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

Abstract

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

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.

Key words: Acari, gnathosoma, mouthparts, feeding mechanisms, sensory structures, digitus mobilis, digitus fixus, morphological adaptations, host piercing, salivary injection, basal sclerite, lever, motivator, stylophore.