Maxillary Palps Research Articles

Scanning of antennae and maxillary palps of anthropophilic Aedes aegypti and ornithophilic Culex pipiens as potential arbovirus vectors

Background and Aim: Efficient mosquito vectors are required to persist and propagate arthropod-borne diseases that seriously affect impoverished populations worldwide. Mosquito sensilla plays a crucial role in host-seeking and disease transmission to humans. This study aimed to distinguish between the several types of sensilla found on the antennae and maxillary palps of Culex pipiens and Aedes aegypti, matching this diversity with host preference and disease transmission. Materials and Methods: Overall, 1300 mosquitoes were collected and examined using dissection and light microscopy. Scanning electron microscopy was used to identify and describe the diverse types of sensilla found on the antennae and maxillary palps of C. pipiens and A. aegypti. Results: In total, 900 C. pipiens and 400 A. aegypti mosquitoes were identified. The antennae and maxillary palps of C. pipiens and A. aegypti carry both sensilla trichoidea and sensilla chaetica. The C. pipiens antenna has long and short grooved peg sensilla, whereas A. aegypti lacks long pegs and expresses only occasional short pegs. The maxillary palps express Capitate pegs in both mosquito species and exclusively show sensilla campaniform in A. aegypti. Conclusion: The lack of long-grooved pegs and the presence of few short pegs, along with campaniform sensilla, limit the host range of A. aegypti and reduce its susceptibility to many infections, unlike C. pipiens. Keywords: Aedes aegypti, Culex pipiens, Scanning electron microscopy, Sensilla.

Molecular Functions and Physiological Roles of Gustatory Receptors of the Silkworm Bombyx mori.

Complete elucidation of members of the gustatory receptor (Gr) family in lepidopteran insects began in the silkworm Bombyx mori. Grs of lepidopteran insects were initially classified into four subfamilies based on the results of phylogenetic studies and analyses of a few ligands. However, with further ligand analysis, it has become clear that plant secondary metabolites are important targets not only for Grs in the bitter subfamily but also for the Drosophila melanogaster Gr43a orthologue subfamily and Grs in the sugar subfamily. Gene knockout experiments showed that B. mori Gr6 (BmGr6) and BmGr9 are involved in the recognition of the feeding-promoting compounds chlorogenic acid and isoquercetin in mulberry leaves by the maxillary palps, suggesting that these Grs are responsible for palpation-dependent host recognition without biting. On the other hand, BmGr expression was also confirmed in nonsensory organs. Midgut enteroendocrine cells that produce specific neuropeptides were shown to express specific BmGrs, suggesting that BmGrs are involved in the induction of endocrine secretion in response to changes in the midgut contents. Furthermore, gene knockout experiments indicated that BmGr6 is indeed involved in the secretion of myosuppressin. On the other hand, BmGr9 was shown to induce signal transduction that is not derived from the intracellular signaling cascade mediated by G proteins but from the fructose-regulated cation channel of BmGr9 itself. Cryogenic electron microscopy revealed the mechanism by which the ion channel of the BmGr9 homotetramer opens upon binding of fructose to the ligand-binding pocket. Research on BmGrs has contributed greatly to our understanding of the functions and roles of Grs in insects.

Transcriptomic and Gene Expression Analysis of Chemosensory Genes from White Grubs of Hylamorpha elegans (Coleoptera: Scarabaeidae), a Subterranean Pest in South America.

Olfaction and gustation processes play key roles in the life cycle of insects, such as finding and accepting food sources, oviposition sites, and mates, among other fundamental aspects of insect development. In this context, chemosensory genes found in sensory organs (e.g., antennae and maxillary palps) are crucial for understanding insect behaviour, particularly the phytophagous behaviour of insect pests that attack economically important crops. An example is the scarab beetle Hylamorpha elegans, which feeds on the roots of several crops important for livestock in its larval stage. In this study, chemosensory gene candidates of H. elegans white grubs identified through the head transcriptome and phylogenetic and tissue-biased gene expression (antennae, head without antennae, and legs) have been reported. Overall, 47 chemosensory genes were identified (2 ORs, 1 GR, 11 IRs, 9 CSPs, and 24 OBPs). Gene expression analysis revealed the predominant presence of IRs in the legs, whereas ORs and the GR were present in the heads and/or antennae. Particularly, HeleOBP9 and HeleCSP2 were significantly expressed in the head but not in the antennae or legs; these and other genes are discussed as potential targets in the context of H. elegans management.

