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ine structure and functional comments of mouthparts in Platypus cylindrusCol., Curculionidae: Platypodinae)
atifa Belhoucinea, Rachid T. Bouhraouaa, Eva Pratsb, Juli Pulade-Villarc,∗
University of Tlemcen, Laboratory Conservation Management of Water, Soil and Forests, AlgeriaUnitat Microscopia Electrònica de Rastreig, Centres Científics i Tecnol‘gics de laq UB (CCiT-UB) c/Lluis Solé i Sabarís, 1-3, 08028 Barcelona, SpainUniversitat de Barcelona, Facultat Biologia, Dept Biologia Animal, Avda Diagonal, 645 08028 Barcelona, Spain
r t i c l e i n f o
rticle history:eceived 19 April 2012eceived in revised form 29 October 2012ccepted 29 October 2012
eywords:mbrosia beetleine structure
a b s t r a c t
Oak pinhole borer, Platypus cylindrus is seen in recent years as one of the biggest enemies directly involvedin the observed decline of cork oak in Mediterranean forests with all the economic implications. Asan ambrosia beetle, it has developed its effective drilling mouthpart enough to make tunnels in hard-wood of the tree. The fine structural aspects of the mouthpart using the field emission scanning electronmicroscopy are analyzed about 23 adults collected in galleries of infested cork oak trees (Quercus suber)in a littoral forest of northwest Algeria. These adults are preserved in alcohol 70%, cleaned and coatedwith gold. The mouthparts of this beetle consist commonly of a labrum, a pair of mandibles, a pair of
outhpartlatypus cylindrus
maxillae and the labium but with adapted structure to excavate galleries in the hardwood. In this roleis also involved the first pair of legs. The function that present the different structures related to theconstruction of the tunnels is discussed. Both of maxillary and labial palpi direct the food to the mouthand hold it while the mandibles chew the food. The distal ends of these palpi are flattened and haveshovel-like setae. Females have larger maxillary palpi than males and this is related to the particularbiology of each sex.
. Introduction
Many species of xylophagous Scolytinae and Platypodinae (Cur-ulionidae), including the genus Platypus Herbst, are regarded asmbrosia beetles (Chararas, 1979). They transport fungi that areultivated on the walls of the clean galleries of laying held ofejections and dug deeply in the wood of the plant-hosts (Batra,967). These xylomycetophagous beetles establish ectosymbioticFrancke-Grosmann, 1967) relationships of mutualism with theungi (Beaver, 1989). Fungi penetrate the plant’s xylem tissues,igest it, and concentrate nutrients (Kuroda, 2001). Ambrosia bee-les do not ingest the wood tissue; instead the sawdust resultingrom the excavation is pushed out of the gallery (Soné et al.,998; Kuroda, 2001). As the ambrosia beetles excavate a tunnel
n the hardwood, these beetles have a well-developed mouthparts mechanical organs.
The drilling activity of these beetles is completed by cleaningalleries by rejecting fibers and wood debris cut mainly by femaleut of tunnels using sharp flattened legs (Jover, 1952; Husson,
1955). The genus Platypus takes its name from this type of legs(pla = flat, typus = leg) (Perrier, 1932). Cleaned galleries are neces-sary to support fungal growth to feed larvae.
The morphology of the mouthparts is related to diet. The typeof mouthparts varies not only among different insect species, butalso among different stages of the same species. Most beetles havemouth parts designed for chewing solid food. Many beetles of thesuperfamily Curculionoidea have a distinct snout that can bore intowood and suck sap. The snout has mouthparts at its end and is usedfor penetration and feeding, and for boring holes for egg-laying (VanZandt et al., 2003; Atkinson, 2004; Moon et al., 2008).
Here, we have observed the micro-structural aspects of themouthparts using the field emission scanning electron microscopy.Previously of this study, the mouthparts of a Korean Platypodi-nae species (Crossotarsus koryoensis Murayama) has been studied(Moon et al., 2008). This is the second contribution to know thefine structure of the mouthparts knowledge of another memberof Platypodinae which is Platypus cylindrus, the only representa-tive in Europe and North Africa (Algeria, Tunisia and Morocco).This ambrosia beetle attacks preferentially Fagaceae trees mainlyQuercus genus.
