Talk:A spider's tactile hairs
The review on „A spider’s tactile hairs“ by Friedrich Barth is a brief but scholarly compendium on the primary sensory organs supplying touch information to a spider: its tactile hairs. The paper focuses on the species Cupiennius salei and puts an emphasis on the biomechanics of hair bending and deflection of hairs, with additional spotlight paragraphs on selected neuroanatomical, electrophysiological and behavioural studies. With regard to the Encyclopedia of Touch, my only major comment concerns the comparison of spiders and other arthropod groups, particularly insects. Given that the present paper’s focus is on the properties of single hairs, and the fact that tactile hairs of spiders are homologous to the tactile hairs of insects, a few sentences on the relationship to findings by Thurm, French and other researchers, who have done very closely related work on the tactile hairs of insects, would be very helpful. Other than that I have only minor points and suggestions.
(1) The author may want to consider cross-links to the other arthropod touch articles in the Encyclopedia of Touch. For example, the articles on insect antennae deal with tactile hairs and hair plates, too. Cross-links and a comment on homology or analogy might be helpful to the reader (I see that the same should be – and is being - communicated to the authors of the insect antennae articles).
(2) In the 2nd paragraph of the section "mechano-sensitive hair sensilla", the statement "…which enriches the small sensory space of spiders” is not clear. Does the author refer to what MacIver and Nelson have termed the "sensorium", i.e., the sampling volume, or rather to the thin “peri-cuticular range" in which tactile hairs can be deflected? The main difference is the role of the spider’s own action.
(3) A number of words lack a hyphen (I think), like first-order lever systems, so-called tubular body, ramp-and-hold stimuli etc. Also, “the scaling up” might better be written as "up-scaling" (the same goes for “the scaling down”).
(4) In two places, differences in magnitude are expressed as “powers of ten”. I suggest to replace this by “orders of magnitude”, as I think this is more commonly used.
(5) The last sentences of section "types of morphology and arrangement" are not clear to me. First it is said that the degree of openness varies a lot, indicating that this implies functional variability. But then it is said that socket diameter, hair length and hair diameter are all mutually correlated, in which case I don’t see how there can be functional variation in the face of a simple scaling rule. I must be missing something.
(6) In the numbered subsections of "Hair shaft and outer lever arm” there is quite a lot of very detailed information. The implication of higher mechanical sensitivity (end of point 1) is not clear to me. Also, points 2 and 3 are not trivial at all, making them a bit challenging to understand, given the brevity of the explanations.
(7) In the middle of section "Physiological responses of sensory cells" there is a statement about a power function with an exponent of 0.5 being something in-between a displacement and a velocity sensor. A bit more explanation might help to make this more comprehensible: What is the difference between a displacement and velocity sensor? Why would an exponent of 0.5 lie half-way between these extremes?
(8) I found the description of the "body raising" reflex not quite clear. For example, I don’t understand how a coxa levator reflex that should lift the leg would contribute to the raising of the body. Or is it that the raising of a single leg triggers the “push-up” generated by the other seven legs? Also, the caption to figure 7 could be clearer if the three “episodes” depicted would be described in sequence.
(9) Figure 8 is not easy to interpret. In a) I wonder how SN activity relates to Myo activity, if SN can be active in the hold phase and also before and after the stimulus, while Myo is silent? In b) I am not sure what the figure is meant to illustrate. The MN neurites are far away from the afferent neurites. Is the main point that they are located in different regions, or that there mus me interneurons to connect them? Maybe Figure 9 alone would be clearer.
Response to Review 1
A comparative approach would be nice in principle but apart from the limited space available there are two aspects speaking against sacrificing the compactness of the present story in favor of an extended comparison. (i) The much appreciated work by” Thurm, French and others “is not very closely related” but largely deals with transduction and encoding. (ii) Another point to consider is that insect and spider tactile hairs are not necessarily homologous structures and there are lots of differences among spider hairs. One fundamental difference between insects and spiders is in the primary processes: The standing transepithelial potential / voltage found in insects does not exist in spiders, which goes along with a different ionic composition of the receptor lymph and fine structural details (Thurm and Wessel 1979; Barth 2002). There are hardly any studies examining the mechanical properties of insect hairs which would lend themselves to a meaningful comparison with the spider tactile hairs treated here. These are different from the insect filiform hairs (and spider trichobothria). However, to show the reader the way to the insect literature the paper by Theiss (1979) is now quoted and also a review paper by Keil (1997). Publications from the Thurm group and dealing with the the transepithelial voltage and cellular transduction are now also included in the References (see text below).
I added the following sentence at the end of the section “Physiological responses of sensory cells”. The reader interested in insect mechanoreceptive hairs is referred to a review article by Keil (1997) and to a paper by Theiss (1979) who found a spring stiffness S in fly macrochaetae very similar to that described here for the spider case. Of particular interest may be the remarkable lack of a standing transepithelial potential in spiders. Such a potential was found to be fundamental for the primary processes (transduction) in insect sensilla but is absent from spiders according to all knowledge currently available (Thurm and Wessel 1979; Thurm and Küppers 1980; Barth 2002).”
