Comparative Morphology Of The Mouth-Part Sensilla Of Six Species Of The Genus Tachycines(Othoptera:Rhaphidophoridae)

Caves are a relatively independent natural environment.Its interior is characterized by darkness,stable temperature,high relative humidity,and lack of food.The unique cave environment makes cave animals not only modifications of physiological and behavioral but also modifications of morphological characteristics.According to the degree of adaptation to the cave environment,cave animals are classified into three major categories: troglobites,troglophiles,and trogloxenes.Among them,the troglobites have the typical characteristics of adapting to cave life,and their adaptability to the cave environment is higher than that of the troglophiles.The trogloxenes only use the cave as a temporary habitat.The habitat of the genus Tachycines insects is complex and diverse,and it can be found in caves,bushes,and crevices on the ground.In this study,three cave-dwelling species(a troglobites species Tachycines shuangcha inhabited in the cave dark zone,and two troglophiles species Tachycines tongrenus and Tachycines plumiopedella inhabited in the cave entrance zone to cave twilight zone)and three surface-dwelling species(Tachycines huaxi,Tachycines sp.1 and Tachycines sp.2)of the genus Tachycines were used as research materials to study the comparative morphology of the overall morphology and sensilla of the mouthparts.The sensillum types,and distribution of mouthparts of the six species were identified by scanning electron microscopy.On this basis,the evolution characteristics and adaptive changes of mouth-part sensilla in cave habitats were explored.The main results are as follows:1.Body length and external morphology of mouthpartsIn this study,the body length,length and width of each part of the mouthparts of six species of Tachycines were measured and compared.The results showed that the proportion of most parts of the mouthparts(mandibles,maxillae,labium,and labial palpus)relative to the body length of three cave-dwelling species were significantly greater than that of three surface-dwelling species.It can be seen that the mouthparts of the cave-dwelling species are longer than surface-dwelling species.In addition,the length of body and mouthparts of the troglobites species T.shuangcha were significantly smaller than those of other species,which may be the adaptive changes derived from the lack of food in the cave dark zone.2.Types and distribution of mouth-part sensillaA total of 27 kinds of sensilla were observed in the mouthparts of the six species,which were: sensilla chaetica subtype 1～4(Sc.1～4),sensilla trichodea subtype 1～6(Sb.1～6),sensilla basiconica subtype 1～10(Sb.1～10),sensilla coeloconica subtype1～4(Sc0.1～4),sensilla palmatum(Sp),sensilla campaniformia(Sca),and B?hm bristles(Bb).Among the 27 types of sensilla,St.5 belongs to the gustatory receptors,Sb.9 and Sb.10 belong to the olfactory receptors,Sco.1～4 belongs to the hygro-thermoreceptors,and other sensilla belong to the mechanoreceptors.The distribution of each sensillum on the mouthparts is similar among the six species.Sc.1,Sb.5,and Sca are distributed on the labrum,mandible,maxillae,labium,and hypopharynx.St.2,Sco.1,Sco.2,and Sb.2 are distributed on the central region of the epipharynx.St.5,St.6,Sco.3,Sb.8,Sb.9,and Sp are distributed on the fifth segment of the maxillary palpus and the third segment of the labial palpus.Sco.4 and Sb.10 are distributed on the third segment of the labial palpus.St.1 is distributed on the labrum,mandible,maxillae,and labium.St.3 is distributed on the labrum,maxillae,and labium.St.4 is distributed on the mandible.Sc.2 is distributed on the epipharynx and mandibles.Sc.3 is distributed on the third to fourth segments of the maxillary palpus and the third segment of the labial palpus.Sc.4 is distributed on the lacinea.Sb.1 is distributed on the labrum,galea,and paraglossa.Sb.3 is distributed on the epipharynx,hypopharynx,and galea.Sb.4 is distributed on the edge of the epipharynx.Sb.6 is distributed on the back of the galea and paraglossa.Sb.7 is mainly distributed on the maxillae and labium.Bb is distributed o the third to fifth segments of the maxillary palpus and the second and third segments of the labial palpus.Among all parts of the mouthparts,the maxillary palpus,the labial palpus,and the epipharynx have the most sensilla types.3.Interspecific differences in mouth-part sensilla of six species of the genus TachycinesBased on the analysis of the type,length,basal diameter,and number of sensilla on the mouthparts of the six species,the interspecific differences of the mouth-part sensilla of the six species were analyzed.In the type of sensilla,Sb.10 and Sco.4 were found only in the troglophiles species T.tongrenus or T.plumiopedella,and the other 25 kinds of sensilla were common to the six species.The distribution area of the two cave species is close to the cave entrance.The unique Sco.4 of the troglophiles species T.tongrenus or T.plumiopedella may help them better perceive the temperature and humidity changes in the cave area and cope with their higher water loss rate.Sb.10 is the unique sensilla of the troglobites species T.tongrenus.The presence of Sb.10 indicates that T.tongrenus may have a stronger chemical sensing range than other species.In terms of the length,base diameter and number of sensilla,the number of gustatory receptors St.5 on the maxillary palpus of the troglobites species T.shuangcha was significantly higher than that of other species,indicating that the troglobites species T.shuangcha inhabited in the cave dark zone has stronger gustatory perception ability than other species.In addition,compared with other species,the mechanoreceptors of T.shuangcha were reduced or degraded.The increase of gustatory receptors and the degeneration of mechanoreceptors may be the characteristics derived from the adaptation to the oligotrophic environment of the cave dark zone.