PETALURA vol. 1, 1993, p. 1



THE SKELETON-MUSCLE ORGANIZATION OF THE
HEAD FIXATION SYSTEM IN ODONATES AND ITS
EVOLUTIONARY IMPLICATIONS: A COMPARATIVE STUDY


Stanislaw Gorb
Department of Insect Physiology
Schmalhausen Institute of Zoology
B. Chmelnickogo str. 15, Kiev
Ukraine, 252601



ABSTRACT

In imaginal odonates the head is only connected with the synthorax through a very narrow neck region. In resting or feeding position the dragonfly head is stabilized by a mechanical coupling of the postcervical sclerites with specialized areas of the posterior surface of the head capsule. In normal flight this cuppling is inactivated and the mobile head is then acting as statolith, while hair sensilla on its posterior side are registrating any positional changes caused by flight manoeuvers. A single levator-muscle is reaching from each postcervical sclerite (SPC) to a prothoracic apodem. In flight this muscle is contracting and thus actively abducting the SPC, that are situated within the soft cervical membrane, to a medial position. The adduction of the SPC is achieved passively through a spring-effect of elastic cuticular parts, when the dragonfly is resting or feeding (in females also during tandem-flight). In the area of contact between the SPC and the head capsule the involved cuticular surfaces do possess significant modifications and specializations. The adhesion in this area of contact is supported by hypodermal secretions. This arrester- or head fixation system is an unique character of imaginal odonates and includes the following structures of the two body segments head and neck:

1.) A skeleton-muscle apparatus, that moves the arrester parts.
2.) Formations with rather complicated microstructures, - fields of microtricha on the back surface of the head (MFH) and on the postcervical sclerites of the neck (SPC).
3.) A secretory apparatus in the hypoderma of the SPC, that produces lipid substance into the area of contact between the MFH and the SPC.
4.) Sensory organs (sensilles), that signalize the position of the SPC relatively to the other neck sclerites and the posterior surface of the head.

The MFH topography was studied in more than 100 species of 26 odonate families. Seven morphological arrester types (calopterygid, coenagrionid, lestid, gomphid, aeshnid, anactid and libelluloid) could be differentiated, based on peculiarities of the MFH morphology. The skeleton-muscle organization (neck sclerites and MFH morphology, arrester muscles and the mechanism of SPC adduction) is discussed from a view point of constructional morphology.

The stability of the head in the SPC+MFH joint may be provided in two ways:

1.) By enlargement of the arrester base; or ...
2.) By formation of a triangle of joints (pair of SPC + MFH and cephaligers + tentorium).

Some evolutionary implications and conclusions are discussed.



INTRODUCTION

Fixation organs of arthropods are rather variable in their construction and functions. An arrester- or fixation-system of the head is unique in the animal kingdom and restricted to imaginal odonates (Fig. 1.).





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This remarkable masterpiece of evolution includes the following structures of the two body segments head and neck:

The arrester system serves for head strengthening when the dragonfly is resting, feeding or when the female is in tandem-position.

Morphological and physiological aspects of the neck region in several insects, e.g. Diptera (STRAUSFELD et al., 1987), have been studied in detail, but studies in Odonata have been scarce. Four cervical sclerites of imaginal Libellula auripennis Burm. (Libellulidae) and two of imaginal Lestes uncata Kirby (= L. dryas Kirby) (Lestidae) were illustrated by SNODGRASS (1909). A comparison of neck sclerites in different insects, including a larval Uropetala (Petaluridae) and an imaginal Epiophlebia superstes Selys (Epiophlebiidae) was briefly described and figured by CRAMPTON (1926). The nomenclature of neck sclerites after SNODGRASS and CRAMPTON with original names was given by ASAHINA (1954). His table included data for six odonate taxa: Anisozygoptera: Epiophlebia (Epiophlebiidae); Zygoptera: Mnais (Calopterygidae); Anisoptera: Orthetrum (Libellulidae), Gynacantha (Aeshnidae), Davidius (Gomphidae) and Tanypteryx (Petaluridae). The functional morphology of the neck in feeding larval and imaginal Aeshna juncea L. (Aeshnidae) was investigated by POPHAM & BEVANS (1979). The general morphology of the dragonfly head capsule was studied by STEINMANN (1967) and ASAHINA (1957). The function of the MFH and SPC as joint organ was proposed for the first time by BERLESE (1909). He suggested that these structures might be organs for stridulation ("laminae stridulatoriae"). TILLYARD (1917) doubted this function and a head-holder function of these organs ("Kopfklemmen") was briefly discussed by MITTELSTAEDT (1950) in his work about dragonfly flight reflexes. This hypothesis of Mittelstaedt has been supported by PRINGLE (1963).