A new species of bristletail of the genus <i>allopsontus</i> silv. (microcoryphia, machilidae) from western crimea

A new species of bristletail, Allopsontus tauricus sp. n., is described from western Crimea. In the external shape of the eyes, the structure of the mandibles, legs, abdominal sternites, ovipositor and male parameres, it seems to be especially similar to A. europeus (Kaplin 1983) from southern Crimea. Their main differences are the relative length of the cerci, the colour of the eyes and the shape of the paired ocelli. The structure of the distal part of the antennal flagellum, the chaetotaxy of the maxillary and lower labial palps in the male also differ significantly, as well as the ratio of the length of the styli to the length of the coxites of abdominal segments VIII and IX in both sexes.

Control of fate specification within the dorsal head of Drosophila melanogaster.

During development, unique combinations of transcription factors and signaling pathways carve the nascent eye-antennal disc of the fruit fly Drosophila melanogaster into several territories that will eventually develop into the compound eye, ocelli, head epidermis, bristles, antenna and maxillary palpus of the adult head. Juxtaposed patterns of Hedgehog (Hh) and Decapentaplegic (Dpp) initiate compound eye development, while reciprocal domains of Dpp and Wingless (Wg) induce formation of the antennal and maxillary palp fields. Hh and Wg signaling, but not Dpp, contribute to the patterning of the dorsal head vertex. Here, we show that combinatorial reductions of the Pax6 transcription factor Twin of Eyeless and either the Wg pathway or the Mirror (Mirr) transcription factor trigger a transformation of the ocelli into a compound eye and the neighboring head epidermis into an antenna. These changes in fate are accompanied by the ectopic expression of Dpp, which might be expected to trigger these changes in fate. However, the transformation of the field cannot be replicated by increasing Dpp levels alone despite the recreation of adjacent Hh-Dpp and Wg-Dpp domains. As such, the emergence of these ectopic organs occurs through a unique regulatory path.

The odorant-binding protein genes obp67 and obp56d-like encode products that guide oviposition site selection in the Asian tiger mosquito, Aedes albopictus.

Aedes albopictus is an important vector of arboviruses and prefers small containers of stagnant water as oviposition sites. One of the mechanisms mosquitoes use to search for suitable oviposition sites is relying on odor cues from prospective sites and their surroundings. The genetic and molecular bases of this behavior are not known for Ae.albopictus. Oviposition site-searching behavior can be separated into 2 stages: container location and water detection. We applied a glue compound to the antennae and the maxillary palps of adult females to mask their ability to detect molecules that may guide them to preferred oviposition sites. Treatment of the antennae significantly reduces the location index (P<0.001), indicating a decreased ability to find oviposition sites, whereas no significant difference was observed in mosquitoes with maxillary palps treated with the same glue compound (P>0.05). The detection time, measured as the duration from contact with the water surface to the deposition of the first egg, was extended in mosquitoes with treated antennae or maxillary palps, supporting the conclusion that olfaction is involved in the detection of oviposition site. Transcriptomic analysis identified differentially expressed olfactory-related genes, including obp67, obp56d-like, obp19d-like and obp67-like. RNA interference (RNAi)-mediated knockdown of obp67 and obp56d-like significantly affected the location index and detection time, respectively. Cas9/guide RNA-mediated knockout of obp56d-like resulted in a prolonged detection time, compared with the wild type (P<0.05). These findings help to elucidate aspects of the olfactory mechanisms involved in Ae.albopictus oviposition site selection, and provide a basis for the development of mosquito surveillance and control strategies.

Sensillar Ultrastructure of the Antennae and Maxillary Palps of the Warble Fly Oestromyia leporina (Pallas, 1778) (Diptera: Oestridae).

Despite the development of molecular techniques, morphological phylogeny still remains integral in underpinning the relationship between some clades of Calyptratae, especially the ones with fast radiation, such as those in Oestridae (Diptera: Brachycera), yet few synapomorphy has been proposed for adults in this family. Using scanning electron microscopy, we investigated the morphological structure and ultrastructure of the antennae and maxillary palps of adult Oestromyia leporina (Hypodermatinae, Oestridae). One type of trichoid sensillum (Tr), three types of basiconic sensilla (Ba I, Ba II, and Ba III), one type of coeloconic sensillum (Co I), and one type of clavate sensillum (Cl) were found on the antennal postpedicel. Surprisingly, this species has the most complex types of sensilla on the maxillary palps that have been reported in Calyptratae so far, with two types of coeloconic sensilla (Co II and Co III) and two types of mechanoreceptors. We then identified three common characteristics on the arista of Oestridae (Hypodermatinae, Oestrinae, Gasterophilinae and Cuterebrinae) that are potential synapomorphies. These characteristics indicate the value of the morphology of maxillary palps and aristae in taxonomy studies of Calyptratae.

