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.