A Study On Hairy Pads Of Drosophila Melanogaster And Its Adhesive Force On Touched Surfaces With Different Roughness

"Survival of the fittest" and a long time evolution of animals create a wide range of special skills,to a certain extent,our human beings are not as good as animals in many ways.As a result,"biomimetics" becomes the necessity for development of modern science and technology and for human progress,such as the compound eyes of insects,the self-cleaning effect of lotus leaf,the sand erosion-resistant properties of shark etc.A variety of topics and styles in biomimetic fields took on constantly these years.We have always been in the way to explore the nature,and beyond ourselves,to make full use of these biological functions and finally benefit our mankind.Geckos,grasshoppers,flies,beetles etc,have extraordinary climbing abilities:they not only are able to walk freely on the level surfaces,but also on a vertical surface or even glazed ceilings.This remarkable function is obviously the "masterpiece" of our nature. What special structures can give them such powerful capacity? It was found in the study of geckos that their four feet are covered with a large number of fiber-like seta from the micro to nanometer-scale in diameter.The further study showed that the van der waals force that between the hundreds of millions of seta and the contact surfaces was the main mechanism of geckos' outstanding adhesion.Moreover,it was reported that the adhesive force of geckos on different rough surfaces was not always the same.Do the insects such as flies and beetles also have the same properties? Unfortunately,the research work on them was less.It had been reported that they also have the similar hairy pads,besides van der waals,whether other mechanisms make contribution or not is still under debate.So, the adhesive abilities,properties and also the real adhesive mechanism need further research and more work on it.This present article intended to study the insects.It explored the influence of the surface roughness on the adhesive force generated by insects by means of the measurement of the adhesive force on different rough surfaces,and then further discussed the mechanisms of the adhesive force of the insects.The drosophila was chosen as the research objective,and the micro-structure of its pad was observed by using Scanning electron microscope(SEM).Meanwhile,the results were compared with the flies' pads.It indicated that(ⅰ) their feet are all covered by similar fiber-like setae;(ⅱ) drosophila has branch-like pulvilli rather than ellipsoid in shape as in flies and its pulvilli's area is smaller,just 1/200 of flies' and(ⅲ) the number of seta on one pulvillus,drosophila just has 35 that are exactly less than flies';(ⅲ) the diameter of each seta is 0.8-1.0μm and the length is about 8-10μm on drosophila's pulvillus,which is a little bit thinker and shorter than flies'.As to the area of the setae' tips,drosophilas' are bigger than flies',the former is 5.65-6.28μm² and the later is about 0.6-1.5μm²Used atomic force microscope(AFM) to analyze the surface roughness of different polishing papers,the results showed that the surface roughness took on rising trend with the increase of particle size of polishing papers.Meanwhile,three parameters used to describe surface roughness are RMS(root mean square average of height deviations taken from the mean date plane),Ra(the arithmetic average of the absolute values of the surface height deviations measured from the mean plane) and Rmax(maximum vertical distance between the highest and the lowest data points in the image).They have better linear correlation and the regression equations were obtained by Least Square Method,the correlation coefficient of them were R_RMS-Ra=0.9996,R_RMS-Rmax=0.9830,R_Ra-Rmax=0.9782 respectively.Also,three self-designed methods and devices,such as cubic boxes,indirect centrifugal force testing and accurate lever-like device measuring,were developed to realize the measurement of adhesive force for living drosophila on different rough surfaces. The measured results were basically accordant and showed that on surfaces with roughness greater than RMS value of 140nm,drosophila had better adhesive ability,when the RMS value is less than 140nm,adhesive force increases with the decease of surface roughness.The lowest force was shown during a narrow region that RMS is round 120nm. It may own to the real contact area between pads and rough surfaces.Finally,the possible mechanisms were analyzed.The theoretical calculation of the maximum van der waals force is 0.0933-0.1167 mN,and the maximum suction is 0.2373-0.2637 mN.As the theoretical maximum adhesion values are all less than the maximum testing force 0.2764 mN,it is clear that drosophila is different from gecko,Van der waals force and suction itself can not be the only mechanism of adhesion generated by drosophila,These results will make certain contribution to the further study of real mechanisms of adhesive force generated by insects.