The slit sense organs of arachnids

Abstract

1. Aiming towards a better understanding of the functional role played by the arrangement of slit sense organs in the exoskeleton of arachnids the diversity of their topography is studied in the walking leg. Data previously collected with a spider and a scorpion are supplemented by such on a whip spider, a whip scorpion, and a harvestman. Thus five arachnid orders are now available for the comparison. 2. There is a wide spectrum of both the richness in 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 are numerous on the spider leg (15) none or only one or two occur in the other arachnids. On the leg of the scorpion and whip scorpion one instead finds an almost equivalent number of groups at comparable sites (12, 13). Whip spider and harvestman clearly have the smallest number of slits on their legs with 58 and 45 as compared to the 325 of the spider leg. 3. An outstanding feature of all the arachnid legs studied is a concentration of the slits proximally on the leg. This is 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 is the leg 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) is a typical though not general property of lyriform organs and groups, but not of isolated slits. 4. A position laterally on the leg close to a joint and an orientation of the slits roughly parallel to the long leg axis is common among both lyriform organs and groups. The harvestman has the least conformistic leg with the groups oriented more or less perpendicular to the long leg axis. Slits in the most ventral region of the leg are very rare. The femora of the harvestman, whip scorpion, and whip spider bear some large isolated slits oriented perpendicular to the long leg axis, which is exceptional. Within some lyriform organs and groups (see f.i. trochanter of whip spider) the axes of the constituent slits divert. 5. These morphological findings are discussed in terms of their physiological significance. The arguments put forth are relevant for an understanding of the topography of campaniform sensilla on the insect leg as well.