pair of bristle-like processes.
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
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