Maxillary palps olfactory sensilla

In addition to the sensilla styloconica, lepidopterous larvae possess gustatory sensilla on the maxillary palps. Eight basiconic sensilla are located on top of each palpus (see Figure 1). Five of them possess a terminal pore, and for that reason these sensilla might be considered as contact chemoreceptors. The remaining three show numerous small perforations all over the cuticle, which indicates an olfactory function (.8). The response spectra of these sensilla are, however, still obscure. 

Potential PBPs (OS-E and OS-F) U02545, U02546 identified on the third antennal segment of adult males (McKenna etal. 1994) PBP-related proteins found in different subsets of olfactory sensilla (Pikielny etal. 1994) First insect olfactory receptors (of unknown odorant selectivity) identified on the maxillary palp (Clyne etal. 1999)... 

In S. gregaria, immunocytochemical experiments showed selective CSP labeling of the outer lymph in diverse chemosensory organs, including contact sensilla of tarsi, maxillary palps, and antennae. However, in antennae, only sensilla chaetica were labeled with no labeling observed in olfactory or coeloconic sensilla (Angeli et al., 1999), suggesting a role for CSPs in contact chemoreception in Orthoptera. 

The second Drosophila appendage that bears olfactory sensilla is the maxillary palp (henceforth simply called palp). The ca. 60 olfactory sensilla on the dorsal and lateral sides of the palp are sw sensilla of the B type. They are structurally similar to antennal TB sensilla and contain two ORNs. In addition to olfactory sensilla the palp also bears ca. 20 large mechanosensory bristles. 

The diverse morphology of the olfactory organ among insect species provides a basis upon which classification has been based. Basically, the olfactory organ in the adult insect comprises two components on the head the antenna and the maxillary palp (Fig. la). Numerous sensilla cover the surface of the antennae and prevent direct contact of ORNs with the external environment. Each sensillum is filled with a potassium- and protein-rich fluid called sensillum lymph and houses one to four ORN dendrites. The small pits on the cuticle surfaces of sensilla allow contact of the ORN dendrite with volatile odorants that dissolve in the lymph. In Drosophila, the third segment of the antenna and of the maxillary palp possess approximately 1,200 and 120 ORNs, respectively. 

Fig. 1 Olfactory organ of insects. Insects have two types of olfactory organ antenna and maxillary palp. Antennae and maxillary palps both bear numerous sensilla. An individual sensillum houses between one and four dendrites of olfactory receptor neurons, a Morphology of the olfactory organ of the adult silkmoth. b Morphology of the olfactory organ of silkmoth larvae...

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