In Algeria, the beetle’s existence has been reported for thefirst time by Lucas near Collo in the east region of the country,under different names (Lucas, 1849), then by Chapuis (1865) in theKabylie region and by Rudolf Trédl (1907) in Algiers. Currently, it is
Fig. 1. Dimorphism of Platypus cylindrus: (a) male, (b) female, (c) male’s abdominal teeth developed on the posterior end of elytra, characteristic of this species. Scale = 1 mm.
Fig. 2. Head in dorsal vision (a), ventral vision (b), (c) and (d) and in lateral vision (e) and (f) of male (a, c, e) and females (b, d, f) of Platypus cylindrus. [an: antenna; ce:compound eye; cl: clypeus; lp: labial palpi; md: mandible; mp: maxillar palpi; ti: tibia.] Scale = 0.2 mm.
76 L. Belhoucine et al. / Micron 45 (2013) 74–82
Fig. 3. Clipeus and labrum area of Platypus cylindrus: (a) male in dorsal vision, (b) female in dorsal vision (c) male in frontal vision, (d) detail in frontal vision, (e) female infrontal vision, (f) detail in fronto lateral vision. [cl: clypeus; la: labrum; lp: labial palpi; md: mandible; mp: maxillar palpi.] Scale = 100 micres.
Fig. 4. Mandibles in frontal vision of Platypus cylindrus: (a) and (b) male, (c) and (d) female, (e) female. [lp: labial palpi; md: mandible; mp: maxillar palpi.] Scale = 200 micres.
L. Belhoucine et al. / Micron 45 (2013) 74–82 77
F ale. [gs
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ig. 5. Maxillae and labium of Platypus cylindrus: (a, b, d, f) female and (c) and (e) mubmentum.] Scale = 100 micres.
onsidered an economically important pest especially in the northestern region, causing annually many cases of deaths of cork oak
rees (Bouhraoua and Villemant, 2010; Belhoucine and Bouhraoua,012).
The aim of this study is to have better knowledge of the mouth-arts morphology to understand their functionality and obtainingeeding source, fungi and mixed wood.
. Materials and methods
Adult ambrosia beetles Platypus cylindrus (Coleoptera: Cur-ulionidae: Platypodinae) were collected in July 2010 from thenfested oak trees (Quercus suber) in the littoral forest of M’Sila,
located 30 km west of Oran in the northwest region of Algeria. Thewood waste material manipulated by adults was collected in theforest from trunks of infested trees presenting abundant sawdustthrough the entrance holes. We collected two types of sawdust: (i)a light yellow colored one from newly infested trees in July 2010,(ii) a brown one from heavily infested trees by the beetle in summer2002 by Bouhraoua.
13 females and 10 males are used in this study to check theSEM pictures. Differentiation between the two sexes is based on
sexual dimorphism characterized by the presence of two unequalteeth developed on the posterior end of male’s elytra (Barbey, 1925;Husson, 1955; Balachowsky, 1963; Villemant and Fraval, 1991)(Fig. 1). The specimens were preserved in alcohol 70% until they
78 L. Belhoucine et al. / Micron 45 (2013) 74–82
male.
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Fig. 6. Maxillae parts of Platypus cylindrus: (a–e) female and (f) and (g)
se, and cleaned with ultrasonic. The whole beetles sampled wereoated to a thickness of approximately 30–40 nm with gold using aputter coater Jeol JFC-1100. SEM pictures were taken with a Stere-scan S-360 (Cambridge Instruments) and Quanta 200 (Fei, Co) bothperated with a voltage of 10 kV. For mouthparts morphologicalerms have been used (Dressler and Beutel, 2010).
. Results
The head of Platypus cylindrus bears a pair of compound eyes sit-ated in the laterals parts more or less oval in lateral vision (Fig. 2end f), a pair of antennae inserted in the face below the compound
Scale: 50 micres (a and f), 10 micres (b, d and g) and 5 micres (c and e).
eyes (Fig. 2e and f), and ventrally a mouthpart (Fig. 2). The ambrosiabeetles have a distinct mouthpart that can excavate galleries in thehardwood. In P. cylindrus, the mouthpart structure is similar in bothsexes, nevertheless a detailed study with a scanning electron micro-scopic observation reveals some differences the most important isthe shape and size of maxillae (Fig. 2e and f).
