Structure of the cuticle of some ptyctimine oribatids (Acari: Oribatida)

The cuticle of Rhysotritia duplicita (Euphtiracaroidea: Euphtiracaridae), Phthiracarus sp., Steganacarus magnus, S. striculus and Tropacarus carinatus (all Phthiracaroidea: Phthiracaridae) was studied by light and transmission electron microscopy. A combination of light and electron microscopy were used to precisely characterize and classify the cuticular layers in oribatid mites and show how they differe in mites and other arthropods. In Rhysotritia duplicata the cuticle of the aspis (upper shield of the prosoma) differs significantly from that of the opisthosoma. Moreover, prosomal cuticle in this species is different from that of all others species studied. There were no structural differences in the cuticle of the prosoma and opisthosoma of species of the superfamily Phthiracaroidea (Phthiracarus, Steganacarus and Tropacarus) in spite of the differences in external apparence. Moreover, in Tropacarus, the rough sculpturing of the cuticle results from the very rough exocuticle, which is unlike the thick smooth cerotegument of the cuticle in Steganacarus.

Several integumental layers can be distinguished under the light microscope when Masson's triple stain is used (Smrž, 1989(Smrž, , 1992(Smrž, , 1994)): Cerotegument -rough lipoid (Krantz, 1978) layer on the cuticular surface, which has frequently incorporated in it material from the environment (mineral or litter particles).It can be separated or removed from the cuticle, e.g., by dissolving it in lactic acid or simple abrasion.Some types of cerotegument are destroyed during the embedding process for histological sectioning.Cerotegument is not stained by Masson's triple stain.
Epicuticle -thin uppermost cuticular layer, hyaline, translucent, without any internal division or structure when viewed under a light microscope.It is not stained by Masson's triple stain, but easily to distinguished from the other layers.
The underlying layer can be homogeneous or divided into several sublayers.These can be termed exocuticle and endocuticle following the usual arthropod cuticular terminology or, together, as procuticle.But some mites exhibit several types of "exocuticle" or "endocuticle", which cannot be homologized with the exocuticle or endocuticle of other arthropods.This is a common feature in mites.This phenomenon is discussed by Alberti (Alberti et al., 1981;Alberti & Coons, 1999).He proposed the more general term "layers of procuticle" based on the very rich and diversified scale of types of cuticles studied.The structural diversity is better described by this terminology.In this paper, the naming of the layers and sublayers is sensu Alberti (Alberti et al., 1981;Alberti & Coons, 1999), with similarity to the conventional, arthropodological terminology indicated in brackets for comparison.The main procuticular sublayers are those previously observed in oribatids using light microscopy after staining with Masson's triple stain (Smrž, 1989(Smrž, , 1992(Smrž, , 1994(Smrž, , 1995)): Exocuticle -amber-coloured layer, not stained by Masson's triple stain.It has no internal structure or lamellae or striae perpendicular to the cuticle surface.
Endocuticle -without any internal structure, very conspicuous carmine colour when stained with Masson's triple stain.
The flexible cuticle between body parts and leg segments is of endocuticular nature and continuous with that of the neighbouring hard cuticular parts.It stains carmine red.
As mentioned below, there are several other types of layers.
Of course, TEM reveals more details.Such observations support the structures described above, although the interpretation of the striation or lamellar structures differs.Moreover, the cerotegument is better preserved in TEM than in histological preparations.The layers with different electron densities, however, correspond with those revealed by Masson staining.Differences between these two methods may also result from the different processes of specimen preparation (e.g.fixative used).As mentioned above, these studies supported the more general terminology -procuticle sublayers.
The ptyctimine oribatids [the cohort Ptyctimina sensu Krantz (1978)] have a unique body division.Their prosoma is capable of ptychoidy, i.e. it is able to close on its opisthosoma like the blade of a knife [therefore "jackknife" mites: Krantz (1978)].The dorsal shield of the prosoma -aspis -protects the appendages of the prosoma (Fig. 1).Zelenková (1986) records differences in the structure of cuticle of prosoma and opisthosoma of Rhysotritia duplicata, as did Štambergová (2000).
I use the terms exocuticle 1, 2 or 3 and endocuticle 1 for the procuticular layers (exo-and endocuticle).These terms denote procuticular layers similar, but different from the traditional exocuticle and endocuticle.They do not describe the sequences of layers in the procuticle.
This study compares cuticular structure and ultrastructure of several members of two superfamilies of ptyctimine oribatids -Euphthiracaroidea and Phthiracaroidea.
Specimens were fixed for TEM in cacodylate-buffered glutaraldehyde (4%), postfixed in 1% osmium tetroxide, embedded in Spurr medium and sectioned using an Ultracut ultramicrotome (Reichert).Sections were stained in lead citrate and uranyl acetate and observed under a Philips EM 300 transmission electron microscope.

