The Johnston9s organs (JO) of mosquitoes are the most complex mechanosensitive organs yet found in insects. Previous findings on behavior of mosquitoes suggest that, together with exceptional sensitivity, their auditory system can discriminate frequencies. Analysis of compound responses of the JO did not provide unambiguous evidence of such discrimination, even less did it help to find its mechanism. Using the feedback stimulation method, we measured the tuning frequencies of the JO sensory neurons. Here we present electrophysiological evidence that male mosquitoes of Culex pipiens possess at least eight groups of auditory neurons which are distinct in their frequency tuning, with individual frequencies ranging from 85 to 470 Hz. Most of the neurons are tuned to 190 – 270 Hz, which corresponds to the difference between a male and a female flight tones. Axons of the JO sensory units propagate graded amplified receptor potentials rather than all-or-none action potentials, are grouped into pairs or triplets and often respond in anti-phase to each other. Some features of the mosquito auditory system suggest an analogy to the retinal mechanisms. Together with our previous findings on frequency tuning in female mosquitoes of different species, this study presents evidence in favor of sophisticated frequency analysis of sound in mosquitoes.

The paired auditory organ of mosquito, the Johnston's organ (JO), being the receiver of particle velocity component of sound, is directional by its structure. However, to date almost no physiological measurements of its directionality was done. In addition, the recent finding on the grouping of the JO auditory neurons into the antiphase pairs demanded confirmation by different methods. Using the vector superposition of the signals produced by two orthogonally oriented speakers, we measured the directional characteristics of individual units as well as their relations in physiologically distinguishable groups – pairs or triplets. The feedback stimulation method allowed to discriminate responses of the two simultaneously recorded units, and to show that they indeed responded in antiphase. Units of different frequency tuning as well as high-sensitive units (thresholds of 27 dB SPVL and below) were found in every angular sector of the JO, providing the mosquito with the ability to produce complex auditory behaviors.

The present article represents the first part of a review of fossil heterostigmatic mites collected from late Eocene Rovno amber. Here, mites of the families Tarsocheylidae, Dolichocybidae, and Acarophenacidae are described, with Tarsocheylidae and Dolichocybidae recorded for the first time from amber. A new genus and four new species are described, namely, Hoplocheylus similis sp. nov. (Tarsocheylidae), Dolichocybe elongata sp. nov. (Dolichocybidae), Proadactylidium fossibilis gen. and sp. nov. and Paradactylidium sineunguis sp. nov. (Acarophenacidae). The extant species Acarophenax lacunatus Cross and Krantz, 1964 and A. assanovi Livshitz and Mitrofanov, 1974 are transferred to the new genus Proadactylidium. A revised key to the genera of Acarophenacidae is presented. The homologies of prodorsal setae in Acarophenacidae are reconsidered and reviewed.

This paper describes the first record of the Sejida (= Sejina) family Sejidae (superfamily Sejoidea) from Burmese amber, based on deutonymphs. Specimens were found in the amber piece along with a wasp (Hymenoptera: Scelionidae), flies (Diptera: Ceratopogonidae) and thrips (Thysanoptera: Adiheterothripidae). This record traces the Sejida (= Sejina) back to the mid-Cretaceous (ca. 100 Ma) and represents not only the oldest valid record of Mesostigmata, but it is also one of the oldest examples of the entire Parasitiformes clade.

The task of directional hearing faces most animals that possess ears. They approach this task in different ways, but a common trait is the use of binaural cues to find the direction to the source of sound. In insects, the task is further complicated by their small size and, hence, minute temporal and level differences between two ears. A single symmetric flagellar particle velocity receiver, such as the antenna of a mosquito, should not be able to discriminate between the two opposite directions along the vector of the sound wave. Paired antennae of mosquitoes presume the usage of binaural hearing, but its mechanisms are expected to be significantly different from the ones typical for the pressure receivers. However, the directionality of flagellar auditory organs has received little attention. Here, we measured the in-flight orientation of antennae in female Culex pipiens pipiens mosquitoes and obtained a detailed physiological mapping of the Johnston’s organ directionality at the level of individual sensory units. By combining these data, we created a three-dimensional model of the mosquito’s auditory space. The orientation of the antennae was found to be coordinated with the neuronal asymmetry of the Johnston’s organs to maintain a uniformly shaped auditory space, symmetric relative to a flying mosquito. The overlap of the directional characteristics of the left and right sensory units was found to be optimal for binaural hearing focused primarily in front of, above and below a flying mosquito.