Ultrastructure Of Antennae And Antennal Sensilla In Scale Insects

Antennae are important peripheral sensory organs for insects,and play an important role in seeking and locating habitat,host,partners,oviposition site,etc.However,the studies about the structures of the antennae and antennal sensilla in scale insects are very limilted.The scale insects belong to Coccoidea(Insecta: Hemiptera: Sternorrhyncha)with near 8000 species that are assigned to 1150 genus and 32 families,and are all phytophagous.The scale insects have a wide distribution and host range.They have various morphologies and behaviors in different groups,and exhibit a great of sexual dimorphism within the same species in adult stage.All of these features make their antennae show a great diversity in morphology,structure,function,development and so on.Thus,it can provide necessary theoretical basises for the studies of scale insects in behavior,development,classification and phylogeny to study the ultrastructure of antennae and antennal sensilla of inscale insects,analysis their functions,compare their changes and development characteristics in different groups,species,instars and sexes.In this paper,the Porphyrophora sophorae Arch.(Margarodidae)and Matsucoccus matsumurae(Kuwana)(Matsucoccidae)in Archaeococcoids,and Didesmococcus koreanus Borchsenius(Coccidae)in Neococcoids,were choosed as main researched insects.Of which,P.sophorae are typical soil-inhabiting scale insects and feed on the root of host plants,M.matsumurae are special pests restricted in pinus spp.,dwell and feed in the bark crevices.The two species all have a “cyst” stage in development.In the “cyst” stage,their antennae are degenerated.D.koreanus belongs to the family Coccidae that is the third biggest family in Coccoidea.D.koreanus sedentarily feed on the surface of host plants and have no “cyst” stage in development.In this paper,the antennae and antennal sensilla of the three scale insects were studied,including the ultrastructures,function and their behavior adaptivity,changed charcatistics,and development rule,in different instars and sexes were observed using light microscopy(LM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results are as follows:1.Antennal sensilla and their innervation in first instar nymphs of P.sophorae.The antennae of first instar nymphs of P.sophorae are six segments,moniliform,120–145 ?m in length,with 20–23 sensilla that in sex types,e.g.,one B?hm's bristle(Bb),one campaniform sensillum(Ca),one Johnston's organ(Jo),13–16 aporous trichodea sensilla(ASt),two coeloconic sensilla(Co),and two multiporous peg(Mp).According to their function,these sensilla can be categorized into three categories: mechanoreceptors,e.g.,Bb,Ca and Jo that have the function of proprioreceptor,and ASt that have the function of tactile sensillum;thermo/hygroreceptors,e.g.,Co only;and chemoreceptors,i.e.,Mp that have the function of olfactory sensillum.The innervation of antennal sensilla in first instar nymphs of P.sophorae has some different characteristics from that in common insects.In which,the neuronal somata of sensilla are not restricted to the epidermis below the sensilla,but extend to more proximal position.The dendrites that innervate the chemoreceptors and thermo/hygroreceptors combine with thecogen cells to form a large nerve tract that extends through the antenna and to the insect's head,and the somata of these neurons are present in the lymph cavity of head.The dendrites that innervate the mechanoreceptors form another nerve tract.The somata of these neurons are located in the lumen of scape and pedicel.2.The structure of antennae and antennal sensilla of the P.sophorae adult females and their adaptive significance to soil-inhabiting behaviorsThe antennae of P.sophorae adult females are very different from that of first instar nymphs.They are composed of nine segments,about 480–650 ?m in length,and in tower shape.All of the antennal segments have a similar short cylindrical shape,except the top segment that has a hemispherical shape.These segments connect to each other compactly at the intersegments,which makes the antennae sturdy.A total of 29–48 sensilla in four types,e.g.,2–5 Bb,5–12 ASt,10–15 Co,and 11–22 Mp are observed on the antennae.These sensilla are mainly located on the scape,flagellomere F3,and flagellomere F7.The other segments have almost no sensillum.The sturdy antennae,central distribution of antennal sensilla,a large number of Co(thermo/hygroreceptors)and Mp(olfactory sensilla)on the antennae,all of these characteristics are well adapted to the behaviers of adult females that finding suitable underground oviposition site in arid or semi-arid areas.3.The antennae and antennal sensilla of the P.sophorae adult males and their their adaptive significance to the behavior of searching adult femalesThe antennae of adult males of P.sophorae are also composed of nine segments,but more developed with about 1350–1660 ?m in length and moniliform in shape.A total of 401–541 sensilla in seven types are observed on the antennae,e.g.,1–4 Bb,62–109 ASt,one Jo,8–15 Co,2–6 Mp and 324–440 multiporous trichoid sensilla(Mt).The sensillum Jo of adult males are very different from that of first instar nymphs.It is more developed and composed of about 60 scolopidia that arranged in a ring in the pedicel,near the epidermis.The sensillum Jo of adult males should has the function of detecting wind speed during the males flight.The Mt are specific olfactory sensilla on the adult male antennae and should have the function of detecting sex pheromones that are released by the adult females.The developed antennae with developed Jo and abundant Mt ensure the adult males of P.sophorae can search and locat adult females in a larger range by flight.4.The changes and development of antennae and antennal sensilla of M.matsumurae in different instars.The antennae of M.matsumurae display three different styles in morphology and sensilla distribution in the first instar(first instar nymphs),third instar(adult females and third instar male nymphs)and fifth instar(adult males),respectively.The antennae of first instar nymphs are six segments,moniliform,and 80–95 ?m in length.Sixteen sensilla of six types are observed on the antennae,e.g.,one Ca,4 SAt,2 intersegmental sensilla(Ins),2 Co,3 Mp and 4 uniporous pegs(Up).Of which,the sensilla Ins