These beetles have a particular mouthparts as a snout laterally(Fig. 2e and f), adapted to bore into wood and suck sap water. Themouthparts are used for penetration and feeding, and for boring
holes for egg-laying. The mouth is a simple hole surrounded byspecialized structures for grasping and grinding. These are com-posed by labrum, a pair of mandibles, a pair of maxillae and thelabium (Fig. 3). The mandibles and the maxillae are the equivalent
L. Belhoucine et al. / Micron 45 (2013) 74–82 79
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Fig. 7. Labium palpi of Platypus cylindrus: (a–c) female and
f jaws with the exception that they move side to side direction.he mandibles constitute the moveable mouthpart together withhe maxillae and the labial palpi (Fig. 3c and d).
The labrum (Fig. 3d) is a simple structure that is equivalent ton upper lip articulated with the clypeus (Fig. 3a and b) by thelypeolabral suture. It is a flat extension located on the midlineehind the clypeus, covering the mandibles. It serves to hold food
n place during chewing wood by the mandibles. On the surfacef the labrum, there are a number of sensory hairs (Fig. 3). Theseagella-shaped sensillae are compactly arranged over the wholeurface of the labrum. The total number of the sensillae found here,s around 40 in both sexes (Fig. 3a and b).
Either side of the labrum, a pair of mandibles appears serving asincers (Figs. 3c and e and 4a and b). They open laterally of the headnd come together medially. The mandibles are heavily sclerotizedtructures having cutting edges that move transversely to grasp orut wood. They are typically the largest mouthpart being used toasticate food. The lateral shape (Fig. 4b–d) is triangular and they
ave longitudinal lines with short sensillae; the distal part has notensillae.
Behind the mandibles are maxillae. They are composed of theollowing parts-cardo, stypes, lacinia, galea, and maxillary palpiFig. 5). The maxillae occupy a lateral position on the head behind
he mandibles. The proximal part of the maxilla consists of a basalardo and the stipes. Both cardo and stripes are loosely jointed tohe head by membrane so that they are capable of movement. Theasal cardo has a single articulation with the head, and the stipes
) male. Scale: (a, c and d) = 50 micres, (b and e) = 5 micres.
has morphology of a flat plate and has two distal lobes – an innerlacinea and an outer galea (Fig. 5b). The lacinia is situated at thedistal end of the stypes (Fig. 5a). The galea is a cupped or scoop-likestructure, which sits over the outer edge of the labium.
More laterally on the stipes is placed the maxillary palpi com-posed by three segments. They are used to direct the food to themouth. The distal ends of the maxillary palpi are flattened and havedifferent setae. Long setae are present in the inner and lateral partoriented toward the center of the mouth (Fig. 5b–e). Short setaeare also present in distal part forming two lines one of them aresurrounding the apical surface and the second is placed shovel-likesetae (Fig. 6a and f). Laterally and in the upper part of shovel-likesetae area, a more or less round area is filled with teeth (Fig. 6a,d–g). The shovel-like setae area is a typical leaf-like structure withnumerous rod-shaped depressions placed radially with others dis-tributed irregularly in the central area (Fig. 6a, b and f). The labialpart is different in shape and size in both sexes. The female’s palpiare bigger than male’s, they are around two times. In shape, themale has two clear divisions but the upper part has triangular shapeand it is smaller than the inferior one (Fig. 5c); in females the supe-rior part is longer than inferior one and it is divided in two unequalparts (Fig. 5d). Teeth area is circular in males (Fig. 6f and g) butoval in females (Fig. 6d and e); teeth are more defined in females
than males and their number is fewer in males. Leaf-like structureis oval in female but more circular in male (Fig. 6a–d, f); averagesize depressions is 11–19 �m in length in both sexes (Fig. 6b and c)and total number is approximately 85 in females 45 in males.
