Trichobothria, Slit Sensillae: Their Evolution and Significance

One of the many lyriform organs on a spider, scorpions possess very similar structures. This image was found at www.microscopy-uk.org.uk/mag/artjan00/spileg.html

Slit sensillae are thin, deep slices in arthropod cuticle leading to sensory neurons. Their roles are numerous, but rely on the deformation of the cuticle around the slit. Depending on where they are found they can function as proprioreceptors (sensing movement), georeceptors (measuring bending by the opisthosoma) and by measuring changes in air pressure can sense vibrations. It has also been shown that differences in vibration reception between the sensillae can lead to 2-dimentional prey location in scorpions. Readers should be aware that these slits can be found isolated or in parallel groups called LYRIFORM organs

SLIT SENSILLA EVOLUTION For a time, it was thought that slit sensillae were a derived feature of recent scorpions, as they were not present in fossil varieties, eurypterids or xiphosurans. To date, I am not aware of any slit organs in fossil specimens, however this may just be a preservational bias as recently they have been found in the Devonian trigonotarbid Palaeocharinus, and the Silurian, aquatic eurypterid Baltoeurypterus. Whilst those of Baltoeurypterus are wider than modern forms, their presence would make it far more parsimonious for all scorpions to have possessed them, and indicates that they served a purpose in aquatic habitats.

High resolution image of trichobothria. Image taken from www.answers.com/topic/trichobothria

TRICHOBOTHRIA All extant arachnids (and many other arthropods for that matter!) possess trichobothria. These are elongate, non-tapering hairs (unlike regular setae) which fit into a cup, allowing movement in most directions. This, coupled with their great sensitivity allows them to detect airborne vibrations. Trichobothria can either be full or petite and key trichobothria (regardless of size) are very important in the systematics of scorpions.

TRICHBOTHRIAL EVOLUTION Despite all extant arachnids possessing trichobothria, it is thought that they have evolved independently in each group as a result of terrestrialisation. As such, discussion of their evolution within the different arachnid groups is outside the scope of this site, however, they are very important in the systematics of modern scorpions.

The terrestrialisation of scorpions probably occurred within the Palaeoscorpiones and certainly by the Mesoscorpionina, yet true trichobothria are first found in the orthostern Palaeopisthacanthus. As trichobothria are obviously very difficult to preserve, these were inferred by the cup-shaped follicle which holds the trichobothrium. That said, the ‘prototypic palaeopisthacanthid’ Corniops mapesii is thought to have possessed smaller trichobothria whose follicles lacked the rimmed cup shape.

An awful lot of systematic work has been done based upon the trichobothria of the pedipalp and each of its segments (chela, patella and femur) and therefore the following may be quite hard to understand until you have read the sections on Systematics and General Evolution. Suffice to say, 6 major types have been identified. These are based on the number of trichobothria on each segment and the position of the major trichobothria. These are labeled P (because this pattern is found in Palaeopisthacanthids), F1 (for archaeobuthids), A (for Buthida), B (for Chaerilida), C (for Iurida) and D (for pseudochactids). To understand these patterns, a knowledge of trichobothrial terminology is needed, but this isn’t as scary as it first looks.

TRICHOBOTHRIAL TERMINOLOGY

Basically, for each segment, trichobothria are numbered from the base (proximally) to the terminus (distally) and to make the numbers more manageable each segment is treated like a separate box, with a dorsal (PREFIXED with d), ventral (prefixed with v), external (e) and internal (i) surface.

A guide to help you visualize what is meant by the e, i, d and v prefixes in trichobothrial terminology


Deviations from this terminology
As well as this, on the patella and manus, it is traditional to group the dorsal and external trichobothria into basal (with a SUFFIX of -b), sub-basal (with a suffix of -sb), sub-terminal (-st) and terminal (-t) forms (surfaces of the palm of the manus being differentiated from those of the fixed finger by capitals, ie. The external, sub-terminal trichobothrium of the palm is Est, whilst for the fixed finger it is est).

Trends
With 18 trichobothria, the palaeopisthacanthids had relatively few compared with modern forms (see below). Admittedly there is some uncertainty as to this number and the images below are based on a composite of Palaeopisthacanthus and Cryptoscorpius, a method which has been criticised.

Trichobothrial arrangement in the Palaeopisthacanthid femur. Based upon Soleglad and Fet (2001)

Trichobothrial arrangement in a palaeopisthacanthid patella. Image based upon Soleglad and Fet (2001)

Trichobothrial arrangement in the manus and fixed finger of a palaeopisthacanthid. Image based upon Soleglad and Fet (2001)

With the exception of femoral trichobothria (which reduce from 4 and are completely lost in scorpionoids), all trichobothria increase during scorpion evolution. Archaeobuthids have a total of 27, pseudochactids have 34 and the Buthida have 39. Likewise the Chaerilida have 37 trichobothria and the Iurida (Scorpionoidea, Chactoidea and Iuroidea) possess 48. This increase in number most probably represents an evolution towards increased specialization in the use of trichobothria.