should be a type of mechanoreceptor with the function of proprioreceptor,and the Up should be a type of chemoreceptor with the function of gustatory sensillum.The adult females and third instar male nymphs of M.matsumurae are all in the third instar.Their antennae are similar in morphology and sensilla distribution,but ther are very different from that of the first instar nymphs.The antennae of adult females are composed of 9 segments,moniliform,and 750–850 ?m in length.In total,82–108 sensilla of seven types are observed on the antennae,e.g.,9–16 Bb,3–7 Ca,50–75 SAt,0–3 Ins,3–10 Co,8 Mp and 5 Up.The antennae of third instar male nymphs are slightly shorter than that of adult females The sensilla distribution are similar to that of adult females,while the numbers of sensilla Ca,ASt1 and ASt3 are less.Adult male antennae are composed of 10 segments,lash-like,and 1250–1400 ?m in length.In total,259–312 sensilla of eight types are observed on the antennae,e.g.,7–15 Bb,2–5 Ca,11–20 SAt,0–3 Ins,2–7 Co,23–29 knobbed seta sensilla(Ks),179–230 Mt,and 8 Mp.The great differences between the first instar and third instar antennae of M.matsumurae in morphology and sensilla distribution should be caused by the degeneration of the antennae in second instar.On the other hand,the first instar and third instar antennae are similar in the sensillar types,specially the same chemoreceptor types.It means these antennae may perform similar functions.It is probably because the first instar nymphs,adult female,and third instar male nymphs have the similar behavior that searching for bark crevices by sensing the chemical substances of host plant.The great differences of antennae between the third instar male nymphs and adult males of M.matsumurae are resulted in the development of the antennae in “pupa” stage,and they are concerned with the great changes in the behaviors of the males from third instar to adult.The main behaviors of the adult males are searching and locating the adult females in a larger range by flight.That require the adult male antennae to be more development and equip some new sensilla.Compared with the third instar male nymphs antennae,two new types of sensilla appear on the adult male antennae.Of which,the sensilla Ks should have the function of flight control,while the Mt should have the function of detecting sex pheromones that are released by the adult females.The antennae of males antennae of M.matsumurae underwent significant changes in “pupa” stage.However,the distributions of sensilla on antennae of third instar male nymphs and adult males indicate that the antennae of adult males are still developed on the structure of the three instar male nymph antennae.In gerenal,the antennae of the male M.matsumurae underwent “pupa” stage in development,but its development characteristics have natural difference from that of holometaboly insects in the pupa stage.It is noticed that,the sensilla Mp of adult males may have the function of sensing plant volatiles like the Mp in other instars.It means that the plant volatiles may also play a role in the process of locating adult females for adult males.5.The changes and development of antennae and antennal sensilla of D.koreanus in different instars.D.koreanus belonged to Eulecaniinae in Coccidae,is an important fruit pest in the north of China.Based on the observation and comparison of the antennae of D.koreanus on the first instar nymphs,the females in second instar,third instar and adult stage,and the males in second instar,“prepupa” stage,“pupa” stage and adult stage,the results are as follows.The female antennae of D.koreanus in first instar,second instar and third instar are all composed of 6 segments,about 100–120 ?m,180–220 ?m and 245–270 ?m in length,respectively.On these antennae,24 sensilla of five types are observed,e.g.,one Ca,11 SAt,2 basiconica sensilla(Ba),7 Mp,and 3 chaetica sensilla(Ch).Of which,the Ba should be a type of thermo/hygroreceptor,and the Ch should be a type of gustatory sensillum.Most of adult female antennae are six segmented,some of these antennae are seven segmented or five segmented.The length of the adult female antennae are 290–340 ?m.The quantity and distribution of sensilla on the adult female antennae are similar with that on the nymphs antennae.Thus it can be seen that the length of the antennae of the female D.koreanus are increased gradually with the instar development,while the number of the antennal segments,the quantity and distribution of sensilla on these antennae have no significant change.The antennal development of female D.koreanus not only accord with that of paurometabola insects,but also are adapted to their behavior that sedentarily feeding on the host plant surface and the less importance of antenna functions.The differentiation of female and male of D.koreanus occurred in second instar.The antennae of second instar males are similar with that of second instar females in length(175–225 ?m),morphology,and sensillar quantity and distribution,but the number of antennal segments of the second instar males are seven.After the second instar,the antennal segments of the male “prepupa” are nine.The antennae of the male “prepupa” are about 190–280 ?m in length,only have two sensilla Ba on the top.The antennae of male “pupa” are nine or ten segments,about 520–610 ?m in length,1 or 2 sensilla Ba on the top.The antennae of adult males are more developed,ten segments,lash-like,and about 800–950 ?m in length,possess 156–214 sensilla in seven types,e.g.,16–34 St,1 Ca,1 Jo,3 Ks,2–5 Ba,7 Mp,and 120–181 Mt.The significant changes of antennae of male D.koreanus in“prepupa” and “pupa” stages reflect the typical development characteristics of the hyperpaurometamorphosis insects,and also are adapted to the dramatic behavior changes between the nymphs and adult males.Meanwhile,the first instar nymphs and adult females antennae of other three species in the Coccidae,the Eulecanium kuwanai kanda(Eulecaniinae),Ceroplastes japonicus Green(Ceroplastinae)and Coccus hesperidum L.(Coccinae)were also observed.The result showed that the distribution patterns of their antennal sensilla are very similar to the first instar nymphs and adult females of D.koreanus,in particularly,the 12 sensilla on the top segment.It is suggested that the antennae of D.koreanus have certain representativeness in the Coccidae.