Haller's organ

Haller's organ (400x)

The Haller organ or Haller's organ is a special sensory organ on the forelegs of ticks (Ixodidae), which contains various hair-like sensillae , which u. a. carry specific chemoreceptors . In particular, it can detect chemical compounds that suggest the proximity of a potential host , and thus serves to find the host. The organ is present in all tick species in all stages of development.

Position of Haller's organ on the first phalanx

It is named after Gottfried Haller (1853–1886), a Swiss private lecturer in zoology, who discovered it in 1881 and assumed it was an organ of hearing.

morphology

The organ is located on the top of the tarsus , the last leg segment, the first pair of legs (front legs). It consists of two adjacent structures.

On the one hand, there is an elongated, more or less deeply deepened pit at the front (distal) of the tarsus. Behind it (proximally) is a strongly hardened (sclerotized) hollow capsule, which is open to the outside via a gap in the chitin shell. In many cases, only the opening on a dome-shaped thickening, which consists of the upper part of the capsule, is visible from the outside. In some genera such as Ixodes , the opening can be quite wide, so that large parts of the inside of the capsule are visible. Both in the pit and within the capsule there is a number of hair-like structures (which differ depending on the species), sensillae , which contain sensory cells . The number of hairs is typically between 15 and 20, of which 6 or 7 are freely visible within the pit. The hairsensils of the capsule can partially protrude outwards. Their arrangement in the capsule probably serves to protect against mechanical damage, it also protects against drying out.

Most of the sensory hair in the capsule has a multitude of small openings. These pores on the surface of the sensilla already suggest the function as a sensory organ with chemosensors. Volatile substances from the environment can diffuse through the pores into the hollow interior (lumen) of a sensillum. Inside there is a collection of receptor neurons that recognize and pass on olfactory stimuli. But not all sensory hairs in the distal pit have pores; some may serve more as moisture receptors.

In addition to Haller's organ, ticks have a number of other chemoreceptive sensory hairs that sit on the palps on the side of the mouthparts and the two front pairs of legs. Compared to blood-sucking insects, the number of these mites is small. The total number of chemoreceptors in all tarsal sensory organs is hardly higher than about 100 in ticks.

Sensors

The receptors enable the tick to absorb chemical compounds such as carbon dioxide , ammonia , hydrogen sulfide and a variety of organic components such as B. perceive benzaldehyde , which occurs in the breath or sweat of their host species. Different receptors have different target molecules and sensitivities. Phenol receptors recognize phenol compounds such as o-chlorophenol, o-bromophenol, o-methylphenol and some derivatives; Lactone receptors recognize γ-valerolactone. There are sensillae with sensitivity to butyric acid , but this sense is far less decisive in finding a host than was assumed after pioneering studies (before the 1950s).

The tick is likely to be able to perceive possible hosts over long distances (10–15 m). In the tropical bovine tick Amblyomma variegatum , the organ also serves to perceive an aggregation pheromone. This is given off by males as soon as they have found a host, so they serve the females and other animals as a kind of "pathfinder". 2-nitrophenol could be identified as one of the components of the pheromone.

swell

• Heinz Mehlhorn (Ed.): Encyclopedic Reference of Parasitology. Springer 2001, ISBN 3-540-66239-1 .
• Pascal Steullet: Perception of vertebrate volatiles in the tropical bont tick, Amblyomma variegatum Fabricius. Thèse présentée à la Faculté des Sciences de l'Université de Neuchâtel pour obtenir le grade de docteur es sciences. 1993.
• RF Foelix, RT Axtell: Ultrastructure of Haller's Organ in the Tick Amblyomma americanum. In: Journal for Cell Research. 124, 1972, pp. 275-292.

Individual evidence

1. ↑ Haller organ in the Lexicon of Neuroscience on Spektrum.de
2. ^ Lecturers at the University of Bern
3. ^ G. Haller: Preliminary remarks on the hearing organ of the ixodids. In: Zoologischer Anzeiger. Volume 4, No. 79, Leipzig 1881, pp. 165-167. on-line
4. ^ SA Leonovich: Phenol and lactone receptors in the distal sensilla of the Haller's organ in Ixodes ricinus ticks and their possible role in host perception. In: Exp Appl Acarol. 2004; 32 (1-2), pp. 89-102, PMID 15139275 .