Added to References: Thurm U, Küppers J (1980) Epithelial physiology of insect sensilla. In: Locke M, Smith >DS (eds) Insect biology in the future. VBW80, Academic Press, New York
Thurm U, Wessel G (1979) Metabolism-dependent transepithelial potential differences at epidermal receptors of arthropods. I. Comparative data. J Comp Physiol 134(2):119-130
Theiß J (1979) Mechanoreceptive bristles on the head of the blowfly:mechanics and electrophysiology of the macrochaetae. J Comp Physiol 32 (132????) :55-68
Keil TA (1997) Functional morphology of insect mechanoreceptors. Microscopy Research and Technique 39:506-531
(1) R: Yes, will be done
(2) R: As is said in the text the diversity of the hair sensilla enriches the small sensory space of spiders which lack true long distance senses like our eyes and ears. The diversity mentioned refers to differences in structure, location, and physiological properties. I refer to all mechanosensory hairs here, including trichobothria. Active sensing (and thereby enlarging the sensory space) is dealt with later on in the text. The point made is that spiders lack true long distance sensing.
(3) R: Corrected
(4) R: I would rather leave “powers of ten”; there is nothing wrong with it and it even is more precise than “orders of magnitude”.
(5) R: Sorry, I have difficulties to follow. In order to avoid misunderstandings the text now reads, with the last sentence deleted: Hair sockets measure between 3 µm and 15 µm in diameter and vary in shape and their degree of openness, which affects the mechanical directional characteristics of hair deflection.
(6) R: In the text it is said: “..higher sensitivity for small deflections…. than for large stimuli…”. The reason is given in brackets, showing the forces needed to deflect the hair by one degree. The bending moment increases much more quickly at small deflection angles than at larger ones. See preceding text.
(7) R: The difference between a displacement and a velocity sensor simply is that they primarily respond to one or the other of these two different physical parameters of the stimulus. To make it easier for the reader the text now reads: “When exposed to ramp and hold stimuli the action potential frequency follows a simple power function y(t)= a x d x t-k in both cases. Here y is the impulse rate, t is the time, a a constant representing the amplification, d stimulus amplitude and k a receptor constant describing how quickly the response to a maintained stimulus declines. In the present case k-values are around 0.5 implying properties in between that of a pure displacement receiver (k=0; response independent of frequency) and that of a pure velocity receiver (k=1; differentiator of first order). “
(8) R: see last 7 lines of §2 of section “Behavioral roles”. It is exactly as the reviewer assumes, a plurisegmental reaction is triggered by the activity in one leg. As to Fig.7: The legend now reads: Figure 7: The body raising behavior of C.salei. (1.) The spider approaches a 10 mm high wire obstacle from the left. (2.) It raises its body as soon as tactile hairs ventrally on the proximal leg and sternum touch it. (3.) Having passed the obstacle the spider returns to its undisturbed walking position. [from (Seyfarth 2000)]
(9) R: (a) looking at Fig.8a again I cannot quite follow the reviewer. There is a certain low amount of spontaneous activity of SN which otherwise mainly responds to the velocity component of the stimulus when the Myo response is seen. (b) As described in the legend the figure shows the neuroanatomical correlate for what is seen in (a). The text of the legend now is slightly extended saying "…….supplying coxal muscle c2. There are local and plurisegmental interneurons which are not shown here. Numbers 1 to 5……"
The article reviews the spider’s hair as a tactile sense organ regarding its morphological, physiological, and behavioral aspects. The contents are invaluable in understanding the sense of touch from a comparative point of view. However, the reviewer recommends that the author will consider some minor points below.
(1) Section 3 "Mechano-sensitive hair sensilla", L1: "100.000 thousands" means 100 thousands?
(2) Figure 3: the photograph (c) seems to be an image of transmission electron microscopy. Please specify it in the caption.
(3) Figure 3d caption: I think this is a "dorsal" view instead of "horizontal" view.
(4) Last part of Section 3.1 (Types of morphology and arrangement): The portion “…correlates with hair diameter and hair diameter with hair length” is unclear. Here it means “…correlates with hair diameter and hair length”?
(5) Section 3.3.1 (Hair shaft or outer lever arm): I think this part requires some figures for better understanding. For example, 2 figures appearing in Barth (2004) are very suitable here.
(6) Reference list: abbreviated style may not be used for literature information. Therefore, "Univ" in Jackson (1986) and Seyfarth (2000) should be "University". Similarly, "Z Naturforsch" in Schmid (1997) should be "Zeitschrift für Naturforschung".
Response to Review 2
(1) R: corrected; it now reads "…with several 100 thousands of hairs…."
(2) R: Legend changed to: "……in longitudinal section (transmission electron micrograph). TB…."
(3) R: I would say it is also horizontal but have no problem to improve to “dorsal”. The legend for Fig.3d now reads: "…..ganglionic mass; dorsal view. Note longitudinal sensory …."
(4) R: see response to Review 1(5)
(5) R: I do understand the concerns of the reviewer. Although I tried to write the text so that the essence will be understood by the non-specialist reader as well, a little physics remains inevitable. Adding figures would necessitate more text as well. And the reader interested in more details is referred to the literature anyway.
(6) R: done