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The visual system of Lycosa tarentula (Araneae,Lycosidae): Microscopic anatomy of theprotocerebral optic centresJacqueline Kovoor a , Arturo Muñoz‐Cuevas a & Joaquín Ortega‐Escobar b

a Zoologie‐Arthropodes, M.N.H.N. , 61 rue de Buffon, Paris, 75231, Franceb Departamento de Psicologia Biológica , Universidad Autónoma de Madrid , Madrid,28049, España E-mail:Published online: 28 Jan 2009.

To cite this article: Jacqueline Kovoor , Arturo Muñoz‐Cuevas & Joaquín Ortega‐Escobar (2005) The visual system ofLycosa tarentula (Araneae, Lycosidae): Microscopic anatomy of the protocerebral optic centres, Italian Journal ofZoology, 72:3, 205-216, DOI: 10.1080/11250000509356673

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Ital. J. Zool., 72. 205-216 (2005)

The visual system of Lycosa tarentula(Araneae, Lycosidae): Microscopicanatomy of the protocerebral opticcentres

JACQUELINE KOVOORARTURO MUÑOZ-CUEVASZoologie-Arthropodes, M.N.H.N.,61 rue de Buffon, 75231 Paris (France)

JOAQUÍN ORTEGA-ESCOBARDepartamento de Psicologia Biológica,Universidad Autónoma de Madrid,28049-Madrid (España)E-mail: joaquin.ortega@uam.es

ABSTRACT

A new revised histological description of the protocerebral visu-al pathways of Lycosa tarentula (Araneae) is presented. Anatomi-cal links are observed between the posterior and median retinaein the first optic neuropils, and between the anterior lateral andposterior lateral in the second optic neuropils. Long and wide in-temeurons join all three indirect retinal laminae; their large soma-ta are located among the third order globuli and their afferent fi-bres enter the "mushroom bodies", which are the third optic neu-ropils for the indirect retinae. Two of the optic neuropils for theanterior median retinae are completely isolated from those of theindirect retinae. A great number of the third order globuli for theanterior median retinae reach the voluminous and complex arcu-ate body, while a smaller number is oriented towards the "mush-room bodies". The visual pathways described are tentatively relat-ed to the behaviour of L. tarentula.

KEY WORDS: Visual system - Optic centres - Optic lobes of thebrain - Histology - Lycosidae - Araneae - Spiders.

ACKNOWLEDGMENTS

Most of the prints presented in this article are due to the excel-lent work of Francine Devienne (C.N.R.S., University Paris VI),many thanks to her.

(Received 4 August 2004 - Accepted 31 March 2005)

INTRODUCTION

The visual behaviour of Lycosidae is relatively wellknown. Visual information is used for spatial orientation(Papi & Syrjämäki, 1963; Papi & Tongiorgi, 1963; Magniet al, 1964; Ortega-Escobar & Muñoz-Cuevas, 1999) to-gether with idiothetic information. Vision is also impor-tant for courtship behaviour, as has been demonstratedby using videotapes by which visual input can be segre-gated from vibratory or tacto-chemical input (see reviewin Uetz & Roberts, 2002). Working with Rabidosa rabi-da, Rovner (1993) showed that different eyes play dif-ferent roles in conspecific interactions.

In Lycosa tarentula, it has been shown that males canbe visually discriminated from females or preys (Orte-ga-Escobar et al., 1996), and that visual input is neededfor path integration during the day (Ortega-Escobar,2002a). In this species, each eye seems to play a differ-ent role; so, anterior median eyes are necessary forhoming using sky polarized light (Ortega-Escobar &Muñoz-Cuevas, 1999) while they are not able to entrainthe locomotor circadian rhythms to changes of thelight/darkness cycle; this entrainment is carried out byall the other eyes (Ortega-Escobar, 2002b).