80 L. Belhoucine et al. / Micron 45 (2013) 74–82
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ig. 8. Scanning electron micrographs of the wood treatment of the ambrosia beetlend sapwood in the form of wood filaments (b) reddish brown sawdust coming fro
The labium is located at the ventral region of the mouthpart ands fused in the midline to form the lower lip. The labium is a singletructure that is equivalent to the floor of the mouth. The labiumith the maxillae assists manipulation of food during mastication.
he main body of the labium is not clearly divided in the three indi-
idual parts common to insects: proximal submentum, the centralentum and distal prementum. The prementum is not visible it is
nsert in the mental emargination. The mentum is strongly differen-iated and it is associated with two short labial palpi 3-segmented
ndrus striped with functional mouthparts (a) clear sawdust resulting from cambiumrtwood with a finer texture. Scale bars indicate 100 �m.
(Figs. 5b and 6c and d). The submentum is very short and com-pletely fused with the gula (Fig. 5e); only marked by a transverserow of setae. The two basal segments in palpi of males are minorthan the distal one (Fig. 7d) but in females they are more or lessequal (Fig. 7c). Moreover, the upper part of the two basal segments
is surrounded by a circular line of long setae (Fig. 7a). The apicalsegment has an oval area with teeth; these are more defined infemales than males and their number is fewer in males (Fig. 7band e).
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P. cylindrus makes galleries directly into the hardwood of the oakree using its functional mouthpart. The frass made by this ambrosiaeetle has its typical shape depending on its origin: sapwood oreartwood. The first one, light, is in the form of wood fibers thether; by cons has a brown powder aspect (Fig. 8). High magnifica-ion electron micrographs reveal the fine structural characteristicsf the internal textures of the two type of sawdust. Average size ofhe wood fibers of light frass is between 650 and 800 �m in lengthnd 38 and s150 �m in width (Fig. 8a). The brown frass takes a finearticles form ovoid in shape (350 �m/160 �m) in size.
. Discussion
The first work studying accurately the mouthparts of Platypodi-ae is related to Moon et al. (2008). In this work the authors studiedlatypus koryoensis (Murayama), actually named as Crossotarsusoryoensis Murayama.
The mouthparts of Platypus cylindrus and Crossotarsus koryoen-is exhibit typical morphology of mycophagous coleopteran beetlesMoon et al., 2008). Both species are very similar in the mouthparts
orphology. They have those characteristics of chewing mouth-arts that can excavate tunnels in the hardwood. Neverthelessome differences have been appreciated; the most important areresent in the maxillae palpi. The segmentation of maxillae palpire more defined in C. koryoensis than P. cylindrus in both sexesnd in C. koryoensis’ males the number of segments are three butnly two in P. cylindrus. Leaf-like structure have more longitudi-al depressions in P. cylindrus than C. koryoensis and they are moreomogenously distributed in C. koryoensis than P. cylindrus; in theentral area is noticed the presence of irregular depressions in P.ylindrus but not in C. koryoensis. Moreover, we have observed otherifferences in labrum area; P. cylindrus have more setae than C.oryoensis.
In Platypus, the morphology of the legs and mouthparts aredapted to the peculiar biology of these beetles. The adults exca-ate galleries in the wood of hardwood trees (Inoue et al., 1998;to et al., 1998; Kobayashi and Hagita, 2000; Kamata et al., 2002;geta et al., 2003, 2004). Male of Platypus cylindrus attacks first theree host and actively begins boring the galleries (Sousa and Inacio005; Bouhraoua and Villemant, 2010; Belhoucine et al., 2011a)reating a penetration corridor of about 1–2 cm in length startingrom the bark wounds (Jover, 1952). Male’s drilling activity using its
outhparts is brief and can be seen upon its arrival on the trunk. Itctively establishes a corridor of penetration by inserting its headerpendicularly to the trunk through cracks in the cork. After, at
east one day of drilling, time required to excavate the first fewentimeters of a gallery in cork or cambium which is more than 2imes its length, the male stops digging and wait at the entranceor a female’s arrival (Bouhraoua, 2003).
After mating the two beetles re-enter the tunnel, the femaleoing first, the male following behind. It is his job to protecthe gallery against the natural enemies as Colydium elongatumColeoptera, Colydiidae) by obstructing holes and to push out allhe bore dust that will now be produced by the female (Husson,955; Hickin, 1963) and subsequently her offspring.
The female continues to extend the gallery system through-ut her life-time. Tunneling proceeds, quite rapidly, in a radialirection and eventually a branched system is created. The tun-el system can take various sizes depending on the circumferencef the trunk and beetles’ attack density from about 30 to morehan 50 cm (Sarauer, 1932; Husson, 1955; Sousa and Inacio, 2005;
ouhraoua, 2003) and can reach up to 1.8 m in length (Tilbury,010).
