Redescriptions and comments on the validity of Acuaria subula and A. skrjabini (Nematoda, Spirurida, Acuariidae), parasites of passerine birds.

Acuaria subula (Dujardin, 1845) is redescribed by light microcopy (LM) and scanning electron microscopy (SEM) on the basis of specimens from its type host, Erithacus rubecula (Passeriformes, Muscicapidae), from Curonian Spit (Kaliningradskaya Oblast', Russia) and Bulgaria. Acuaria skrjabini (Ozerskaya, 1926) is redescribed by LM and SEM on the basis of specimens from Passer domesticus (type host) and P. hispaniolensis (Passeriformes, Passeridae) from Bulgaria. Contrary to previous opinions recognizing A. skrjabini as a junior synonym of A. subula, the present study confirms that they are distinct species. They can be distinguished on the basis of the ratio between the length of cordons and the body length, the ratio between the length of muscular oesophagus and glandular oesophagus, and the ratio between the total length of oesophagus and the body length. In addition, the plates forming the cordons in these two species exhibit different morphological characters. Another difference between these two species is associated with the particular irregular mosaic ornamentation of the cuticle on the ventral and lateral sides of body around the region of vulva of A. subula and its absence in A. skrjabini. Data on their host and geographical ranges are surveyed. The type series of Acuaria buttnerae Chabaud et Petter, 1961, described as a parasite of Calandrella brachydactyla (Passeriformes, Alaudidae) in France, is re-examined; the latter species is recognized as a junior synonym of A. skrjabini (new synonymy).