Identification and Functional Analysis of Odorant Binding Proteins in Apriona germari (Hope).

Apriona germari (Hope) presents a significant threat as a dangerous wood-boring pest, inflicting substantial harm to forest trees. Investigating the olfactory sensory system of A. germari holds substantial theoretical promise for developing eco-friendly control strategies. To date, however, the olfactory perception mechanism in A. germari remains largely unknown. Therefore, we performed transcriptome sequencing of A. germari across four distinct body parts: antennae, foreleg tarsal segments, mouthparts (maxillary and labial palps), and abdomen terminals, pinpointing the odorant binding protein (OBP) genes and analyzing their expression. We found eight AgerOBPs (5, 19, 23, 25, 29, 59, 63, 70) highly expressed in the antennae. In our competitive binding experiments, AgerOBP23 showed strong binding abilities to the pheromone component fuscumol acetate, eight plant volatiles (farnesol, cis-3-hexenal, nerolidol, myristol acetate, cis-3-hexenyl benzoate, (-)-α-cedrene, 3-ethylacetophenone, and decane), and four insecticides (chlorpyrifos, phoxim, indoxacarb, and cypermethrin). However, AgerOBP29 and AgerOBP63 did not show prominent binding activities to these tested chemicals. Through homology modeling and molecular docking, we identified the key amino acid sites involved in the binding process of AgerOBP23 to these ligands, which shed light on the molecular interactions underlying its binding specificity. Our study suggests that AgerOBP23 may serve as a potential target for future investigations of AgerOBP ligand binding. This approach is consistent with the reverse chemical ecology principle, establishing the groundwork for future studies focusing on attractant or repellent development by exploring further the molecular interactions between OBP and various compounds.

A conserved odorant receptor underpins borneol-mediated repellency in culicine mosquitoes.

The use of essential oils derived from the camphor tree to repel mosquitoes is an ancient practice that originated in Southeast Asia and gradually spread to China and across Europe via the Maritime Silk Road. The olfactory mechanisms by which these oils elicit avoidance behavior are unclear. Here we show that plant bicyclic monoterpenoids and borneol specifically activate a neural pathway that originates in the orphan olfactory receptor neuron of the capitate peg sensillum in the maxillary palp, and projects to the mediodorsal glomerulus 3 in the antennal lobe. This neuron co-locates with two olfactory receptor neurons tuned to carbon dioxide and octenol that mediate human-host detection. We also confirm that borneol elicits repellency against human-seeking female mosquitoes. Understanding the functional role of the mosquito maxillary palp is essential to investigating olfactory signal integration and host-selection behavior.

The role of the major chemosensory organs in the host-seeking activity of Anopheles coluzzii (Diptera: Culicidae).

Anopheles coluzzii (Coetzee & Wilkerson) and its sibling species Anopheles gambiae s.s. (Giles) are highly anthropophilic and among the major malaria vectors in sub-Saharan Africa. Mosquitoes use various senses to find hosts, but rely primarily on olfaction. Therefore, the mosquito olfactory system has been studied extensively, including a variety of studies comparing chemosensory gene expression between An. coluzzii and its zoophilic sibling species Anopheles quadriannulatus (Theobald). These studies revealed species-specific chemosensory gene expression in the antennae and maxillary palps, which raised the question of a potential role for the palps in determining species-specific host preferences. To answer this question, we mechanically ablated the antennae, maxillary palps, and labella, and ran both control and ablated mosquitoes through a dual-port olfactometer. While we aimed to identify the organs responsible for vertebrate host choice, the ablated mosquitoes exclusively responded to human odor, so we were unable to do so. However, we were able to refine our understanding of the roles of these organs in host-seeking activation (leaving the release cage) as well as odor response (entering an odor port). As expected, the antennae are the most important organs to both behaviors: activation was roughly halved and vertebrate odor response was abolished in antennae-ablated mosquitoes. Maxillary palp ablation had little impact on activation, but reduced odor response to a similar degree as the exclusion of CO2. Finally, while labellar ablation dramatically reduced activation (probably associated with the inability to feed), it had little impact on odor response, suggesting that any labellar role in host choice is likely not olfactory.