The eyes of L. tarentula have been the object of adetailed histological study (Kovoor et al, 1992, 1993)showing that the anterior median eyes have two differ-ent regions: one, frontally directed, with polygonal cells(photoreceptors A) bearing three to five rhabdomeresand arranged in a mosaic, while ventral cells (photore-ceptors B), rectangular in section, and bearing rhab-domeres on two opposite faces, are aligned in parallellines; successive lines of photoreceptors are orientedperpendicularly to each other. This arrangement is thebasis for navigation by using polarised light (Kovoor etal, 1993; Ortega-Escobar & Muñoz-Cuevas, 1999).

Optical physiology measurements also suggest that thisspecies has a very good spatial resolution in its posteriormedian eyes (Kovoor & Muñoz-Cuevas, 1996/1997) onlysurpassed by the Salticidae anterior median eyes. Rhab-doms of all eyes change daily (Kovoor et al., 1995) andthis change is internally controlled (Kovoor et al, 1999)probably by neuroendocrine cells placed at the opticlobes (Kovoor et al, 2005).

The first synaptic zones (laminae) of the optic lobeshave been previously studied (Kovoor et al, 1992). Be-fore joining the laminae, nerves of the posterior retinaeform "mixed" bundles in which axons from PM and PLare associated. The laminae (SI) corresponding to eachtype of retinae are clearly separated from each other,but the presence of 15% PL axons in PM nerves and 6%PM axons in PL nerves very probably results in someextent of functional synergy between the posterior reti-nae. The degree of convergence between the photore-ceptor and second order neuron numbers is high forthe posterior eyes (from 3-6 to 3-9) and low for the an-terior ones. The level of integration of small retinaethrough the laminae seems therefore to be much betterthan that of large retinae.

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The microscopic anatomy of the central nervous sys-tem of spiders was first investigated by Saint-Remy(1890, unpubl. thesis, Univ. of Poitiers). Then followedfundamental studies by Hanström (1921, 1926, 1928,1935) particularly on the optic centres of the brain. Thelatter Author formulated a hypothesis concerning the re-lationships between the anatomy of the CNS and behav-iour. Although modern structural studies have not borneout the hypothesis (Babu, 1965, 1985; Meier, 1967;Strausfeld et al, 1993), we used Hanström's work as abasis for our histological study of the optic neuropils.The first synaptic zones of the optic lobes have beenpreviously studied (Kovoor et ai, 1992), while the pre-sent study completes the histological data by coveringthe second and third synaptic zones in the protocerebraloptic lobes. The necessity of such a precise histologicalanalysis arose from a previous work on the structure ofthe visual system and the activity rhythm of L. tarentulain which it was suggested that a circadian clock is likelyto be present in the optic lobes of this spider and shouldbe histologically identified, although yet unknown andnot alluded to in the case of any other spider.

MATERIALS AND METHODS

The species studied, Lycosa tarentula (L. 1758), was collectedin the field, at Canto Blanco (Madrid, Spain) or reared in the lab-oratory by J. O-E. Adult females (n = 25), males (« = 8) and juve-niles in = 18, from the second to the fifth developmental stage)were studied. Prosomas of adults and whole juveniles were fixedin aqueous Bouin's fluid (24 h at the most), dehydrated in 95%ethanol and 1-butanol, and kept in the latter for 2 to A monthsbefore embedding in paraplast or paraffin wax. Sections, 5-6 urnthick, in the three orthogonal planes were stained by classicalmethods: one-step trichrome, Masson-Goldner trichrome and Hei-denhain's azan. Histochemical reactions and other staining meth-ods were used to visualize a) polysaccharidic substances and an-ionic groups (glycogen, mucosubstances and glycoproteins): PASreaction, alcian blue, aldehyde fuchsin; b) proteins: Danielli's cou-pled tetrazonium reaction, lead hematoxylin and phosphotungstichematoxylin stainings which visualize particularly well the nervefibres, and the ferric ferricyanide reaction for reducing metabolicproducts such as pigments. Procedures are detailed in Gabe's(1968) and Lillie & Fullmer's (1976) handbooks.

Glial cells and connective fibres are deliberately absent fromour description which concerns nerve fibres only. Figure 1 is a di-agrammatic representation of a parasagittal section of the proso-ma of L. tarentula showing the region of the central nervous sys-tem chosen for study.

