Some animals live in environments, where there is (almost) no light available. It makes no sense to see in the dark, but it is important for a specimen to know, where it actually is, where it is going to, whether there is enough food and what the conspecifics are doing. Predators need to be recognized in time, and a sexual partner must be found. There is also need for an efficient communication between specimens of a species. How can all this be performed by mites of the Astigmata, which usually live inside decomposing soil habitats in a more or less permanent darkness?
Olfactory sense organs in mites of the Histiostomatidae
Histiostoma sachsi (Histiostomatidae, Astigmata) is such a mite, living inside cow dung or compost. It might have a rudimentary ability for a light perception, but has not visible or functional eyes. It cannot produce any sounds. It can only feel and smell. Seemingly very limited abilities, but the contrary is fact: Due to evolution this mite is perfectly adapted to its life-style. It can feel objects by touching on them using its body setation (= body hairs). And it smells by means of very specialized body hairs, which are called solenidia and appear in different types, shapes and functions. These mites don’t smell on the level of us humans, which would be very insufficient. If at all, it should be compared with a dog. I am always fascinated when seeing blind dogs and how perfectly they can interact with their environment, despite their handicap. That’s may be how the efficiency of olfactory perception abilities of such a mite must be imagined. They do not only perceive scent particles from other animals, plants and soil components. Even olfactory signals from their conspecifics will be correctly and differentiatedly interpreted. And that not only marginally. Olfactory signals represent indeed the major mode of their intraspecific communication.
Chemical communication of mites of the Histiostomatidae
Communication always requires contributions from both sides, a signal and an answer. These mites smell the signal of a conspecific using their solenidia, and they answer by the secretion of biochemical components. For these purposes, they possess a huge and complex gland system located on the upperside of their backs. Volatile excretions aggregate inside a big and rounded reservoir and finally leak to the outside via a pore, called oilgland opening. These gland systems are located symmetrically on both sides, each with one reservoir and one pore.
The meaning of the sent volatile message simply depends on the composition of the correspondingbiochemical components. Even diffferent stereochemical configurations of the same molecule can have different meanings. Citral for instance is a major component and has in different stereoisomers different functions. Such cummunicative volatile signals are usually named pheromones. And mites of the Histiostomatidae can indeed produce different kinds of pheromnes via the same gland system. Aggregation pheromones inform specimens about a suitable place to stay together with their conspecifics, for example due to a sufficient amount of food resources. Alarm pheromones solicit mites nearby to flee from an unpleasant situation. Sexual pheromones attract adult partners to each other in order to perform the mating procedure. But the gland secretions can even more. As allomones, they communicate with specimens of other species. They function as defenses against predators or other dangerous cohabitants.
Deutonymphs need to find a carrier for dispersal
Another form of communicative interspecific interactions is performed by a specific juvenile instar, the deutonymph. It looks morphologically quite different from all other instars (heteromorphic situation), does not need or possess a functional mouth, has a thicker cuticle as protection against drying out and a complex sucker organ on its underside in order to attach itself to an insect or another bigger arthropod. Deutonymphs of the astigmatid mites search for bigger carrier-arthropods to get carried from one habitat to another (dispersal strategy is calledphoresy). While doing so, they again use their specifically modified leg setation (hairs) on the first pairs of legs to perceive scents for the detection of a suitable and passing by carrier. Basically it is still unknown, whether the term „communication“ is indeed appropriate in this context as we don’t know yet about a mutual interaction between deutonymphs and their carriers, before the phoretic ride begins.
Olfactory orientation of the deutonymph of Histiostoma sachsi, copyrights Stefan F. Wirth, February 2019.
Specific way of walking in deutonymphs
In detail, different kinds of behaviors can be observed in deutonymphs, when searching a carrier. The detailed behavioral patterns in this context can slightly differ between even closer related species. Deutonymphs of Histiostoma sachsi as all deutonymphs show a characteristic mode of walking, in which especially the first pair of legs plays an important role. During each step, performed by four pairs of legs, the first legs are lifted up much higher than all other hind legs. While doing so, they slightly tremble up and down. A behavior that mostly supports a better basic orientation inside a „jungle-„micro-landscape, being filled up with soil particles and decomposing plant tissues. But what H. sachsi deutonymphs additionally need in order to find their carriers is repeatedly to rest between the walking activities. Thus the first legs, which normally are still walking legs, are made free and that way available for the perception of carrier-scent-components only. These namely are the legs that bear the highest densiy of solenidia.
Two different behavioral modes for an efficient orientation towards a carrier
Two different modes of resting with olfactory searching activities could be observed: In periodic intervals the deutonymph attached to the ground by using its sucking structures. They were then more or less laying on their entire undersides with only their forebodies slightly lifted up. By alternating moving the first legs up and down, olfactory information could be perceived from all directions without having the own body as a barrier to backwards. To improve its orientation situation, the deutonymph additionally turned on its own axis around, being stabilized by its sucking structures, which are flexible enough to follow these movements. When the deutonymph intended to continue its walk, it first needed to detach from the ground, which happened via muscle contractions that caused an abrupt detachment of the corresponding suckers. But main aim of the deutonymph is to find an elevated place, where the probability of a passing by carrier is especially high and from where a bigger insect (or other arthropod) can easier be ascended. There the second behavioral mode was performed. The deutonymph only fixed the edge of its hind body to the ground, again using the suckers on its underside, which are located close to this edge. This time the entire mite body stood in an upright position. The first legs again „waved“ alternating up and down and could under these especially elevated conditions even perceive scents from bigger distances. By occasionally slightly and alternating turning their upright bodies to both sides, olfactory information could be easier detected from all directions.
Carrier of H. sachsi still unknown
The frequency of such movements in mites increases typically as closer a suitable carrier approaches. But this was not yet observed or documented for Histiostoma sachsi. Its carrier inside the compost substrate is still unknown, which is why I so far could’t perform corresponding experiments. The species‘ describer, Scheucher (1957), found her mite specimens in cow dung and also didn’t identify the corresponding carriers there.
The observations presented in my video are part of my research project about morphologies and behaviors of deutonymphs in the Histiostomatidae.
Berlin, February 2019. All copyrights Stefan F. Wirth.