Fine Morphology Of Mouthparts And Their Sensilla In Five Species Of Stored-product Beetles

Mouthparts are among the most important sensory and feeding structures in insects and comparative morphological study may help explain differences in feeding behavior and breadth of diet among species. Stored-product beetles cause significant damage significant damage and major post-harvest losses in global production by the adults and larvae feeding on stored grains and contaminating food products. To facilitate future study of feeding mechanisms in this insect, the ultrastructural morphology of mouthparts, including the distribution of sensilla in the adult maize weevil Sitophilus zeamais Motschulsky, bruchid weevil Callosobruchus chinensis(Linnaeus), red flour beetle Tribolium castaneum Herbst, and also the adult and larva saw-toothed grain beetle Oryzaephilus surinamensis(L.) and confused flour beetle Tribolium confusum Jacquelin du Val, was observed using a scanning electron microscope.The chewing mouthparts of these species generally resemble those of other coleopteran species. The overall blueprint of mouthparts includes labrum, mandible, maxillae, labium and epipharynx. Although there are some common features among these stored-product insects, the fine morphology, types and distribution of sensilla are clearly different in the species studied.The maize weevil mouthparts are located at the tip of the elongated rostrum. The sclerotized mandibles with 4 incisors each are asymmetric, serving as pincers. The maxilla ha longitudinal clefts with a four-segmented palp at each side of the labium. Four types of sensilla basiconica occur in the distal cavity of the maxillary palp and digitiform sensilla are located on the distal maxillary segment. The labium is a three-segmented palp. Two types of sensilla basiconica with a 4+3 model are present on the distal cavity of the labial palp, whereas a kind of sensilla basiconica is at the base of the labial palp.For the bruchid weevil, the labrum is rectangular and 2 types of sensilla campaniformia can be found on the epipharynx. Each mandible has one incisor and the molar is triangular. The maxillary palp has four-segmented, and on the distal segment, the number of sensilla exhibit differences between males and females. The number of digitiform sensilla on the lateral wall is 9 for the female and 7 for the male. The labial palp is three-segmented with a total of 7 sensilla on the tip representing 3 types.The labrum of saw-toothed grain beetle is rectangular and the epipharynx has 3 types of sensilla coeloconica. The mandibles have 2 incisors each and the molar is oval. The Maxillary palp is four-semented. On the last segment, 2 digitiform sensilla distributed on the latral wall and 22 sensilla were on the distal tip representing 6 types. The labial palp is three-segmented with a total of 7 sensilla representing 3 types. The larva’s labrum is semicircle and the epipharynx has only 6 sensilla coeloconica I, blunt with single pore. Each mandible has 3 incisors and the rectangle molar has a protuberance. The maxillary palp is three-segmented with no digitiform sensillum on the lateral wall of the last segment, and the tip has 13 sensilla divided into 6types. The labial palp is also three-segmented and 11 sensilla representing 4 types can be found on the tip.In confused flour beetle, the labrum is rectangular and 2 types of sensilla coloconica and 1 type of sensilla basiconica can be found on the epipharynx. The mandibles have 1 incisor each and the molar is triangular. 121 sensilla in six types distribute on the tip of the maxillary palp and 4 digitiform sensilla are on the lateral wall of the distal segment. The labium is a four-segmented palp, among which the distal segment has 1 digitiform sensilla on the lateral wall and a total of 32 sensilla in 6types on the tip. The larva’s labrum is rectangular and the epipharynx has a total of 10 sensilla represending 3 types. The mandibles have 2 incisors each, and the trapezoidal molar has no protuberance but wrinkles. The maxillary palp is four-segmented with 1 digitiform sensilla on the lateral wall of the last segment and another 13 sensilla divided into 5 types on the tip. The labial palp is three-segmented and the tip has 12 sensilla divided into 5 types. The mouthpart morphology of the adult confused flour beetle is quite similar to that of the red flour beetle. The epihparynx has the same 2 types of sensilla but their distribution and number are different. In addition, only the confused flour beetle has sensilla coeloconica II and the incisor is more pointed.There are significant differences among five species of stored-product beetles belonging to four different familes. They can be distinguished by the shape of labrum, the number and type of epipharynx sensilla, the shape of mandibles, the number of incisors, and the type and number of sensilla on the tip of the labial and maxillary palps. These structural features are useful in distinguishing the different families of Polyphaga. The differences in the number and types of sensilla on the tip of the labial and maxillary palps between larvae and adults are strongly linked to the nature of their food.