RESULTS

Second synaptic regions of anterior median retinae (AM S2)

In AM nerves, wide (A) and thin (B) axons are recog-nizable; AM laminae also show two types of fibres (Fig.2). They correspond to two distinct arrangements of thephotoreceptors in the anterior median retinae as de-scribed in an earlier publication (Kovoor et al., 1993).In the more or less twisted efferent fibre tract, the dis-tinction between A and B fibres is difficult to resolve

Maxilla

Fig. 1 - Schematic drawing of the Central Nervous System of Z.tarentula, in situ in the prosoma. Parasagittal section, from cam-era lucida drawings. AL SI, lamella of anterior lateral retina; ALS2, medulla of anterior lateral retina; AM SI, lamella of anteriormedian retina; AM S2, medulla of anterior median retina; MB,"mushroom body"; ON, optic nerves; PL SI, lamella of posteriorlateral retina; PL S2, medulla of posterior lateral retina; PM SI,lamella of posterior median retina; PM S2, medulla of posteriormedian retina.

under the light microscope. The medullae (AM S2) be-gin at the mediodorsal end of the optic lobes, at 200-220 pm from the laminae. They form a dense oval massof afferent and efferent fibres in synaptic contactthrough their ramified endings; fibres of two differentdiameters, some very thin (less than 1 um), can be dis-tinguished (Figs 3, 4). The somata of third order neu-rons are located in two clearly separated sites, one situ-ated anteriorly, and the other posterior to the medullae.A part of second order fibres are the axons of globulicells situated in the optic lobe cortex, anterior and later-al to the medullae AM S2 and forming an islet togetherwith about twenty particularly large monopolar cellsshowing neurosecretory activity (Fig. 5; Kovoor et al.,2005). The axons of these cells form a straight bundleoriented forward and ending among second order in-terneurons from the posterior retinae, in front of themost anterior dorsal part of the "mushroom bodies", ob-servable in horizontal sections of the prosoma at thislevel (Fig. 6). Processes of those AM S2 neurons are al-so oriented towards the "mushroom bodies". More nu-merous second order interneurons of AM S2 are foundin the median posterior cortex, close to the medullae.These neurons send processes to the "central body" (ar-cuate body), generally considered to be the third synap-tic zone (lobula) of the anterior median retinae.

Second synaptic zones of the indirect retinae (PM S2, PLS2, AL S2)

The medullae of the posterior median (PM S2) andlateral (PL S2) retinae are superposed in the optic lobes,

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«täipFig. 2-6 - 2 - View of an optic lobe. Horizontal section of the prosoma. First synaptic zone (lamina) of an anterior median retina (AM SI)and part of the second synaptic zones (medullae) of posterior lateral (PL S2) and median (PM S2) retinae. A , wide fibres, B , narrow fi-bres corresponding to two classes of photoreceptors in AM retina. One-step-trichrome, green filter (scale bar, 50 um). 3 - Parasagittalsection of an optic lobe showing the medulla of an anterior median retina (AM S2); the afferent fibres come from two opposite sites (ar-rows). Masson-Golgner trichrome, green filter (scale bar, 50 pm). 4 - Detail of medulla of an anterior median retina. Fibres makingsynaptic contacts clearly are of two types, wide (arrows) and narrow (arrow heads). Plumbic haematoxylin, green filter (scale bar, 25pm). 5 - Anterior site of third order globuli (G3) the processes of which (arrow) form a part of the medulla of an anterior median retina(AM S2); neurosecretory cells (star) are next to the globuli. Masson-Goldner trichrome, green filter (scale bar, 50 pm). 6 - View of an op-tic lobe in horizontal section of the prosoma. In the middle: anterior lateral end of the "mushroom body" (MB); at the top: medulla of aposterior median retina (PM S2); at the bottom: neurosecretory cells (NS), and a long mass of third order globuli (G3) linked to posteri-or retinae, bordering the optic lobe. Periodic acid-Schiff-Groat's hematoxylin-methyl blue, green filter (scale bar, 100 pm).