The dragonfly arrester system has been especially investigated in the fields of external and internal morphology (GORB, 1988; 1989; 1990a: 1990b; 1991a), histology (GORB, 1990b) and physiology (GORB, 1991b; 1991c). Nevertheless some questions have not been treated in these papers: Distinctions of muscle topography in several taxa; the mechanism of SPC adduction; correlation between size and topography of the MFH with the morphology of the head capsule, in different taxa. In the present paper I am discussing these questions, and also peculiarities of the skeleton-muscle organization of the arrester system, from a viewpoint of constructional morphology, together with some evolutionary considerations.





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SPECIES EXAMINED

The external morphology of more than 100 species of 26 families of the order Odonata has been examined:

Hemiphlebia mirabilis Selys (Hemiphlebiidae); Synlestes weyersi Selys (Chlorolestidae); Lestes barbarus Fabricius, L. dryas Kirby, L. sponsa Hans., L. virens Charp. (Lestidae); Argiolestes minimus Tillyard, Dimeragrion percubitale Calvert, Heteragrion chrysops Hagen in Selys, Oxystigma williamsoni Geijskes, Philogenia cassandra Hagen in Selys (Megapodagrionidae); Palaemnema clementina Selys (Platystictidae); Mecistogaster linearis Fabricius, Megaloprepus caerulatus Drury, Pseudostigma aberrans Selys (Pseudostigmatidae); Platycnemis pennipes Pall. (Platycnemididae); Argia vivida Hagen in Selys, Coenagrion pulchelum Van der Lind., Erythromma najas Hans., Enallagma cyathigerum Charp., Ischnura elegans Van der Lind. (Coenagrionidae); Isosticta spinipes Selys (Isostictidae); Nososticta solida Hagen in Selys, Prodaisineura vittata Selys, Epipleoneura lamina Williamson, Psaironeura remissa Calvert (Protoneuridae); Diphlebia lestoides Selys (Diphlebiidae); Rimanella arcana Needham (Amphipterygidae); Libellago lineata Burmeister, Rhinocypha fenestrella Rambur (Chlorocyphidae); Bayadera indica Selys, Euphaea variegata Rambur (Euphaeidae); Euthore fasciata Hagen in Selys, Cora cyane Selys, Chalcopteryx rutilans Rambur, Polythore ornata Selys (Polythoridae); Heliocharis amazona Selys (Dicteriadidae); Iridictyon trebbaui Racenis (Calopterygidae); Epiophlebia superstes Selys (Epiophlebiidae); Petalura gigantea Leach, Phenes raptor Rambur, Tachopteryx thoreyi Hagen, Uropetala carovei White, Tanypteryx pryeri Selys (Petaluridae); Aeshna affinis Van der Lind., Ae. mixta Latr., Ae. grandis L., Anaciaeschna isosceles Müll., Anax goliath Selys, A. imperator Leach, Gynacantha nervosa Rambur, Staurophlebia reticulata Burm., Polycanthagyna melanictera Selys, Acanthaeschna victoria Martin, Tricanthagyna trifida Rambur, Aeschnophlebia anisoptera Selys, Austroaeschna multipunctata Martin, Brachytron pratense Müll., Boyeria vinosa Say, Oligoaeschna modiglianii Selys (Aeshnidae); Stylogomphus suzukii Matsumura in Oguma, Sinogomphus flavolimbatus Oguma, Aphylla producta Selys, Cyclophylla breviphylla Belle, Phyllogomphoides sp., Progomphus obscurus Rambur, Zonophora batesi Selys, Sieboldius albardae Selys, Gomphidia kruegeri Martin, Ictinogomphus acutus Laidlaw (Gomphidae); Chlorogomphus magnificius Selys, Cordulegaster annulatus Don., Anotogaster sieboldii kuchenbeiseri Foerster, Zoraena diastatops Selys (Cordulegastridae); Eusynthemis brevistyla Selys (Synthemistidae); Epophthalmia vittigera Rambur (Macromiidae); Cordulephya pygmaea Selys, Oxygastra cutisii Dale, Gomphomacromia paradoxa Brauer, Somatochlora flavomaculata Van der Lind., Epitheca bimaculata Charp., Cordulia aenea L. (Corduliidae); Anatya anomala Kirby, Brachidiplax sobrina Rambur, Uracis imbuta Burm., Celithemis eponina Drury, Leucorrhinia pectoralis Charp., Libellula depressa L., L. quadrimaculata L., Orthetrum cancellatum L., Sympetrum flaveolum L., S. meridionale Selys, S. vulgatum L., Dythemis velox Hagen, Onychothemis abnormis Brauer, Diastatops pullata Burm., Zenitoptera americana L., Camacinia gigantea Brauer, Tramea chinensis Burm., Zyxomma petiolatum Rambur, Celebothemis delecollei Ris, Zygonyx iris Selys, Aethriamanta brevipennis Rambur (Libellulidae).

