Entomological Review, Vol. 81, No. 9, 2001, pp. 1049-1058. Translated from Entomologicheskoe Obozrenie, Vol. 80, No. 1, 2001, pp. 209-218. Original Russian Text Copyright © 2001 by Filimonova. English Translation Copyright © 2001 by MA1K "Nauka/lnterperiodica " (Russia). Internal Anatomy of Females of Myobia murismusculi (Schrank) (Acari: Trombidiformes, Myobiidae) S. A. Filimonova Zoological Institute, Russian Academy of Sciences, St. Petersburg, 199034 Russia Received May 18, 1999 Abstract—The general morphology of the main internal organs, their fine structure and arrangement in the body of parasitic mite Myobia murismusculi have been investigated for the first time by means of light and electron micros- copy. Similarly to other Trombidiformes, this mite has no free body cavity, with the entire space between the or- gans being occupied with ventriculus and midgut caeca. Malpighian tubules are absent, excretion is carried out by a pair of eoxal glands. The respiratory system consists of typical tracheae. Large cells, probably analogous to those of the insect fat body, are found to be closely associated with midgut. The female reproductive system is composed of receptaculum seminalis, unpaired ovary, long complex oviduct, and cuticlelined vagina. The oviduct is subdi- vided into three regions according to the fine structure of the epithelial lining and secretory product. Two proximal regions produce material for external egg-shell layers, while the largest distal part has an internal cavity filled with unknown secretory product. The family Myobiidae is one of the numerous fami- lies of trombidiform mites, which comprises exclu- sively parasites living on the surface of the mammal- ian skin. At the present stage of knowledge, Myobii- dae are considered to be one of the families of the cheyletoid complex. Several species of the family Cheyletidae, including both free-living and parasitic mites, have been examined by Akimov and Gorgol (1990) by optical microscopy. As a result, a general scheme of their internal anatomy was composed. Since most of trombidiform mites are extremely small, an ultrastructural investigation is necessary nowadays in most cases to describe their internal anatomy. How- ever, the application of this method to cheyletoid spe- cies has been confined to few studies of the structure of Demodex folliculorum (Desh and Nutting, 1978; Desh, 1980), a highly specialized intradermal parasite with great number of anatomical transformations of internal organs. There is more information on the family Tetranychidae, which forms, together with the cheyletoid complex, the subcohort Raphignatha of the cohort Eleutherengona (Kethley, 1982). Papers de- voted to Tetranychidae cover problems concerning the fine structure of all main internal organs (Alberti and Crooker, 1985). As for myobiids, there are only two papers devoted to their internal anatomy. One of these deals with the organization of mouthparts (Paran, 1982), and the other, with the structure of the excretory organ and its connection with the alimentary canal (Paran, 1979). The present study, which is an attempt to fill the gap in the literature, does not pretend to describe myobiid anatomy thoroughly, but chiefly deals with three main systems of organs: digestive, excretory, and reproductive. It should be regarded as preliminary material for further comparative anatomical investiga- tions of various groups of trombidiform mites. MATERIALS AND METHODS Adult females of Myobia murismusculi (Schrank) were collected from white laboratory mice cultured at the Zoological Institute, Russian Academy of Sci- ences. Whole mites were fixed, just after being col- lected from the host, in a 2.5% solution of glutaralde- hyde in 0.1M phosphate buffer and postfixed in a 1% solution of Os0 4 in the same buffer. The material was embedded in epoxy-resin mixture, epon 812. Sets of semithin sections in three perpendicular planes were made to reveal the mutual arrangement of inter- nal organs. The sections were stained with a mixture of Methylene Blue and Azure II (pH 6.8). Thin sec- tions were also prepared in different planes, stained by the standard method, and examined with a TESLA BS-500 electron microscope. A total of 20 females were examined. RESULTS Adult females of Myobia murismusculi are about 400 urn long and no more than 100-110 urn wide. 1049