Mandibles are used to cut wood and reduce it in size to haven the end a fine texture of sawdust variable in size and color.
n 45 (2013) 74–82 81
Depending on the location of the excavation and the age of thetree, the sawdust is black in color corresponding to the bark (corkdry or mother), but more generally light brown (cambium or sap-wood) to reddish brown (heartwood). Clear sawdust resulting fromcambium and sapwood is typically fibrous, pale in color and com-posed of many filaments (Fig. 8a). When heaped in bark fissuresthe fibers have the appearance of piles of wood wool. The sawdustcoming from heartwood, by cons, takes a finer texture (Fig. 8b).Wood is denser in this part of the tree, reason why the beetletakes more time to dig a gallery. This structure is obtained witha high infestation of the tree; the insect penetrates further intothe plant. Sawdust is ejected by the female and the male have toremove it out of the gallery. Releases of sawdust accumulate inlarge quantities under the trees. Indeed the male cannot evacuatethe wood fibers once the couple has sunk deep into the gallery.This debris then accumulates as small cylinders about 2–5 mmin length inside the tunnel (Bouhraoua and Villemant, 2010). Thetibia of anterior legs (Fig. 2e and f) smooths galleries conditioningthem to cultivate fungi and laying eggs. The female usually laysher first eggs about 4 weeks after entry and continues to lay fur-ther batches at irregular intervals throughout her 10–20 monthslife (Tilbury 2010; Bouhraoua and Villemant, 2010). The male con-stantly clean galleries and at the end of his life, moves at theentry of the gallery to prevent predators’ entrance (Bouhraoua andVillemant, 2010). Spores of ambrosia fungi are basically transportedin females’ specialized structures called mycangia and introducedinto the tunnel (Belhoucine et al., 2011a, b). The fungi quicklybecome established on the walls and form a thick layer thatprovides nourishment for the adult beetles and the developinglarvae.
In P. cylindrus, the mandibles appear as large pincers on the frontof the mouth. The mandibles are heavily sclerotized (Fig. 4), rathercompact structure having cutting edges which move horizontally tograsp, crush, or cut wood to make galleries through the hardwood.Then the function of mandibles is double: (i) to make the galleryand (ii) to select fungi mixed to masticated wood because the beetlefeed on fungi only. Thus, for the most part the wood is not actuallyconsumed (Atkinson, 2004).
Maxillary and labial palpi are found around the mouth servingto move food into the mouth. Both hold each pair of lobed sensorypalpi (Figs. 5–7), which may detect olfactory stimulation (Moonet al., 2008). The mandibles and maxillae are a pair of modifiedlimbs which direct the food to the mouth and hold it in place whilethe mandibles chew the food into small fragments (Moon et al.,2008). In maxillae the lacinia is situated at the distal end of thestypes (Fig. 5a and b). It is serrated or toothed and serves to aid theeating process both by holding and masticating the food.
It has been reported that the maxillae and labial palpi directthe food to the mouth or assist manipulation of food during mas-tication. Therefore, the shovel-like setae at the distal ends whereteeth are located, of both maxillary (Fig. 6a, d–g) and labial palpi(Fig. 7) seem to have some relations with additional functions offood ingestion.
Fine structural examination reveals that the maxillary compo-nents of this ambrosia beetle, not only the cardo and stripes butalso the lacinea and galea have their optimum adaptations to aidtheir dietary behaviors both by holding and masticating the food(Moon et al., 2008). Certainly the female parts are better and moreadvanced than those of males which can be explained by the roleof each one in the wood.
5. Conclusion
The differentiation of the mouthparts structure between maleand female’ P. cylindrus reflects the role and duration of boring of
8 Micro
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ach one, in wood. They are more developed in females as theyrovide the main task of breeding and feeding and the boring time
s very long. By cons, the mouthparts of male despite its strongandibles, they are very little used in boring.
cknowledgement
We thank Armando Equihua-Martinez (Instituto de Fitosanidad,olegio de Postgraduados, Montecillo, Texcoco, Estado de México)
or his comments and corrections.
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