Fine Morphology And Evolution Of The Mouthparts In Cicadomorpha

Mouthparts are among the most important sensory and feeding structures in insects and differ according to differences in feeding behavior and habits.The abundant morphology and various function makes mouthpart a perfect material for comparative morphological study.Cicadomorpha are highly diverse and form a major component of Hemiptera,among which many species are economically important.Piercing-sucking mouthpart is a well developed feeding apparatus for Cicadomorpha,and plays an important role in selecting host plants,feeding and transmitting plant virus.Comparative morphological study of the mouthparts may not only help explain differences in feeding behavior and diet breadth among species,but also found new evidences to reveal the evolutionary relationships among the remaining controversial taxa.In the present work,the mouthpart morphology of 44 species in Cicadomorpha was detailed observed with scanning electron microscope.The basal components of piercing-sucking mouthpart and their fine morphology were defined,and the variation among different species and groups was compared,especially on labrum,labium and stylet fascicle.Based on the morphology observation,85 mouthpart characters were selected to study the phylogenetic relationship in Cicadomorpha.The results were as follows:The mouthparts of Cicadomorpha insects were typical piercing-sucking mouthpart,consisting of a small labrum,a tubular labium and stylet fascicle consisting of two outer mandibular stylets and two interlocked maxillary stylets.The short and small labrum is always attached to the anterior margin of the anteclypeus and overlies the labial segments.It is cone-shaped with flat extension at lateral margin or not.In Cicadoidea,the labrum is shovel-shaped with obvious lateral extension,and abundant sensilla on it.In Cercopoidea,the lateral extension becomes narrow and not obvious,no sensillum but more cuticular processes present on labrum.In Membracoidea,labrum is triangular pyramidal with no extension,and covered with cuticular processes.The labium is three-segmented with a deep longitudinal groove in the anterior side.The length,width and height of each segment vary in different species.A variety of sensilla could be found on mouthpart especially on labium and their types and quantity were much larger on dorsal surface than ventral side.In most species,the quantity of sensilla gradually increased from the base of labium to the end.The sensilla identified in Cicadomorpha include sensilla chaetica,sensilla basiconica,sensilla trichodea,sensilla coeloconica,sensilla cavity,sensilla digitiformia,sensilla campaniformia,leaf-like sensilla and pore-like sensilla.In addition,five kinds of cuticle processes could be observed,including spines,squamaceous projection,palmate projection,papillae and petal shaped projection.The types,distribution and quantity of sensilla and cuticle processes varied among different groups.They always concentrate on the labial tip,while their distributions are different.In Cicadoidea,abundant long sensilla trichodea distribute in groups on dorsal side of raised tip,which is covered with scale-like cuticular processes.The labial tips of Cercopoidea are flat with two pairs of sensilla basiconica on ventral side and several groups of sensilla on dorsal side.In Cercopidae,the distribution pattern of sensilla on dorsal side is 4+4,while is 1+5+4 in Aphrophoridae.For Membracoidea,only one pair of sensilla basiconica locates on the central part of labial tip,and other sensilla are around the sensory field.The terminals of mandibular stylets were always decorated with odontoid process on both sides and ridges on the outer surface,whose morphology and quantities differed among species.Only one or two ridges can be found in Cicadoidea,and their odontoid processes are quite small.The ridges increase to 4-7 in Cercopoidea and odontoid processes are wider and obvious,and in particular,cross grains can only be found on madibular stylets of froghoppers.In Membracoidea,the ridges of most species are more than 10 and odontoid processes on both sides are quite prominent.From the oval cross section of stylet fascicle,each stylet could be clearly found possessing different numbers of dendritic canals.The number of dendritic canal in mandibular stylet was three in Membracidae,while only one in Cicadellidae,Cercopoidea and Cicadoidea.For maxillary stylet,there was only one dendritic canal in Cercopoidea,Cicadoidea,Cicadellinae,Macropsinae and one species in Idiocerinae,but in Membracidae and other species of leahopper,there were two dendritic canals in maxillary stylet.In most cases,the angle between mandibular stylet and maxillary stylet was 45°.The maxillary stylets are interlocked by a connecting system composed of T-shaped,hooked,and straight processes,but in Cicadellidae,only two processes could be found.In all species,these processes could be divided into dorsal lock and ventral lock,which formed the bigger food canal and smaller salivary canal.From the comparision between the nymphs and adults of Meimuna mongolica(Distant),significant general transformations during nymphal development include increases in the overall size of the appendages and their component segments or processes,and increases in the number and variety of setal types.Conspicuous transformations that occurred on particular mouthpart components include changes in:(a)the size and shape of the labrum from square to long and shovel-shaped;(b)the third labial segment is oblong-elliptic with an expansion in subapical region in nymphal stages but slightly tapered to the apex with a posterior longitudinal groove in the adult;(c)increases in type and quantity of sensilla with the stage of development;(d)loss of the finger-like sensilla on the distal part of the labium in the adult;(e)the ridges at the tips of the mandiblar stylets become more prominent in later stages of nymphal development,while odontoid protrusions more prominent in the female than in the male of the adult;(f)the insection on both sides of maxillary stylet becomes increasingly apparent in nymphs,while in adult,they changed into serrated structure;(g)the cross section of the stylets is subcircular in nymphal stages but oblong elliptical in the adult,and mandibular stylet markedly thickened;(h)The total length of the mouthparts differ significantly between developmental stages,and increase exponentially from the first instar nymph to adult.The phylogenetic relationship of Cicadomorpha was analyzed on the basis of the mouthpart ultramorphology characters with maximum parsimony,and a consensus tree was obtained.The results showed that Cicadoidea,Cercopoidea and Membracoidea were monophyly groups,and the sister group of Membracidae was Cicadellidae.The relationship of these three superfamilies was(Cicadoidea + Cercopoidea)+ Membracoidea.In Cicadellidae,most of the subfamilies could be gathered in one branch.Continuous variation tendency can be found from Cicadoidea to Membracoidea: The mouthparts change from long to short;The shapes of labrums change from shovel-shaped to triangular pyramidal;The quantity of sensilla on labrum decreases but the cuticular processes increases;The length of labium decrease as well as the quantity of sensilla;The sensilla on the tip of labium change from clustered distribution to circled;The mandibular stylets becomes thinner and their tips become sharper;More ridges and stronger odontoid processes emerge on mandibular stylets;The interlocked structures of maxillary stylets decrease;The breach on middle ridges of maxillary stylets gradually appear.From this study,the fine morphology of mouthpart in Cicadomorpha has been observed,and their difference and evolutionary trend has been obtained.It was convinced that mouthpart characters can be used for classification and phylogeny analysis.Moreover,we deduced the reason for morphological difference between groups,and discussed the function of mouthpart structures in feeding process.