276
Harald Tichy and Ewald Ging1
The position of hygro-thermoreceptive sensilla in the exosceleton appears to reflect a tradeoff between exposure and protection. Some sensilla rise from the floor
of a crater (Figs. 2AB, 3D, Carausius morosus, Altner et al. 1978), or are entrenched in a groove (Fig. 3E, Bombyx mori, Steinbrecht 1989; Steinbrecht et al.
1989; Steinbrecht and MUller 1991; Zimmermann 1991) or located in a heavily
walled pit (Fig. 3B, Apis mellifera, Yokohari et al 1982; Yokohari 1983). Some
are hidden in a hole (Fig. 3C, Locusta migratoria, Altner et al. 1981). The sensillum of the cockroach displays the tightest fitting protective wall, so tight that it
can be taken for a thin outer wall of the sensillum with a ring-shaped opening just
under the cap (Fig. 3A, Periplaneta americana, Yokohari 1981; Tominaga and
Yokohari 1982). In the spider, the tarsal organ encloses seven nipple-shaped sensilla as a group (Figs. 2D-F, 3F, Cupiennius salei, Anton and Tichy 1994) in its
tiny air-filled cavity.
In insects, the dendrites of two of the three sensory cells are unbranched. In
some cases they extend out to the apex of the peg and fill its lumen completely
(Figs. 3A-D, 4B); in others, they terminate well short of it (bombycid moth, Fig.
3E). The dendrite of the third sensory cell takes different forms and extensions. It
is unbranched in the honeybee, ending at some distance from the apex of the peg
(Fig. 3B) and branched in the migratory locust, ending below the base of the peg
(Figs. 3C, 4B). In the cockroach, the third dendrite branches and sends numerous
slender extensions into the cuticular peg (Figs. 3A, 4B). It is folded or even transformed into lamellae that terminate below the cuticle in the stick insect (Fig. 3D)
and the bombycid moth (Fig. 3E),
In the moth, a few of the slender lamellae may enter the cuticular peg. A special
location is found in the satumiid moth Antheraea, where the lamellae lie within an
elongated socket which rises up to 60 J.lm above the antenna! surface (Fig. 4B,
Haug 1986; Zimmerman 1991). For the spider sensilla the situation is different.
The apical pore is filled with dense material (Figs. 3F, 4B). All sensory cells have
unbranched dendrites that terminate at the tip pore. Which of the dendrites is the
source of the thermoreceptive activity observed in the recordings? In insects, two
of the three cells are quite long, lie very close together, and extend far out into the
peg. It is reasonable to assume that they have similar function and belong to the
two hygroreceptors. Consequently, the third dendrite is considered to be thermoreceptive. Transduction in thermoreceptors has been attributed to the temperature
dependence of both a sodium/potassium pump and the conductances of sodium
and potassium channels (Braun et al. 1990). The number of these molecular receptors is expected to rise with dendrite size. A larger number will increase the
signal-to-noise ratio and thereby improve absolute temperature sensitivity. Apart
from the size the functionally most significant feature of the thermoreceptive dendrite is its position. Whereas the dendrite of the spider warm cell extends to the tip
of the sensillum and terminates just inside a single pore opening (Figs. 2F, 3F,
4B), the dendrites of insect cold cells end beneath the base of poreless sensilla
(Figs. 2B, 3A-E, 4B). That is, their tips are not positioned above the surface of the
antenna. In the satumiid moth Antheraea, however, the cold-cell dendrite rises
above the antenna! surface by being contained in an elongated socket that bears
the sensillum (Fig. 4B).
Harald Tichy and Ewald Ging1
The position of hygro-thermoreceptive sensilla in the exosceleton appears to reflect a tradeoff between exposure and protection. Some sensilla rise from the floor
of a crater (Figs. 2AB, 3D, Carausius morosus, Altner et al. 1978), or are entrenched in a groove (Fig. 3E, Bombyx mori, Steinbrecht 1989; Steinbrecht et al.
1989; Steinbrecht and MUller 1991; Zimmermann 1991) or located in a heavily
walled pit (Fig. 3B, Apis mellifera, Yokohari et al 1982; Yokohari 1983). Some
are hidden in a hole (Fig. 3C, Locusta migratoria, Altner et al. 1981). The sensillum of the cockroach displays the tightest fitting protective wall, so tight that it
can be taken for a thin outer wall of the sensillum with a ring-shaped opening just
under the cap (Fig. 3A, Periplaneta americana, Yokohari 1981; Tominaga and
Yokohari 1982). In the spider, the tarsal organ encloses seven nipple-shaped sensilla as a group (Figs. 2D-F, 3F, Cupiennius salei, Anton and Tichy 1994) in its
tiny air-filled cavity.
In insects, the dendrites of two of the three sensory cells are unbranched. In
some cases they extend out to the apex of the peg and fill its lumen completely
(Figs. 3A-D, 4B); in others, they terminate well short of it (bombycid moth, Fig.
3E). The dendrite of the third sensory cell takes different forms and extensions. It
is unbranched in the honeybee, ending at some distance from the apex of the peg
(Fig. 3B) and branched in the migratory locust, ending below the base of the peg
(Figs. 3C, 4B). In the cockroach, the third dendrite branches and sends numerous
slender extensions into the cuticular peg (Figs. 3A, 4B). It is folded or even transformed into lamellae that terminate below the cuticle in the stick insect (Fig. 3D)
and the bombycid moth (Fig. 3E),
In the moth, a few of the slender lamellae may enter the cuticular peg. A special
location is found in the satumiid moth Antheraea, where the lamellae lie within an
elongated socket which rises up to 60 J.lm above the antenna! surface (Fig. 4B,
Haug 1986; Zimmerman 1991). For the spider sensilla the situation is different.
The apical pore is filled with dense material (Figs. 3F, 4B). All sensory cells have
unbranched dendrites that terminate at the tip pore. Which of the dendrites is the
source of the thermoreceptive activity observed in the recordings? In insects, two
of the three cells are quite long, lie very close together, and extend far out into the
peg. It is reasonable to assume that they have similar function and belong to the
two hygroreceptors. Consequently, the third dendrite is considered to be thermoreceptive. Transduction in thermoreceptors has been attributed to the temperature
dependence of both a sodium/potassium pump and the conductances of sodium
and potassium channels (Braun et al. 1990). The number of these molecular receptors is expected to rise with dendrite size. A larger number will increase the
signal-to-noise ratio and thereby improve absolute temperature sensitivity. Apart
from the size the functionally most significant feature of the thermoreceptive dendrite is its position. Whereas the dendrite of the spider warm cell extends to the tip
of the sensillum and terminates just inside a single pore opening (Figs. 2F, 3F,
4B), the dendrites of insect cold cells end beneath the base of poreless sensilla
(Figs. 2B, 3A-E, 4B). That is, their tips are not positioned above the surface of the
antenna. In the satumiid moth Antheraea, however, the cold-cell dendrite rises
above the antenna! surface by being contained in an elongated socket that bears
the sensillum (Fig. 4B).