Scuttle fly Megaselia scalaris (Loew) (Diptera: Phoridae) endoparasitoid as a novel biocontrol agent against adult American cockroaches (Periplaneta americana)

The American cockroach, Periplaneta americana (Linnaeus, 1758) (Blattodea: Blattidae), is one of the most common pests that thrive in diverse environments and carries various pathogens, causing critical threats to public health and the ecosystem. We thus report in this study the first observation of decapitated American cockroaches as a result of infestation with scuttle fly parasitoids. Interestingly, behavioral alterations in the form of zombification-like behavior could be observed in cockroaches reared in the laboratory before being decapitated, implying that the insect targets cockroach heads. To identify this parasitoid, cockroaches’ corpora were isolated in jars, and apodous larvae were observed. Larvae developed into small coarctate pupae, and adults emerged. The scuttle flies were collected and exhibited tiny black, brown, to yellowish bodies. The fly was initially identified based on its morphological properties as a member of the order Diptera, family Phoridae. To provide further insights into the morphological attributes of the phorid species, the fly was examined using a scanning electron microscope (SEM) and then identified as Megaselia scalaris accordingly. SEM analysis revealed the distinctive structure of M. scalaris concerning the head, mouth parts, and legs. Specifically, the mouth parts include the labrum, labellum, rostrum, and maxillary palps. Although further investigations are still required to understand the complicated relationships between M. scalaris and American cockroaches, our findings provide a prominent step in the control of American cockroaches using M. scalaris as an efficient biological control agent.

Molecular and cellular organization of odorant binding protein genes in Drosophila

Chemosensation is important for the survival and reproduction of animals. The odorant binding proteins (OBPs) are thought to be involved in chemosensation together with chemosensory receptors. While OBPs were initially considered to deliver hydrophobic odorants to olfactory receptors in the aqueous lymph solution, recent studies suggest more complex roles in various organs. Here, we use GAL4 transgenes to systematically analyze the expression patterns of all 52 members of the Obp gene family and 3 related chemosensory protein genes in adult Drosophila, focusing on chemosensory organs such as the antenna, maxillary palp, pharynx, and labellum, and other organs such as the brain, ventral nerve cord, leg, wing, and intestine. The OBPs were observed to express in diverse organs and in multiple cell types, suggesting that these proteins can indeed carry out diverse functional roles. Also, we constructed 10 labellar-expressing Obp mutants, and obtained behavioral evidence that these OBPs may be involved in bitter sensing. The resources we constructed should be useful for future Drosophila OBP gene family research.

A new species of the vitismin cockroach genus Perspicuus Koubová, 2020 from the Late Cretaceous (Santonian) of Hungary

Mesozoic amber cockroaches are rare compared to sedimentary imprints and are only known from Myanmar, Lebanon, France, Russia and USA. Perspicuus csincsii sp. n. from the Late Cretaceous (Santonian) ajkaite amber of Hungary with very long cerci, cup-like terminal palpomere of the maxillary palp and fully carinated legs, is clearly distinguished from all other Vitisminae species by the unique, black forewing with small light-coloured dots/stripes and the absence of pubescence on the wings (autapomorphies). The second cockroach described from the ajkaite adds important data to the distribution pattern of Vitisminae during the Late Cretaceous and further highlights the similarity of the ajkaite and burmite biotas.

Larval internal morphology of the introduced blowfly Chrysomya megacephala (Fabricius, 1794) (Diptera: Calliphoridae)

The blowfly Chrysomya megacephala (Diptera: Calliphoridae) colonizes different environments and is found in many global regions. Its importance includes its involvement in the decomposition of organic matter, forensic entomology, maggot therapy, bioprospecting, source of biodiesel, industry, stabilization of heavy metals in soil, pollination, and as a vector of diseases and causing secondary myiasis. Internal morphological studies contribute to understanding insect's function and provide basic data for applied areas with interest in this specie. The objective of this study was to describe the internal morphology of C. megacephala larvae. Post-embryonic development occurred over 240 h, equally divided into larval and pupal stages. After hatching, the larvae averaged <1 mm in length and 0.02 mg in weight, growing to 13.83 mm and 50.4 mg in the third instar. We utilized whole, third-instar larvae to perform histology. Sensory organs were observed in the cephalic region, antennae, maxillary palps, and ventral organ. The alimentary canal comprises a foregut, midgut, and hindgut, including tracheal insertions and accessory organs, such as salivary glands. The midgut is the largest organ of the alimentary canal and fat body are present in body cavity and intersegmental space. Thus, our data about the internal morphology of C. megacephala larvae describe the structure and position of the alimentary canal, salivary gland, sensory organs, and fat body.