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Fig. 7-10 - 7 - Succession of synaptic zones in a prosomal horizontal section. Medullae of posterior retinae (PL S2, PM S2) are separatedfrom each other by connective tissue fibres (arrow heads). AM SI, laminae of anterior median retinae; PL SI, lamina of a posterior lat-eral retina. One-step-trichrome, green filter (scale bar, 100 pm). 8 - Aspects of anterior and posterior lateral laminae (AL SI, PL SI) andmedullae of posterior retinae (PL S2, PM S2). G2, G3, second and third order globuli. Periodic acid-Schiff-Groat's hematoxylin-methylblue, green filter (scale bar, 50 urn). 9 - Detail of a medulla of an indirect retina. Fibrils joining in small nodules (arrow heads; scale bar,20 urn). 10 - Lamina of an anterior lateral retina (AL SI) and its globuli (G2). Some fibres enter the medulla of a posterior lateral retina(PL S2, arrow). Periodic acid-Schiff-Groat's hematoxylin-methyl blue (scale bar, 25 |im).

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Fig. 11-15 - 11 - Relationships between the lamina (AL SI) of an anterior lateral retina and the medulla of a posterior lateral retina (PLS2). Wide fibres (arrows) pass through this region. Masson-Goldner trichrome, green filter (scale bar, 50 urn). 12 - Wide fibres linked tothe lamina of an anterior lateral retina (AL SI). Aldehyde fuchsin-one-step-trichrome, green filter. G2, second order AL globuli (scale bar,50 um). 13 - Mass of third order globuli (G3) along with large interneurons. PM S2, medulla of a posterior median retina. Periodic acid-Schiff-Groat' hematoxylin-methyl blue, green filter (scale bar, 50 pm). 14 - Origin of wide interneurons, large somata located amongthird order globuli (G3) linked to the medullae of indirect retinae. Acid periodic-Schiff-Groat' hematoxylin-methyl blue, green filter(scale bar, 25 um). 15 - Bundle of wide interneurons linked to the lamina of a posterior median retina (PM SI). Acid periodic-Schiff--Groat' hematoxylin-methyl blue (scale bar, 50 pm).

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posterior to the laminae, and clearly separated fromeach other by connective tissue fibres (Fig. 7). The reg-ular arrangement in palisade shown by the first orderneurons in the laminae is lost in the medullae: PM S2and PL S2 appear as numerous small dense nodules setin several non parallel rows forming a kind of cord, 40to 50 um thick, undulating throughout the whole thick-ness and width of the optic lobe for 420 pm, in females,or 380 um, in smaller males (Fig. 8). The fine arborisa-tions of at least two axons terminate in each nodule:one belongs to a second order neuron while the otheris the efferent fibre of a third order neuron whose affer-ent fibre reaches the "mushroom body", a third synapticzone for the indirect retinae. Somata of third order neu-rons are piled in a thick rind lining the median contactzone of both optic lobes. A single efferent fibre seemsto split into two (or even three) nodules (Fig. 9). Theanterior half of these cells is connected with the secondand third synaptic regions of AL and PL retinae; and theposterior half to PM S2 and PM S3 (Fig. 18).

The second synaptic zone of the anterior lateral reti-nae (AL S2) is not completely isolated from PL S2, al-though a set of second order neurons connecting AL SIis found ventro-medially between AL SI and AL S2, sep-arated from neurons serving PL SI and PL S2, which arelocated laterally (Fig. 8). Synaptic endings in the poste-rior ventral part of PL S2 are indeed processes of sec-ond order AL neurons. (Figs 8, 10). Some others ofthese neurons serve PL S2 more internally (Fig. 11).Moreover, large fibres (diameter: 10 to 14 pm) directlyjoin AL SI (Fig. 12); they are, as it seems, processes ofthird order neurons included in the cortex borderingthe median contact zone of the optic lobes (Figs 13,14). These fibres, together with others of the same cali-bre and origin, are connected to PM SI and PL SI andrun as a dense bundle in the axis of each optic lobe. Inthe same region, numerous afferent and efferent fibresare also found, among which are third order PM S2 andPL S2 fibres running transversally or obliquely (Fig. 15).