As comparative material, specimens of other insect orders, such as Neuroptera (Nemoptera sp., Myrmecoleon sp., Ascalaphus libelluloides), Mecoptera (Panorpa communis) and Embioptera (Haploembia solieri), have been studied too.



METHODS

1.) Light microscopy: Whole-mount preparations of the MFH and SPC, embedded in canada-balsam, have been examined, drawed and photographed with a binocular microscope MBI-11 (magnifications: *200, *400, *600).

2.) Scanning electron microscopy (SEM): The MFH and neck region were air-dried, mounted on holders, sputter-coated with gold and examined with the scanning electron microscopes JEOL JSM-35C, JEOL JSM-T20 and TESLA-BS-301.

3.) Morphometry: 12 values of the head in 26 species of 6 families have been measured (Fig. 2.). In all cases 3-7 specimens of each species have been examined. According to STEINMANN (1967) the values of the dragonfly head are sufficiently constant to warrant such a morphometric analysis.





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The measurement pattern adequately described the general characters of the head capsule as well as the size and topography of the cephalic fields of microtrichia. Correlation coefficients for every pair of values of Lestes- and Sympetrum-samples have been calculated with a personal computer, and clusters according to the closest-neighbour-method were created.

4.) Abbreviations: BS - basisternum, CEP - cephaliger, CX - coxa, LM (or M1) - abductor muscle of the postcervical sclerite, MFH - microtrichia field of the head, NM - neck membrane, OCC - occiput, PN - pronotum, SEC - eucervical sclerit, SPC - postcervical sclerit, TR - tentorium.



RESULTS

1.) Neck sclerites morphology: The dragonfly imago has a well developed neck region. The lateral walls of the neck are built by a pair of eucervical sclerites (Fig. 3.). These SEC are symmetrical relatively to the longitudinal body axis and are anteriorly transformed into two acute processes, called cephaligers. The cephaligers converge towards the centre of gravity of the head. In this point the head tentorium connects with the neck through a pair of mobile joints (Fig. 4.). Ventrally the SEC connect with the basisternum through the neck membrane. The microsculpture of this neck membrane is rather complicated and differs in representatives of different families. In the CEP-region the SEC connects with the neck membrane, which provides mobility for the SEC. The SEC can be moved by a neck muscle system over an axis that crosses the dorso- and ventro-proximal edges of the sclerites. Near their dorso- proximal edge the SEC are connected with the postcervical sclerites. They are isolated from the SEC and connect with NM from three sides. Such an articulation promotes high mobility of the SPC. The SPC are differentiated into a sclerotized basis and an elastic distal part. The distal part is rounded or elongated, and covered by microtrichia, by hair and campaniform sensilles.

