Mesostigmata Dermanyssina mouthparts evolution: from predatorism to parasitism

A. Di Palma - Department of Agricultural, Environmental and Food Sciences, University of Foggia, Italy

47
©SKA

Dermanyssina represents the most species-rich, ecological diverse and geographically wide spread cohort of Gamasida. They are found worldwide and include free living predaceous or fungivorous soil inhabiting forms, as well as pollenivores and facultative or obligatory external or internal parasites of vertebrates and invertebrates.

©zookeys.pensoft.net

The gnathosoma is certainly one of the most distinctive features in mites and the adaptations of these mouthparts to different nutritional patterns are enormous and comparable to the ones observed in insects. Several hypotheses suggest that fluid feeding is a derived condition in mites; and observations among gamasid mites suggest that ingestion of particulate food could be the plesiomorphic condition.

The gnathosoma is composed basically of two main components: the cheliceral frame and the infracapitulum (=subcapitulum). The cheliceral frame is a membranous part where the chelicerae are inserted, in a movable way, by means of the “cheliceral sheaths”.

The infracapitulum is located ventrally to the chelicerae and is mainly composed of the palpcoxae that meet the tegulum dorsally located (terminology after Evans and Loots, 1975). The resulting gnathosomal tube is divided into a dorsal and a ventral part by the cervix (=subcheliceral plate), which connects the mesial walls of the palpcoxae. Dorsally, the chelicerae lie over the infracapitulum, which contains the mouth and the pharynx. The mouth is defined as the opening at the base of the labrum leading into the pharynx.

The region anterior to the mouth is called “pre-oral channel”. The labrum is an unpaired projection covering the pre-oral channel and the mouth. Hence the infracapitulum is a cone like projection with the pre-oral channel at the tip and the chelicerae dorsally located. The pre-oral channel dorsally sealed by the labrum and posteriorly connected to the pharynx and the esophagus, is responsible for sucking up fluids.

In free living species, the labrum and the lateral walls of the pre-oral channel are provided with numerous tiny ridges or processes that are considered to act as a sieve preventing the entrance of solid particles of food. Such system is reduced in species that live on blood or pre-orally liquefied food. The infracapitulum presents ventrally an infracapitular gutter with characteristic transverse denticulate ridges and a well developed structure: the tritosternum. At the tip of the infracapitulum there are paired projections: the laciniae and the horn-like corniculi plus a pair of salivary styli where the secretions of salivary glands discharge.

Plesiomorphically the chelicerae show a terminal chela and are composed of three articles: a basal (first) article where protractor and retractor muscles are inserted, a middle (second) article that ends with a fixed digit of the chela and a movable digit (third article) ventrally located to the fixed one. The ancestral function of the chelicerae was probably to grab and manipulate the food to the mouth, but capability of piercing, cutting and ripping the food developed rapidly.

Predatory Dermanyssina usually grab their prey by means of their chelicerae, hence the cheliceral digits cut an opening into the prey and the alternating action of the chelicerae and their chewing activity together with the secretion of the salivary glands, through the salivary styli, result in partial digestion of the prey.
Sclerotized corniculi and palps help in holding the prey during feeding activity. The fluid food flows between and around the chelicerae to reach the pre-oral channel bordered by the lips. However, the amount of fluid produced during feeding activity may exceed the amount that the mite can swallow thanks to the pharyngeal pump, hence the overflow reaches the posterior region of the gnathosoma where the bases of the first legs and the folded cuticle redirect it laterally and anteriorly to reach the tip of the infracapitulum passing through the infracapitular gutter and the tritosternum. Cheliceral morphology, shape and dentition have been correlated with the type of prey and even with the possibility, for predators, to switch to plant-feeding.

Obligate phytophagous are rare among Dermanyssina and almost exclusively on fungi, pollen or nectar. They show stout chelicerae with few teeth in the fixed digit and a concavity in the movable (Fam. Ameroseiidae). It is assumed that fungal hyphae and spores are chopped or punctured while held in the concavity.

The chelicerae of the facultative bloodsuckers still have some of the characteristic of the predators (powerful claws and well developed teeth) and feed mainly on a mixed diet: i.e. they take blood by gnawing dried clots formed at the sites of scratches on the host body.

Zooparasites in Dermanyssina show adaptations that are obvious in obligate blood feeders. The chelicerae appear to be the most adaptive in this respect and specialized for host penetration. They tend to be elongated, slender, more or less pointed without conspicuous teeth and involved in host piercing. Moreover, they might develop a concavity in the inner (paraxial) surface so that when placed together they form a channel (sometimes held together by a coupling device) while the terminal chelae (fixed digit) become more or less reduced. The functional morphology of such channel is still controversial: i.e. either for piercing or a tube for sucking up fluids, or both function at the same time. On the other hand, the strainer system in the pre-oral channel tend to be absent so as the number of denticles in the infracapitular gutter and the tritosternum may be vestigial or absent. Finally, the apical paired projections (laciniae) are poorly developed as well.
Species strongly adapted to parasitism show a vestigial fixed digit while the stylet like movable digit shows strong teeth and a pronounced capacity for tearing or piercing the host tegument to allow the feeding.

References are available on request

9th Symposium on AvCoV & AMPV – 4th Annual Meeting of COST Action FA1207