The maxillary palps of Tephritidae are selectively tuned to food volatiles and diverge with ecology

The maxillary palp is an auxiliary olfactory organ in insects, which, different from the antennae, is equipped with only a few olfactory sensory neuron (OSN) types. We postulated that these derived mouthpart structures, positioned at the base of the proboscis, may be particularly important in mediating feeding behaviors. As feeding is spatio-temporally segregated from oviposition in most Tephritidae, this taxonomic group appears quite suitable to parse out sensory breadth and potential functional divergence of palps and antennae. Scanning electron microscopy and anterograde staining underlined the limited palpal olfactory circuit in Tephritidae: only three morphological subtypes of basiconic sensilla were found, each with two neurons, and project to a total of six antennal lobe glomeruli in Bactrocera dorsalis. Accordingly, the palps detected only few volatiles from the headspace of food (fermentation and protein lures) and fruit (guava and mango) compared to the antennae (17 over 77, using gas-chromatography coupled electrophysiology). Interestingly, functionally the antennae were more tuned to fruit volatiles, detecting eight times more fruit than food volatiles (63 over 8), whereas the number of fruit and food volatile detection was more comparable in the palps (14 over 8). As tephritids diverge in oviposition preferences, but converge on food substrates, we postulated that the receptive ranges of palpal circuits would be more conserved compared to the antennae. However, palpal responses of three tephritid species that differed in phylogenetic relatedness and ecologically niche, diverged across ecological rather than phylogenetic rifts. Two species with strongly overlapping ecology, B. dorsalis and Ceratitis capitata, showed inseparable response profiles, whereas the cucurbit specialist Zeugodacus cucurbitae strongly diverged. As Z. cucurbitae is phylogenetically placed between B. dorsalis and C. capitata, the results indicate that ecology overrides phylogeny in the evolution of palpal tuning, in spite of being predisposed to detecting food volatiles.

First data on early instar Afrotropical Miltogramminae (Diptera: Sarcophagidae) reveal unexpected morphological diversity

Abstract First instar larvae of the genera Hoplacephala Macquart, 1846 and Lamprometopia Macquart, 1846 are described for the first time, along with the first instar larva of an Afrotropical species of the genus Phylloteles Loew, 1844. Their morphology is analyzed using a combination of various microscopy techniques, confirming their close phylogenetic affinity with other members of the tribe Sarcotachinini. Substantial morphological differences are observed between the investigated genera, as well as in comparison to other known larvae of Miltogramminae. Noteworthy unique character states are identified in the shape of the cephaloskeleton, the sculpture of the integument, and the shape of the papilla of the thoracic and abdominal segments. The larva of Hoplacephala sp. stands out as the most peculiar, featuring a massive cephaloskeleton, integument armoured by cuticular scales, and modification of body papillae into fleshy processes. The first instar larva of Phylloteles picifrons Pape, 1985 possesses a large, bladder-like structure with unknown function dorsally on the first thoracic segment. All documented larvae are characterized by a maxillary palpus forming a distinct protuberance with elongated second sensillum basiconicum and the absence of a dorsal bridge of the cephaloskeleton. This last character state indicates a parasitic or parasitoid life history for all studied taxa.

Taxonomic review of the monotypic genus Dinahia Bechyn, 1946 (Coleoptera: Chrysomelidae: Chrysomelinae).

The monotypic genus Dinahia Bechyn, 1946, endemic to the Atlantic Forest (Rio Grande do Sul, Santa Catarina, and So Paulo), is reviewed. Dinahia privigna Bechyn, 1946 is redescribed, and, for the first time, the aedeagus and spermatheca of this species are described and illustrated herein. Additionally, a diagnosis for the genus, a nomenclatural history, photographs of both nontype specimens and the syntype, a geographic distribution map, and a discussion of Dinahia and similar genera according to the bibliography (Calligrapha Chevrolat, 1836; Cosmogramma Erichson, 1847; Desmogramma Erichson, 1847; Grammomades Achard, 1923; and Zygogramma Chevrolat, 1836) are provided. This discussion incorporates a comparative table encompassing characters extracted from the antennae, maxillary palps, prosternal, mesoventral, and metaventral processes, V tarsomeres, and claws. Additionally, it includes an illustrated replacement couplet for step 22 in Sampaio & Fonseca's (2023) identification key, both to elucidate Dinahia's differentiations and similarities from these five genera.