2.) Arrester muscles: The SPC is abducted by a SPC-levator-muscle (LM). Its origin is an apodem of the pronotum, its insertion is the cuticular region between the SEC and the SPC. The topography of the muscle insertion was similar in all studied specimens. The pronotal apodem is differently developed and successively elongated in the sequence Cordulegaster, Sympetrum, Cordulia, Aeshna and Gomphus. Consequently the topography of the muscle origin changes too. The directions of forces relatively to the SPC, caused by the LM, are different in different taxa. The LM may be directed postero-dorso-medially (Sympetrum, Cordulegaster, Lestes), dorso-medially (Aeshna, Cordulia, Calopteryx) or anterio-dorso-medially (Gomphus) in species with a long pronotal apodem (Fig. 3.).

As soon as the abduction of the SPC to the longitudinal body axis (by the LM) is completed, the sclerites (SPC) do not contact anymore with the MFH, consequently the head is free and mobile. To establish the SPC-MFH contact, the head must be closer to the neck. The head-neck muscle system, which is similar in most insects, is achieving this effect.





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This muscular complex consists of three pairs of muscles that are moving the head in the horizontal plane (yaw), sagittal plane (roll) and transversal plane (pitch) (MITTELSTAEDT, 1950; PRINGLE, 1963; STRAUSFELD et al., 1987).

3.) The mechanism of SPC adduction: An antagonistic muscle to the SPC - abductor has not been found. The sclerit adduction is passively established through the elasticity of peculiarly organized cuticular structures. The SPC and SEC are posteriorly connected by an elastic cuticular part and are moved independently. This cuticular part can adduct the SPC like a spring. The spring-effect is increased by a pair of hard cuticular "legs" of the SPC-SEC region in Calopteryx, Lestes, Sympetrum, Gomphus and Cordulegaster (Fig. 5.). When the SPC are abducted by the LM, these "legs" are twisting. When the LM are relaxing, the "legs" are untwisting and adduct the SPC. Besides, the SPC-"legs" of Sympetrum do have two supplementary parts of elastic cuticula, - posteriorly and anteriorly. They are probably also involved in sclerit adduction. A soft and elastic cuticula of the distal part of the SPC is damping external forces and stresses on the head capsule, when the SPC is connecting with the MFH.

4.) MFH topography: The MFH and SPC are functionaly connected structures and are consequently correlated in form, size and type of microtrichia. On the other hand some great differences in MFH size and topography have been noticed between the different odonate families, which are probably correlated with several pecularities of the head capsule.

5.) Morphological types of the MFH:

The different types of fixation systems are nearly always family-characteristic, rarely genus- or species-characteristic.



DISCUSSION

Imaginal dragonflies do possess a well developed neck region compared to dragonfly larvae or other insect orders (CRAMPTON, 1926; POPHAM & BEVANS, 1979). A good mobility of the head is necessary for the signalization of body-turns when the dragonfly is in high-speed and intensively manoevering flight (MITTELSTAEDT, 1950). The start of flight muscle activity is inducing the inactivation of the head fixation by contraction of the SPC-levator-muscle. The head is acting as statolith and contacts with hairy plates of the neck and pronotum. This mobility of the head is a result of reducing the base of the head-neck joint nearly to a single point. Consequently this articulation does only have a weak mechanical stability. In several behavioural situations the arrester system can eliminate this shortcoming by enlarging this base (GORB, 1991b; 1991c) (Fig. 9.). Sensory controll of the head fixation is achieved by trichoid sensilla of the head capsule and campaniform sensilla of the SPC. The phenomenon of head fixation is an exceptional case among arthropods. The only other case described in the entomological literature is the head fixation of the Isoptera, managed by mandibular processes that fit into hollows of the prothorax.





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This kind of fixation is increasing the stability of the head-prothorax articulation when the mouthparts are cutting a plant stem (ILYCHEV, 1987).

The odonate arrester system is unique because of the following features:

The structure of the microtrichia on head and neck is an important morphological base for the type of fixation. The presence of different types of microtrichia and consequently of different types of fixation, is explaining the poor correlation of head shape and size with the MFH size. The stability of adhesion is achieved by different principles of fixation (different microtrichia types), and not by enlargement of the area of contact. Important factors for this stability also seem to be the secretion at the SPC-region and the correlation between the microtrichial density on the SPC and MFH (GORB, 1990b).

