Comparative Study On Structure,Properties And Composition Of Lepidopteran Silk

Silkworms spin silk,spiders spin webs,bees build nests and so on.Animal silk production is one of the most peculiar and mysterious phenomena in nature.Silk production behavior is a characteristic of many arthropods and some mollusks.More than 500,000 species of insects from16 orders have the ability to secrete silk.Insects produce silk for a variety of purposes,including hunting,protection,reproduction,and migration.Silk production is an important means for the survival and reproduction of insects,and plays a crucial role in the maintenance of ecological stability and biodiversity.There are about 200,000 species of Lepidoptera in the world,which is the second largest order after Coleoptera.Most lepidopteran insects have the habit of spinning silk.Most moths are capable of producing cocoons,but the cocoons of different species of lepidoptera vary greatly.Some lepidopteran cocoons are regular and dense,some lepidopteran can only spit a small amount of silk combined with other materials such as wood chips,dead leaves,body hair,soil particles and so on to form cocoons.Insects from the families Bombycidae and saturniidae have strong cocoon-forming ability and can generally form dense cocoons.Although the butterfly larvae in the order Lepidoptera also undergo the metamorphosis of pupation,most butterflies do not spin cocoon but form naked pupae,and only a few butterflies,such as Pteridae,can spin silk to form cocoons.At present,the researches on the Lepidopteran silk mainly focus on Bombycidae and Saturniidae,while the researches on the silk of other Lepidoptera are relatively few.1.Morphological observation of lepidopteraIn this study,a total of 22 species belonging to 15 families of Lepidoptera were collected.Morphological observation showed that 11 Lepidoptera species could spin cocoons.Other species of lepidoptera secrete silk to help them form a stable living environment without forming an intact cocoon.In the course of morphological observation,the insects were classified according to the sericin content,the presence of cocoon,the content of calcium oxalate and the presence of conjugate.In the end,the silk from 22 species of lepidopteran insects were divided into four categories: non-cocoon silk,sericin-less cocoon silk,sericin-rich cocoon silk and silk-conjugate cocoon.Among them,there were 11 kinds of non-cocoon silks,from Cossidae,Pyrausta nubilalis,Perina nuda,Brahmaea porpuyrio Chu et Wang,Spodoptera exigua,Helicoverpa armigera,Panacra mydon mydon,Danaus chrysippus,Idea leuconoe,Parnara guttata and Papilionidae A.There are 7 kinds of sericin-less cocoons,including Cnidocampa flavescens,Phauda flammans Walker,Clania variegata Snellen,Galleria mellonella,Antherea pernyi,Saturniidae A and Saturniidae C.The sericin-rich cocoons are from four species,including Trabala vishnou gigantina,Actias selene,Actias ningpoana,and Saturniidae B.silk-conjugated cocoons include three species: Cossidae,Clania variegata Snellen and Miltochrista striata.2.Structure and properties of silk from lepidopteran insectsFurthermore,the secondary structure and mechanical properties of silk from the collected insects were studied.The secondary structures of silk were determined by infrared spectroscopy.The secondary structures of silk were determined by infrared spectroscopy for 22 species,including 6 species of Microlepidoptera,5 species of other Macrolepidoptera,7 species of Bombycoidea,and 4 species of butterflies.According to the statistics,the ?-folding content of Parnara guttata is the highest,which reaches 40.2%,followed by Trabala vishnou gigantina36.2%.In the Bombycoidea,the ?-folding content is relatively close among many species,which is about 20%.The lowest was Papilionidae A,which contained only 9.7%?-folding.The mechanical properties were further measured and the breaking strength and elongation among different Lepidopteran insects were compared.There were great differences among various Lepidopteran insects.The best breaking strength was 573.706 MPa for Trabala vishnou gigantina,while the worst was Saturniidae A,with breaking strength of 236.08 MPa.In terms of elongation,the best one is the Idea leuconoe,with 22.43%,while that of Saturniidae B is only 8.7%.We also found that according to previous classification of lepidopteran insects,sericin-less cocoons had better elongation,and sericin-rich cocoons had higher ?-folding content and higher breaking strength.The elongation of sericin-less cocoons ranged from 15.83-22.43%,while that of sericin-rich cocoons ranged from 7.2% to 11.45%.The elongation of sericin-less cocoons was significantly higher than that of sericin-rich cocoons.In terms of fracture strength,the breaking strength of Antherea pernyi,Saturniidae A and Idea leuconoe in sericin-less cocoons is between236-373 MPa,while that of Trabala vishnou gigantina,Actias selene,Actias ningpoana is between449-573 MPa in sericin-rich cocoons.The breaking strength of sericin-rich cocoons is obviously higher than that of sericin-less cocoons.3.Composition and sequence of silk proteins in Lepidoptera insectsThe composition and sequence of silk proteins in lepidopteran insects were studied.At first,we compared the different content of sericins among different lepidopteran silks,The sericin were identified in 11 lepidopteran species including Pyrausta nubilalis,Perina nuda,Actias ningpoana,Saturniidae B,Galleria mellonella,Actias selene,Trabala vishnou gigantina,Phauda flammans Walker,Eriogyna pyretorum,Cossidae and Saturniidae A However,most of these lepidopteran silks have very low abundance of sericins,and only Trabala vishnou gigantina,Actias selene,Actias ningpoana,and Saturniidae B,have relative high abundance of sericins.being in the top 10.Seroin proteins were identified in Actias ningpoana,Galleria mellonella,Cossidae,Panacra mydon mydon and Eriogyna pyretorum.Protease inhibitors were identified in Pyrausta nubilalis,Eriogyna pyretorum and Phauda flammans Walker.Fibroin heavy chains were identified in all 15 species of Lepidoptera,fibroin P25 protein was identified in Galleria mellonella,Trabala vishnou gigantina,Phauda flammans Walker and Cossidae.and fibroin light chain has been identified in Phauda flammans Walker,Pyrausta nubilalis,Perina nuda,Trabala vishnou gigantina,Galleria mellonella,and Danaus chrysippus.We found that fibroin light chain,fibroin P25,and N-and C-termini of fibroin heavy chain are highly homologous.but the repeat region of fibroin heavy chain are various in different lepidopteran insects.The silks of the three kinds of sericin-rich cocoons,Trabala vishnou gigantina,Actias selene,Actias ningpoana,have shorter repeat motifs and higher polyalanine content,thus have higher strength.While sericin-less cocoons have longer repeat motifs,lower polyalanine,and better elongation due to the increased helical structure ratio.