Steganacarus striculus (Figs 8-9)
Both body parts are covered by the same type of cuticle consisting of a conspicuous epicuticle and procuticle composed of an amber coloured striated ("exocuticle 3") and thin red coloured ("endocuticle") sublayers (Figs 8,9).The cuticular structure of the body and legs resembles that of previous species.The cuticle has superficial small pits covered by a rough cerotegument, which is easily separated from the cuticle surface (Fig. 11).Because of the cerotegument the rough nature of the body surface is more conspicuous (Fig. 10).TEM revealed very fine lamellae running parallel to the surface and allochthonous particles, of inorganicand organic origin in the cerotegument (Fig. 12).

Phthiracarus sp.
The same type of cuticle covers both part sof the body and resembles that of S. striculus.It consists of a conspicuous epicuticle and procuticle composed of amber coloured striated "exocuticle 3" and a thin red coloured "endocuticle" sublayers.

Tropacarus carinatus (Figs 13-14)
The composition of the prosomal and opisthosomal cuticle is similar to that of the previous species, but the upper procuticular sublayer ("exocuticle 3") is conspicuously thick, the underlying one ("endocuticle") relatively very thin.Moreover, "exocuticle 3" is remarkably shaped into high ridges, rather "waves", on its upper surface.They cover the whole body (Fig. 14) forming the characteristic pits on the body surface (Fig. 13).

DISCUSSION
As mentioned above, the terminology of Alberti (Alberti et al., 1981, Alberti & Coons, 1999) is suitable for the describing variability in oribatid (maybe, all mites) cuticle.On the other hand, some features (striation, staining in LM, lamellae or lamellar parabolic fibres in TEM) appear to be characteristic of some layers.The exocuticle sublayers are very diverse and better differentiated by the terminology "exocuticle 1, 2, 3".Therefore, both types of terminology are used in this paper for orientation and comparison.The traditional, strict division of cuticle into exo-and endocuticle (cf.insect cuticle: Imms, 1973) is insufficient.There are more sublayers of procuticle, which are often very similar to each other (cf.Evans, 1992;Alberti & Coons 1999).
The cuticle of phthiracaroid mites has the same structure on both body parts, although their surfaces differ in intact mites: a) nearly smooth in Phthiracarus sp. and Steganacarus striculus b) rough in Steganacarus magnus and Tropacarus carinatus.
The internal structure, however, seems to be similar in all studied species.In LM, the perpendicular striation is clear.The parallel lamellae, however, are conspicuous only in TEM.On the other hand, there are some differences in the role of cerotegument and cuticle in the sculpturing of the body surface in the latter group: the cerotegument gives rise to the rough superficial sculpture in Steganacarus magnus, whereas in Tropacarus carinatus it is the exocuticle.
The leg cuticle of studied species has of the same structure, probably because it has the same major functioninsertion of muscles.In unfavourable conditions, ptyctimine oribatids, can retract thein legs under thein aspis.Hence, protective modifications of cuticle or cerotegument appear to be redundant.An enormous number of free cells was prezent in the legs (oenocytes sensu Smrž, 1995;cf. Romer & Gnatzy, 1981) of all species studied.
clearly visible in LM, and the latter in TEM.These differences may have resulted from the different wals in which the specimen were prapared.TEM revealed the very fine details necessary for accurate descriptions, but Masson's triple stain distinguished the main characteristics and useful details after preparation of the specimens for examination under high magnification followed by image analysis.Therefore, their combination provides a useful method for the clasification of cuticular structures.