Comparing rather closely related genera like Aeshna and Anax or Lestes and Coenagrion, one will notice that only a single MFH is present in Aeshna and Lestes, while two MFH of identical size are present in Anax and Coenagrion, on each side of the posterior surface of the head respectively. Double MFH do provide an enlargement of the area of adhesion with the SPC. This type of arrester system allows two possible positions for the SPC (vental and dorsal). Probably intermediate positions for the SPC are also possible, between the ventral and dorsal MFH. The long and flatened microtrichia of the dorso-ventally heightened SPC of gomphids do contact directly with the smooth posterior surface of the head, because gomphids do not possess any MFH. This type of fixation is working just like the fixation of a surface with fine microtrichia plus hypodermal secretion in the praetarsus of many insects, such as Hemiptera, Coleoptera, Mecoptera and Diptera (HASENFUSS, 1978; BAUCHENß, 1979; BAUCHENß & RENNER, 1977; WIGGELSWORTH, 1987; RÖDER, 1984; 1986; GHASI-BAYAT, 1979; GHASI-BAYAT & HASENFUSS, 1980a; 1980b; STORK, 1980; 1981a; 1981b; 1983). By analogy with some Orthoptera (BERNAYS, 1986; BERNAYS & HAMAL, 1987) we may suppose that different forms of head capsules are diet-induced. Different types of head capsule and probably also different types of chasing behaviour (ZAIKA & VORONOVA, 1977; GORB, 1991b) resulted in different constructional types of the arrester system.





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From a viewpoint of constructional morphology there are several constraints concerning the MFH-topography (BAREL & ANKER et al., 1989). To improove head stability during head fixation, a maximum increase of the arrester base is necessary, which could be achieved by a lateral shift of the MFH; but in this case the SPC and its muscles would be essentially lengthened and SPC adduction could not be achieved by cuticular springs. Another constructional constraint, especially in Zygoptera, seems to be the eye-size. The most important constraint is the topographic position of the MFH on the dorso-ventral axis, in relation to the occiput. If the MFH is situated lower than the occiput, then the system in fixed position does have a maximum stability, because the fixation points are forming a triangle (left MFH - CEP - right MFH). This kind of MFH position is only present in some evolutionary advanced taxa, such as Coenagrionoidea, Anax and some libellulids. Most other taxa do have an extremely ventrally placed MFH, which is often even situated close to the hypostomal suture. In contrast to the triangle formation, there is some tangential mobility when the head arrester system is in "fixed" position. In the gomphid-type of arrester system there are other reasons for a free mobility of the head in "fixed" position. In this particular case the arrester construction allows shifting of the SPC along the cuticular fold on the posterior surface of the head capsule. In all these types of arrester system the most important constructional constraints for the dorso-ventral position of the MFH are the size and topography of the mouthparts. The mouthparts of aeshnids and especially of petalurids are somewhat larger than those of libellulids. In taxa with relatively larger mouthparts the extreme ventral position of the MFH is on the level of the ventral edge of the occiput, close to the hypostomal suture. When the MFH are situated considerably lower than the occiput, the arrester base is smaller than it is when the MFH are situated on the same level as the occiput (Fig. 9.; compare the arrester base of libellulids with gomphids and aeshnids for the same values of head width).


Consequently the stability of the arrester joint can be achieved in two different ways:

This second type is evolutionary younger and characteristical for the Libelluloidea.

Evolution of the fixation mechanisms and cuticular structures:

  • a) Elastic "hinge"-type: Present in the Chlorolestidae and Lestinae of the Lestinoidea.
  • b) hook-type: Present in the evolutionary advanced taxa Sympecmatinae (Lestidae), Coenagrionoidea and Libelluloidea.
  • c) "Pulvilles"-type with greatly increased surface: Present in the Aeshnidae and Gomphidae etc.

The general character distribution of the fixation system is rather congruent with the "traditional" phylogenetic tree of odonates. The Epiophlebiidae ("Anisozygoptera") did retain the highest number of plesiomorphic characters. Within the Anisoptera petalurids and gomphids have preserved many plesiomorphic characters. Within zygopteres there are two different evolutionary directions:

  • a) Diminution of the microtrichial fields on the head capsule (MFH) in Lestinoidea.
  • b) Development of two microtrichial fields on each side of the head.





