Three-dimensional Morphological And Flight Functional Analyses Of The Hind Wing Axillaries Of Scarab Beetles

Scarabaeoidea belongs to Insecta,Coleoptera,Polyphaga and they are usually called scarabs.Their forewings are elytra,which mainly take a protective role.Therefore,their hind wings play a major role in flight.The hind wings of scarabs can be divided into three parts:membranous wing and wing veins,the wing base,the wing articulations(axillaries).Among them,the hind wing articulations play an important role in flight.As the link between the chest and the hind wing,they can transfer the flight power generated by the contraction and expansion of the flight muscle to the hind wing,thus driving the flight of insects.The insects of the Scarabaeoidea have their own unique flight methods.Most scarabs fly with their elytra open,while some scarabs fly with their elytra closed,like Cetoniinae(except Trichiini)and some Scarabaeinae(Gymnopleurini).How these two different flight modes affect the morphology of the hind wing axillaries is unknown.In this study,micro CT scanning was performed on the axillaries of 114 scarabs from 11 families and 17 subfamilies,and three-dimensional models of the axillarries were reconstructed by computer three-dimensional reconstruction method.Then comparative morphology and three-dimensional geometric morphometrics were used to analyze the landmarks data of axillaries.The analyses methods include principal component analysis(PCA),canonical variate analysis(CVA),and cluster analysis.The main results are as follows:1.The scarabs were divided according to different family levels,the results of PCA showed that the first axillary(1Ax),the second axillary(2Ax),and the third axillary(3Ax)all had overlapping parts that cannot be distinguished,especially in the family of Scarabaeidae,there was a significant degree of variation in the morphology of the axillaries.However,the CVA results showed significant differences(p<0.01)among the family level(Scarabaeidae,Lucanidae,Passalidae,and Geotrupidae).The species under the Scarabaeidae were divided according to their subfamily level.The PCA results showed that there was a partial overlap between the Rutelinae and the Melolonthinae,but the Cetoniinae and the Scarabaeinae can be distinguished.The CVA results showed significant differences among these four subfamilies(p<0.01).2.According to the two different flight modes of elytra open or closed during flight,the results of PCA and CVA showed that these two groups of scarabs can be distinguished and had significant differences(p<0.001)in the three axillaries,indicating that the morphologies of the axillaries were affected by the selection pressure of flight.This difference was mainly reflected in:the head of 1 Ax of scarabs fly with closed elytra had a tendency to expand outwards,the neck was broad and the middle part has a tendency to bulge,the tail tended to shorten,especially the distal arch.The upper part of the middle ridge of 2Ax tended to extend and the lower part tended to shorten;the distal lobe tended to contract inward,and the proximal lobe tended to expand outward.The head of 3Ax tended to widen gradually,the neck tended to narrow gradually,and the tail tended to shorten.3.In order to explore whether the impact of flight selection pressure is significant or minor,scarabs were divided into five groups for cluster analysis.The analysis results of 1Ax and 2Ax showed that:dung beetles fly with elytra open and dung beetles fly with elytra closed formed a branch;flower chafers fly with elytra open and other scarabs fly with elytra open formed a branch,It showed that dung beetles’ 1Ax and 2Ax were less affected by the flight selection pressure,but these two axillaries of the flower chafers may be more affected by the flight selection pressure.The analysis results of 3Ax showed that the flower chafers fly with elytra closed and the flower chafers fly with elytra open formed a branch;the dung beetles fly with elytra open and other scarabs fly with elytra open formed a branch,which showed the flower chafers’ 3Ax was less affected by flight selection pressure,while dung beetles’ 3Ax was more affected by flight selection pressure,and these three axillaries may also be affected by other factors.4.The comprehensive analysis results of comparative morphology and three-dimensional geometric morphometrics showed that the inner end of 1Ax’s head,the mid-neck and the distal arch were more affected by flight selection pressure;the median to anterior part of the ridge and the distal lobe of 2Ax was more affected by flight selection pressure;the head leading edge and the tail were more affected by flight selection pressure.In this study,the morphological differences of the three hind wing axillaries of scarabs were summarized,the correlation between the morphology of the axillaries and the flight mode was explored.It was found that scarabs fly with elytra closed tend to be more capable of flying,and this change in the morphology of the axillaries may be more conducive to flight.The hind wing axillaiy was fine and three-dimensional structure,and the two-dimensional image can not present its complete structure very well.Therefore,the advantage of this method is that the three-dimensional models this study obtained can fully show the axillaries structural features.At the same time,in the application level,this study may provide some reference for the future functional morphology bionics.