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Harald Tichy and Ewald Gingl
the three or four sensory cells innervating the sensillum, two possess unbranched outer
dendritic segments (1,2) extending up to the apex of the cuticular peg. The outer segment of
the third takes on various forms. It is shorter; sometimes unbranched (B,C), sometimes
forked with slender extensions (A) or even with lamellae (D,E) that project into the peg (E)
or end below its base (D). The fourth cell, when found (A,B), maintains its ciliary structure
for the length of its outer segment and terminates well short of the peg ( 4 ). A small inner
receptor lymph cavity (i/c) encloses the central portion of the dendrites, between their inner
and outer segments (1 -4), and is surrounded by sheath cells. Beside these cells, or just outside the dendritic sheath, the more voluminous outer receptor lymph cavity (ole) is present.
In Cupiennius, a pore at the tip of the nipple-shaped sensilla leads directly into the lumen of
the dendritic sheath (F). Each sensillum is capped by a thin layer of amorphous material,
found also in the pore and then encasing the terminals of the three unbranched dendrites (J3). This layer merges with the dense lymph bathing the outer segments further inside
4 Humidity Transduction: Functional Interpretation of
Sensillum Structure
Of paramount interest in hygroreception is the manner in which humidity acts on
the hygroreceptive sensory cells. Hygroreceptors were proposed·to operate as mechanical hygrometers, psychrometers, and electrochemical hygrometers (for reviews see Tichy and Loftus 1996; Steinbrecht 1999). The underlying models are
quite different and were developed for different sensilla. Might it just be that the
structural diversity even among insect hygroreceptive sensilla is too great for a
single model to apply in all cases? Structural characteristics of the interior of the
sensillum wall, the dendritic sheath, the receptor lymph cavities, and the dendritic
segments are essential to the understanding of the models.
Pores are generally absent in the thermo-hygroreceptive sensilla of insects. In
the spider, the sensillum displays an apical pore filled with dense material. The
material is piled up on the outside, apparently discharging through the pore
(Fig. 3F).
In insect sensilla, a sheath encloses the dendrites in a lumen and, together with
the surrounding sheath cells, separates them from the outer lymph cavity (Fig. 3AE). The sheath may have an important function. It may seal off the hygroreceptors
from the water inside the body. In the spider's tip-pore sensilla, the dendritic
sheath extends up to the pore, where it fuses laterally with the cuticle wall
(Fig. 3F).
In insect sensilla two lymph cavities, filled with extracellular fluid, are present
(Fig. 3A-E). The inner cavity surrounds the sensory cells in the region of their
cilia. From there the cavity proceeds distally and becomes the lumen of the dendritic sheath. In this narrow canal a lymph surrounds the dendritic outer segments
from their ciliary region up to the tips. The lymph appears to form layers, differing
in electron density and wrapping the dendrites.
Harald Tichy and Ewald Gingl
the three or four sensory cells innervating the sensillum, two possess unbranched outer
dendritic segments (1,2) extending up to the apex of the cuticular peg. The outer segment of
the third takes on various forms. It is shorter; sometimes unbranched (B,C), sometimes
forked with slender extensions (A) or even with lamellae (D,E) that project into the peg (E)
or end below its base (D). The fourth cell, when found (A,B), maintains its ciliary structure
for the length of its outer segment and terminates well short of the peg ( 4 ). A small inner
receptor lymph cavity (i/c) encloses the central portion of the dendrites, between their inner
and outer segments (1 -4), and is surrounded by sheath cells. Beside these cells, or just outside the dendritic sheath, the more voluminous outer receptor lymph cavity (ole) is present.
In Cupiennius, a pore at the tip of the nipple-shaped sensilla leads directly into the lumen of
the dendritic sheath (F). Each sensillum is capped by a thin layer of amorphous material,
found also in the pore and then encasing the terminals of the three unbranched dendrites (J3). This layer merges with the dense lymph bathing the outer segments further inside
4 Humidity Transduction: Functional Interpretation of
Sensillum Structure
Of paramount interest in hygroreception is the manner in which humidity acts on
the hygroreceptive sensory cells. Hygroreceptors were proposed·to operate as mechanical hygrometers, psychrometers, and electrochemical hygrometers (for reviews see Tichy and Loftus 1996; Steinbrecht 1999). The underlying models are
quite different and were developed for different sensilla. Might it just be that the
structural diversity even among insect hygroreceptive sensilla is too great for a
single model to apply in all cases? Structural characteristics of the interior of the
sensillum wall, the dendritic sheath, the receptor lymph cavities, and the dendritic
segments are essential to the understanding of the models.
Pores are generally absent in the thermo-hygroreceptive sensilla of insects. In
the spider, the sensillum displays an apical pore filled with dense material. The
material is piled up on the outside, apparently discharging through the pore
(Fig. 3F).
In insect sensilla, a sheath encloses the dendrites in a lumen and, together with
the surrounding sheath cells, separates them from the outer lymph cavity (Fig. 3AE). The sheath may have an important function. It may seal off the hygroreceptors
from the water inside the body. In the spider's tip-pore sensilla, the dendritic
sheath extends up to the pore, where it fuses laterally with the cuticle wall
(Fig. 3F).
In insect sensilla two lymph cavities, filled with extracellular fluid, are present
(Fig. 3A-E). The inner cavity surrounds the sensory cells in the region of their
cilia. From there the cavity proceeds distally and becomes the lumen of the dendritic sheath. In this narrow canal a lymph surrounds the dendritic outer segments
from their ciliary region up to the tips. The lymph appears to form layers, differing
in electron density and wrapping the dendrites.