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ACKNOWLEDGEMENTS

Prof. Dr. L.I. FRANTSEVICH, Dr. P.A. MOKRUSHOV, Dr. V.N. FURSOV, S.S. KULCHITSKY (Schmalhausen Institute of Zoology, Kiev, Ukraine); Dr. L.N. PRITYKINA (Palaeontological Institute, Moscow, Russia); Dr. L.A. ZHILTSOVA (Zoological Institute, Sankt-Petersburg, Russia); Prof. Dr. B.F. BELYSHEV, Dr. Yu. HARITONOV (Biological Institute, Novosibirsk, Russia); Dr. S.N. BORISOV (Institute of Zoology and Parasitology, Dushanbe, Tadjikistan); Dr. R.S. PAVLJUK (Museum of Zoology, Lwiw University, Ukraine); Prof. Dr. N.N. SHCHERBAK (Museum of Zoology, Kiev, Ukraine); Dr. J. DE MARMELS (Universidade Central de Venezuela); Prof. Dr. M. SAMWAYS (Univerity of Natal, South Africa); Mr. M. SUGIMURA (Dragonfly Museum, Nakamura, Japan); Dr. M.M. EIDELBERG (Nikitsky Botanical Garden, Jalta, Ukraine); kindly gave me specimens of different taxa. I also thank V.A. KRIVOSHEEV (Botanical Institute, Kiev) and N.T. STRASHKO for their assistance in SEM-microscopy, B.O. MIKHALEVICH for original BASIC-programms for cluster analysis, E.V. GORB (University of Kiev) for helping with the translation of this manuscript, and an anonymous referee. I am very grateful to Mr. G. BECHLY (University of Tübingen, Germany) for his advices and assistance with this paper.



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Submitted 31. May 1993
Reviewed and accepted 20. July 1993





PETALURA vol. 1, 1993, p. 13



FIGURE LEGENDS


Fig. 1.)

Sketch of the head fixation system of odonates (dorsal view):


Fig. 2.)

Pattern of measurements used for cluster analysis:
a = total head width; b = distance between the medial borders of the MFH (arrester base); c1 = maximum width of occiput (base of the head-cephaliger joint); c2 = occiput height, d = eye length; e = eye height; m1 = distance between the upper borders of the head and the occiput; m2 = distance between the lower border of the occiput and the mouthparts; p = distance between the upper borders of the head and the MFH; r = distance between the posterior and the anterior border of the head (head length); s1 = height of the MFH; s2 = width of the MFH.


Fig. 3.)

Neck region of imaginal odonates (lateral view of the interior side):

The lateral walls of the neck are formed by a pair of eucervical sclerites (SEC). These SEC are transformed anteriorly into two acute cephaligers (CEP). The SEC connect with the postcervical sclerite (SPC) near the dorso-proximal edge of the SEC. The origin of the SPC-abductor muscle (LM) is the pronotal apodem, its insertion is the cuticular region between the SEC and the SPC.


Fig. 4.)

The cephaligers (CEP) are symmetrical relatively to the longitudinal body axis. These processes converge anteriorly to one point, which is the centre of gravity of the head. In this point the head tentorium (TR) connects with the neck through a pair of mobile joints. Drawing from Sympetrum sanguineum Mill..


Fig. 5.)

Cuticular structures that achieve SPC-adduction:

Posteriorly the SPC and SEC are connected by parts of elastic cuticula, which achieve the adduction of the SPC by a spring-mechanism. The spring-effect is increased by a pair of hard cuticular "legs" (L1, L2) of the SPC-SEC region. Besides, the SPC-"legs" of Sympetrum do have two supplementary parts of elastic cuticula (SA + SC, SP), - posteriorly and anteriorly.


Fig. 6.)

Cluster diagramm of the head characters:
The characters of the head capsule are well correlated with the arrester base, but only weakly correlated with the size of the microtrichia fields (s1, s2).


Fig. 7.)

Correlation of the arrester base (B), the base of the head-cephaliger joint (C1) and the occiput height (C2) with the head width (A).


Fig. 8.)

Correlation of the arrester base with the head width in ...


Fig. 9.)

Free or fixed position of the head fixation system, caused by arrester action in different behavioural situations:


Fig. 10.)

Morphological types of the MFH in Anisoptera:


An enlarged arrester-base and the formation of a triangular contact (two MFH and the cephaligers) increase the stability of the arrester joint.




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