Morphological adaptations of mite chelicerae to different trophic activities (Acari)

The authors briefly review the current knowledge on the structural organization and functional significance of the chelicerae and their sensory structures among mites of economical interest in agriculture that have been recently investigated for their gnathosoma. The chelicerae of the polyphagous predatory mite Typhlodromus exhilaratus Ragusa (Gamasida, Phytoseiidae) have a rather unspecialized organization and are of chelate type. They function mainly in grasping the prey and other food, while the other mouthparts are adapted to prey wounding, salivary injection and juice suction. In the Honey Bee parasite, Varroa destructor Anderson & Trueman (Gamasida, Varroidae), the chelicerae have almost completely lost the fixed digit while the movable digit is thinner and elongated. The chelicerae participate directly, along with the other mouthparts, only in the perforation of the Honey Bee integument. In the plant-feeder, Penthaleus major (Dugès) (Actinedida, Penthaleidae), the movable digit is composed of a long and stout stylet-like structure, and a peculiar basal sclerite which activates the digit movements. During cheliceral protrusion, the stylets may work jointly or independently from each other in piercing the plant surface and causing deep and large wounds. The protruded stylets may justapose each other; they may delimit an intercheliceral channel and may be involved in salivary injection. The fixed digit is a large somewhat soft structure which covers dorsally the pre-oral and supralabral chamber, and is passively involved in juice sieving and sucking. The movable digit of the fungivorous Tarsonemus nodosus Schaarschmidt (Actinedida, Tarsonemidae) is needle-like and articulated with a transverse lever to the second cheliceral article. The basal cheliceral articles are fused together forming a stylophore. The fixed digits partly envelop the movable digits and they are consolidated with the subcapitulum. The stylar morphology suggests an independent piercing action of the movable digits. The phytophagous Two-Spotted Spider mite, Tetranychus urticae Koch (Actinedida, Tetranychidae), and False Red Spider mite, Cenopalpus pulcher (Canestrini & Fanzago) (Actinedida, Tenuipalpidae), have extensively protractable styliform chelicerae. The movable digits are very elongated and stylet-like, originating proximally from the stylophore (fused first cheliceral articles); the fixed digits are partially fused to each other. During cheliceral protrusion, the stylets are forced to be interlocked to each other to delimit an intercheliceral canal which is directly connected to the salivary duct. At the same time, the fused fixed digits are involved in salivary pump functioning. So, the chelicerae play a double role in piercing the plant surface and discharging saliva into the host tissue. In the tiny phytophagous mites of the superfamily Eriophyoidea (Actinedida), the gnathosoma has a strongly derived organization. Fixed and movable digits are both styliform, laminar shaped, and included in a strict frame of nine stylets of different origin which function together in host –– 126 –– INTRODUCTION Up to the present, the Acari have become of increasing economic importance to human activities. Owing to their small size and ecological needs, they can live in diverse microenvironments. They display a great variety of trophic behaviours (EVANS, 1992), utilizing every kind of substrate and being involved in the cycle of organic matter. They have medical and veterinary as well as agricultural relevance for the direct and indirect (pathogen transmission) injuries they cause to man, domestic animals, edible fungi, stored products and plants. On the other hand, some species are useful for the positive roles they play against injurious insects, mites and nematodes. Therefore, they find considerable economic interest so as to be protected and actually applied in integrated pest management and biological control programs. As a rule, structural and functional adaptations of the gnathosoma have evolved on the basis of mite feeding mechanisms and they can help in understanding mite trophic relationships. Of course, the injuriousness and symptoms resulting from feeding depend on the mouthpart frame-work, which surely needs a morphological approach to be understood in a thorough way. In particular, the chelicerae have undergone the most outstanding modifications in being chelate in typically predatory mites and, passing through the parasitic and fungivorous habits, they assume finally the specialized and derived stylet-like organization of the plant-feeding mites (KRANTZ & LINDQUIST, 1979; LINDQUIST, 1998; ALBERTI & COONS, 1999; RAGUSA & TSOLAKIS, 2000). The present paper gives a brief account of the fine morphology of some acarine chelicerae, including remarks on their sensory structures, and updating and deepening previous short notes (NUZZACI, 1985 and 1994). In particular, it deals with the movable and fixed cheliceral digits, showing the piercing, salivary injection and juice suction. The short sliding movements of the cheliceral shafts are activated by the unpaired motivator. The chelicerae seem to be mainly involved in deeping the mouthparts into the wounds and partially in salivary injection. In each of the investigated species, chelicerae are provided with putative chemoreceptive sensilla, in addition to other mechanoreceptors, which seem to be strictly involved in food perception and acceptance. As yet, however, an alternative perceptive function cannot be excluded.