SLIT SENSE-ORGANS OF ARACHNIDS - COMPARATIVE-STUDY OF THEIR TOPOGRAPHY ON WALKING LEGS (CHELICERATA, ARACHNIDA)

The functional role of diverse slit sense organ arrangements in the exoskeleton of arachnids was studied in the walking leg. Data previously collected with a spider and a scorpion were supplemented by data on a whip spider [Admetus pumilio], a whip scorpion [Mastigoproctus brasilianus] and a harvestman [Amilenus aurantiacus]. The 5 arachnid orders were compared. There was a wide spectrum of both extent of total slit sensillar supply and the relative share contributed to it by isolated slits, groups of single slits, and lyriform organs, respectively. Whereas lyriform organs were numerous on the spider leg, none or only 1 or 2 occurred in the other arachnids. The legs of the scorpion and whip scorpion had an almost equivalent number of groups at comparable sites. Whip spiders and harvestmen had the smallest number of slits on their legs with 58 and 45 as compared to the 325 of the spider leg. An outstanding feature of all the arachnid legs studied was a concentration of the slits proximally on the leg. This was most pronounced in the harvestman and whip spider, i.e., those cases with the smallest total number of slits (percentage proximal to tibia: spider 70%; harvestman 100%; whip spider 93%). The trochanter was the segment most richly supplied. A location right at or close to the distal end of the respective leg segment (i.e., to proximal part of articulation) was typical though not general for lyriform organs and groups, but not for isolated slits. A lateral position on the leg close to a joint and an orientation of the slits roughly parallel to the long leg axis was common among both lyriform organs and groups. The harvestman had the least typical leg with the groups oriented more or less perpendicular to the long leg axis. Slits in the most ventral region of the leg were very rare. The femora of the harvestman, whip scorpion and whip spider bore some large isolated slits oriented perpendicular to the long leg axis, which was exceptional. Within some lyriform organs and groups the axes of the constituents slits were divergent. Morphological findings were discussed in terms of their physiological significance. The arguments were relevant for an understanding of the topography of campaniform sensilla on the insect leg as well.