Ultrastructure of the sensilla on the larval antennae and mouthparts of Chilo infuscatellus (Snellen, 1890) (Lepidoptera: Crambidae)

The sugarcane shoot borer Chilo infuscatellus (Snellen, 1890) is a major sugarcane pest in Lepidoptera, but little information is available on its larval ultrastructure. Here, the sensilla of the antennae and mouthparts of the mature larva of C. infuscatellus were investigated using scanning electron microscopy. The antenna is three-segmented and possesses two types of sensilla, as well as three sensilla basiconica and two sensilla chaetica on the apical surface of the middle pedicel; three sensilla basiconica on the apex of the distal flagellum. The mandibulate mouthparts consist of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a hypopharynx. The labrum possesses six pairs of sensilla chaetica; the membranous epipharynx bears three pairs of flattened sensilla chaetica, three pairs of epipharyngeal sensilla, and a pair of oval sensilla digitiformia. The mandible is equipped with two sensilla chaetica on the posterolateral surface. The maxillae are well developed, each formed by a cardo, a stipes, a galea and a two-segmented maxillary palp. Each galea carries six sensilla basiconica and two large sensilla styloconica. The maxillary palp is furnished with 9 sensilla basiconica, one sensillum digitiformium and one sensillum placodeum on the distal segment. On the labium, each labial palp has a sensillum chaeticum with apical bifurcation and a thick sensillum styloconicum. This paper briefly discusses the possible function of sensilla on antennae and mouthparts.

Anatomical organization of the cerebrum of the praying mantis Hierodula membranacea.

Many predatory animals, such as the praying mantis, use vision for prey detection and capture. Mantises are known in particular for their capability to estimate distances to prey by stereoscopic vision. While the initial visual processing centers have been extensively documented, we lack knowledge on the architecture of central brain regions, pivotal for sensory motor transformation and higher brain functions. To close this gap, we provide a three-dimensional (3D) reconstruction of the central brain of the Asian mantis, Hierodula membranacea. The atlas facilitates in-depth analysis of neuron ramification regions and aides in elucidating potential neuronal pathways. We integrated seven 3D-reconstructed visual interneurons into the atlas. In total, 42 distinct neuropils of the cerebrum were reconstructed based on synapsin-immunolabeled whole-mount brains. Backfills from the antenna and maxillary palps, as well as immunolabeling of γ-aminobutyric acid (GABA) and tyrosine hydroxylase (TH), further substantiate the identification and boundaries of brain areas. The composition and internal organization of the neuropils were compared to the anatomical organization of the brain of the fruit fly (Drosophila melanogaster) and the two available brain atlases of Polyneoptera-the desert locust (Schistocerca gregaria) and the Madeira cockroach (Rhyparobia maderae). This study paves the way for detailed analyses of neuronal circuitry and promotes cross-species brain comparisons. We discuss differences in brain organization between holometabolous and polyneopteran insects. Identification of ramification sites of the visual neurons integrated into the atlas supports previous claims about homologous structures in the optic lobes of flies and mantises.

Molecular basis of host plant recognition by silkworm larvae

Herbivorous insects can identify their host plants by sensing plant secondary metabolites as chemical cues. We previously reported the two-factor host acceptance system of the silkworm Bombyx mori larvae. The chemosensory neurons in the maxillary palp (MP) of the larvae detect mulberry secondary metabolites, chlorogenic acid (CGA), and isoquercetin (ISQ), with ultrahigh sensitivity, for host plant recognition and feeding initiation. Nevertheless, the molecular basis for the ultrasensitive sensing of these compounds remains unknown. In this study, we demonstrated that two gustatory receptors (Grs), BmGr6 and BmGr9, are responsible for sensing the mulberry compounds with attomolar sensitivity for host plant recognition by silkworm larvae. Calcium imaging assay using cultured cells expressing the silkworm putative sugar receptors (BmGr4-10) revealed that BmGr6 and BmGr9 serve as receptors for CGA and ISQ with attomolar sensitivity in human embryonic kidney 293T cells. CRISPR/Cas9-mediated knockout (KO) of BmGr6 and BmGr9 resulted in a low probability of making a test bite of the mulberry leaves, suggesting that they lost the ability to recognize host leaves. Electrophysiological recordings showed that the loss of host recognition ability in the Gr-KO strains was due to a drastic decrease in MP sensitivity toward ISQ in BmGr6-KO larvae and toward CGA and ISQ in BmGr9-KO larvae. Our findings have revealed that the two Grs, previously considered to be sugar receptors, are molecules responsible for detecting plant phenolics in host plant recognition.