Third synaptic regions of the indirect retinae: "mushroombodies"

"Mushroom bodies" {corpora pedunculata) are thethird synaptic regions related to the three pairs of indi-rect retinae of L. tarentula. They consist of two sym-metrical fibrous masses, about 500 pm long, lying paral-lel to the axis of the optic lobes, that is, inclined atabout 30° (in females) or 35° (in males) against the hor-izontal plane of the spider body. Both masses are madeup of a round anterior part, the "head", 275 pm in di-ameter, followed by the "shaft", an elongated, slightlynarrower portion with a lateral, acuminate end piece,the "haft". Head and shaft of the "mushroom bodies" lieon the ventral face of the optic lobes. Both shafts com-municate with each other in the posterior third of theirlengths through the transversal "bridge" (anterior visualcommissure; Fig. 16). Anterior and posterior lateralblunt end pieces ["hinterer blinder Stiel" of Hanström

(1921)] may be seen on the same parasagittal section ofthe prosoma (Fig. 17). In the same section, on the dor-sal face of the optic lobe posterior to PM S2, a compos-ite cluster of cortical cells also appears, including thirdorder neurons of AM S2 and about twenty neurosecre-tory cells, the processes of which are connected to wideinterneurons (Kovoor et al., 2005).

The somata of third order neurons are densely piledup along the median face of the optic lobes and theirprocesses extend to the second and third synapticzones Their anterior halves are connected to PL S2, theposterior ones to PM S2 (Figs 6, 8, 13).

Numerous fibres from PM S2, PL S2 and AL S2 reachthe "head" of the "mushroom bodies". The conical bluntlateral end piece of the head receives processes of thethird order neurons of PM S2 (Fig. 17). The ventral faceof the head proper receives one branch of the fibreslinked to PL S2 and also AL S2, the dorsal face receivesthose linked to PM S2 (Fig. 18). Afferent fibres, but alsoparticularly wide postsynaptic fibres (diameter =15 pm)penetrate the "mushroom body" deeply from the distalend of the "shaft", where they ramify and establishsynaptic relations with other fibre arborisations (Figs 19--21). These large and wide fibres which pass throughthe midprotocerebrum are likely to originate from largecell bodies located in the dorsal cortex, anterior to thearcuate body ("central body") (Figs 23, 28).

Optic lobes and both sides of the protocerebrumcommunicate through a double series of nerve fibreswhich form the anterior commissure (bridge). Fibres ofthe visual anterior commissure proper come from (orgo to) the median part of the "shaft" (Fig. 16). Other fi-bres composing the anterior bridge come from differentprotocerebral regions.

Third synaptic region of the anterior median retinae: thearcuate body ("central body", AM S3)

Axons of third order neurons linked to AM S2 con-verge on a particular neuropil named the arcuate body("central body"). It is a double crescent shaped mass,640 pm long, transverse to the prosoma at the dorsalposterior margin of the latter which is moulded by itsconvex side. The tips of the crescent are directed for-wards. Along its concave internal side, nerve fibres ofthe posterior visual commissure run bilaterally (Fig. 22).

The arcuate body is made up of an anterior dorsal, anda posterior ventral, lobe of about the same volume, 200pm thick and 270 pm wide at the most; these are not ful-ly superposed in the horizontal plane: their contact zoneslopes 25°; two thirds of the internal surface of the poste-rior ventral lobe is overlapped by that of the other lobe(Fig. 23). The brain cortex, 50 pm thick, covers the ante-rior dorsal lobe entirely, its concave side excepted; theventral posterior lobe is only half-covered (Fig. 23).

