282
Harald Tichy and Ewald Gingl
Locusta migratoria (Ameismeier and Loftus 1988), Cupiennius salei (Anton and Tichy
1994 ). Outlines of dendrites taken from C. Note log-scale within bars; on the left side of the
bars, values are given numerically. B Sketch of form and position of thermoreceptive dendrite in insects and spider hygro-thermoreceptive sensilla. C Digitized tracings from published transmission electronmicrographs of transverse sections of the dendrites of different
arthropod thermoreceptors. The outlines are drawn to scale, so that differences in size of the
membrane area can be appreciated. Speophyes lucidulus (Corbiere-Tichane 1971, Fig. 3a),
Antheraea pernyi (Zimmermann 1991, Fig. 4d), Periplaneta americana (Yokohari 1981,
Fig. 13c), Locus/a migratoria (Ameismeier and Loftus 1988, Fig. 1), Cupiennius salei
(Anton and Tichy 1994, Fig. 2b)
The outer receptor lymph cavity is closer to the cuticle and more spacious. Nowhere is an observable avenue of easy communication between the outer and the
inner cavities. The sheath and the cells surrounding it appear to be a barrier between the two. In the tip-pore sensilla of the spider there is only one receptor
lymph cavity, the inner cavity (Fig. 3F). The cavity narrows as it enters the dendritic sheath and proceeds outwardly towards the pore. The electron density of the
lymph increases along the way but thins again at the pore opening. The fluid
would seem to discharge slowly there and pile up amorphous material at the entrance, thus placing a limit on the access of the lymph to the atmosphere. The pore
may be involved in stimulus conduction and/or transduction.
4.1 Mechanical Hygrometer Model
For insects the most favored model views the cuticular wall as a hygro-mechanical
transducer. Humidity-dependent shrinking due to water loss and swelling due to
water uptake are believed to cause changes in the geometry of the wall that in tum
leads to deformation of the dendritic membranes and voltage changes across them
(Fig. 5). Here, the intimate association of the dendritic membranes with the cuticular wall would be important.
If changes in humidity indeed lead to structural changes of the wall, surface
structures should change as well. We used the scanning force microscope to analyze the effect of changes in humidity on the surface of the peg sensilla in the honeybee Apis me/lifera (Resch et al. 1998). Female hair, which was used as reference
material, was found to increase in length by 1.0% when Hr was increased from 0
to 100%. These data are in agreement with the original specification of the supplier (Lambrecht Company 1996). In the honeybee this same technique revealed
no humidity induced structural changes of the surface on top of the peg. Recent
experiments on the cockroach (Yokohari 1999) displayed a 65-nm increase in both
the diameter and length of the peg when relative humidity was raised from 0 to
100%. This value was obtained by placing a piezoelectric element on the apex of
the peg. Unfortunately, no full account of this work was published, so that procedure and analysis cannot be evaluated.
Harald Tichy and Ewald Gingl
Locusta migratoria (Ameismeier and Loftus 1988), Cupiennius salei (Anton and Tichy
1994 ). Outlines of dendrites taken from C. Note log-scale within bars; on the left side of the
bars, values are given numerically. B Sketch of form and position of thermoreceptive dendrite in insects and spider hygro-thermoreceptive sensilla. C Digitized tracings from published transmission electronmicrographs of transverse sections of the dendrites of different
arthropod thermoreceptors. The outlines are drawn to scale, so that differences in size of the
membrane area can be appreciated. Speophyes lucidulus (Corbiere-Tichane 1971, Fig. 3a),
Antheraea pernyi (Zimmermann 1991, Fig. 4d), Periplaneta americana (Yokohari 1981,
Fig. 13c), Locus/a migratoria (Ameismeier and Loftus 1988, Fig. 1), Cupiennius salei
(Anton and Tichy 1994, Fig. 2b)
The outer receptor lymph cavity is closer to the cuticle and more spacious. Nowhere is an observable avenue of easy communication between the outer and the
inner cavities. The sheath and the cells surrounding it appear to be a barrier between the two. In the tip-pore sensilla of the spider there is only one receptor
lymph cavity, the inner cavity (Fig. 3F). The cavity narrows as it enters the dendritic sheath and proceeds outwardly towards the pore. The electron density of the
lymph increases along the way but thins again at the pore opening. The fluid
would seem to discharge slowly there and pile up amorphous material at the entrance, thus placing a limit on the access of the lymph to the atmosphere. The pore
may be involved in stimulus conduction and/or transduction.
4.1 Mechanical Hygrometer Model
For insects the most favored model views the cuticular wall as a hygro-mechanical
transducer. Humidity-dependent shrinking due to water loss and swelling due to
water uptake are believed to cause changes in the geometry of the wall that in tum
leads to deformation of the dendritic membranes and voltage changes across them
(Fig. 5). Here, the intimate association of the dendritic membranes with the cuticular wall would be important.
If changes in humidity indeed lead to structural changes of the wall, surface
structures should change as well. We used the scanning force microscope to analyze the effect of changes in humidity on the surface of the peg sensilla in the honeybee Apis me/lifera (Resch et al. 1998). Female hair, which was used as reference
material, was found to increase in length by 1.0% when Hr was increased from 0
to 100%. These data are in agreement with the original specification of the supplier (Lambrecht Company 1996). In the honeybee this same technique revealed
no humidity induced structural changes of the surface on top of the peg. Recent
experiments on the cockroach (Yokohari 1999) displayed a 65-nm increase in both
the diameter and length of the peg when relative humidity was raised from 0 to
100%. This value was obtained by placing a piezoelectric element on the apex of
the peg. Unfortunately, no full account of this work was published, so that procedure and analysis cannot be evaluated.