4 DISCUSSION
As largely consistent with reports for other pests, the olfactory system
of G. cantor is also an ideal target for pest control (Wu et al.,
2022). In this study, by analyzing
the oral appendages of G. cantor with scanning electronic
microscopy, we found several significant differences between males and
females. On the one hand, in the gross morphology of the mandible, we
found that the length and basal width of the mandibles of females were
significantly more developed than those of males, but there was no
significant difference in basal thickness. This suggests that the
functional differences between the ossified parts of the mandible of
males and females are mainly the depth of groove production and have
little effect on their shared feeding behavior. This is because the
palate of females is involved in the grooving process and has a greater
demand for mechanical sensing, especially for sensing the force required
by the palate during the grooving process, so it is related to the sex
difference in basal width and length. This has also been reported in
insects such as the desert locust Schistocerca gregaria Forskal
(Katel et al., 2021), in which the mandible participates in the grooving
process, and there are some behavioral differences between males and
females. On the other hand, in the ultrastructural morphology of the
mandible, we found that the number of ST I in the female mandible was
significantly higher than that in the male mandible. As an olfactory
sensor, ST I can help females identify and confirm oviposition grooving
more quickly through its olfactory in addition to feeding function. This
result indicated that this olfactory
sensor could help females recognize more chemical odors, which is
consistent with the results of previous studies on other insects (Dong
et al., 2020).
In addition, the oral appendages of adult G. cantor (labial
palpus and mandibular palpus) are mainly involved in host selection,
oviposition, mating and other behaviors (Zhang et al., 2018; Hall et
al., 2019). In the gross morphology of the labial palpus and mandibular
palpus, we also found that there was obvious dimorphism, which is
consistent with Aphis
gossypii Glover and Diaphorina citri Kuwayama (Ettay et
al.,2001;Bento et al.,2017), especially in terms of length. Each
subsegment and overall length showed obvious differences, and only the
basal width of the fourth segment of mandibular palps did not show
significant sex differences. This is related to the dominant role of
labial and mandibular papillae in female oviposition.
It is worth mentioning that our results on the ultrastructural
morphology of the labial palpus and mandibular palpus also confirmed
this conjecture, and we found that the SCh V, SCh IV and Bb of female
beetles are more developed than those of male beetles. A large number of
studies have shown that the main function of the SCh is to focus on
force and vibration induction, and the sex difference in the Bb inG. cantor is mainly related to its function of sensing the
position and movement frequency of labial and mandibular cumulus hair
during oviposition (Sun et al., 2011). Therefore, we hypothesized that
this was because G. cantor needs more developed mechanical
sensors to detect the vibration frequency and force of the oral
appendages in the process of oviposition localization and recognition.
Other sensilla showing female superiority mainly distributed in the
labial and mandibular palpus of G. cantor also include ST II, STB
IV, SC, SP and SCa. The functions of ST II have been confirmed in at
least two different types of olfactory sensilla cells in the antennae of
male cotton bollworms (Lopes et al., 2002; Dong et al., 2020). Similar
functions of this sensor have also been confirmed in our study of the
labial palpus and mandibular palpus of G. cantor . Moreover, some
studies have shown that STB IV in Liriomyza sativae Blanchard is
a kind of contact chemosensory apparatus and has taste function (Zhang
et al., 2017). Based on this, we conducted a further investigation and
found that STB IV shows female superiority in external morphology and
quantity both in the labial palpus and mandibular palpus of G.
cantor. In addition, SC, SP and SCa have been reported in a variety of
insects, such as Dyseriocrania subpurpurella and Pissodes
yunnanensis (Faucheux et al., 2008), showing that the main functions
are carbon dioxide sensing and temperature and humidity sensing. We
found that these functions also apply to the reproductive behavior ofG. cantor because the oviposition process is highly complex,
which requires the female to be sensitive to the appropriate
temperature, humidity and carbon dioxide concentration in the
environment, which is also in line with their relatively developed
results in females.
In summary, the present study investigated the external morphology of
the sensillum types of the oral appendages of G. cantor using
scanning electron microscopy to contribute to a better understanding of
host selection and courtship behavior in G. cantor and to future
studies linking these behaviors to electrophysiological mechanisms. The
observed differences in sensillum distribution and function will greatly
facilitate the design of more effective standardized control methods
against this pest.