Neurons of the arcuate body cortex are of severaltypes (Fig. 26). The largest somata (nucleus diameter =10 pm) are grouped in a series of clusters comprisingabout a dozen cells, the axons of which are grouped in

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Fig. 16-19 - 16 - "Mushroom body" and anterior visual commissure (AVC). Ha, haft; He, head; Sh, Shaft. Oxydization-aldehyde fuchsin-one-step-trichrome, green filter (scale bar, 50 um). 17 - Afferent fibres related to posterior retinae (PL S2, PM S2) enter the lateral end of"mushroom body" head (MB); wide fibres (*) are also joining the "mushroom body" (arrow head). Phosphotungstic hematoxylin, greenfilter (scale bar, 50 um). 18 - Bundles of fibres from the medullae of the indirect retinae (AL S2, PL S2, PM S2) joining "mushroom body"head (MB). NS, cluster of neurosecretory cells. Phosphotungstic hematoxylin, green filter (scale bar, 50 pm). 19 - Wide interneurons (ar-row) entering the shaft of the "mushroom body" (Sh). Note the dense texture of the haft (Ha). Oxydization-aldehyde fuchsin-one-step--trichrome, green filter (scale bar, 50 pm).

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threads passing between the lobes to the midbrain (Figs23-25). In a male of L. tarentula, the more or less verti-cal course of a part of these fibres forming a thread, 25pm in diameter, could be followed until it entered thesuboesophageal ganglion. At least two other types ofneurons are identifiable in the arcuate body cortex;their ramified processes meet those of afferent fibres ata different level, inside the two lobes; each lobe showsa stratified structure characterized by a succession offive (A-E) distinct layers in the anterior lobe (Fig. 27),and two (F-G) in the posterior one (Figs 29, 30). Thetexture of the layers corresponds to the presence oframified fibrils oriented in multiple directions and to theaccumulation of numerous synaptic junctions betweendendrites belonging to afferent neurons and neurons ofthe arcuate body proper (Fig. 27).

Fibres of the dorsal tract linked to AM S2 enter the ar-cuate body near the lateral ends of the crescent (Figs27, 3D where they meet fine ramifications of cells ofthe arcuate body proper.

Along about 180 pm of its length, the dorsal cortex ofthe optic lobes shows grouped neurons (nucleus diam-eter = 10 pm), easily distinguishable from globuli cells,along with sporadic neurosecretory cells (nucleus diam-eter = 14 pm) which send compact bunches of axonstowards ventral, lateral, and posterior regions of thebrain and especially to the suboesophageal ganglion(Figs 28, 32).

DISCUSSION

Indirect eyes synaptic zones

In an earlier publication (Kovoor et al, 1992) we de-scribed the vast space occupied by the first synapticzones of the indirect retinae in the optic lobes, especial-ly the posterior median retinae which are the largest ofall. The visual pathways of the posterior retinae are notcompletely isolated from each other: PM laminae com-

Fig. 20-21 - 21 - Large fibres deeply penetrate the shaft distal end of "mushroom bodies" where they ramify and make synaptic contactswith other fibre arborisations. Masson-Goldner trichrome, green filter (scale bar, 50 pm). 21 - Detail of Figure 20. Minute black dots in-side the "mushroom body" are synapses sites (scale bar, 20 urn).

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PROTOCEREBRAL OPTIC CENTRES IN LYCOSA TARENTULA (ARANEAE) 213

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Fig. 22-26 - 22 - View from the "mushroom body" (MB) to the arcuate body (AB) and the posterior visual commissure (PVC). Horizon-tal section of the prosoma, Acid periodic-Schiff-Groat' hematoxylin-methyl blue (scale bar, 100 jam). 23 - Parasagittal section of the pos-terior region of the brain showing a thick dorsal cortex, the end part of the "mushroom body" (MB) and the arcuate body (AB) split intwo lobes with long bundles of fibres running in between. One-step-trichrome, green filter (scale bar, 100 urn). 24 - In a transverse sec-tion, small islets of axons in a range between the two lobes of the arcuate body. Acid periodic-Schiff-Groat' hematoxylin-methyl blue,green filter (scale bar, 50 urn). 25 - Detail of Fig. 24. Islets of axons (arrow heads) (scale bar, 25 pm). 26 - Several types of neurons inthe arcuate body cortex, and the more or less thick fibres emerging from some of them. Phosphotungstic hematoxylin, green filter(scale bar, 25 pm).

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32

Fig. 27-32 - 27 - Anterior dorsal lobe of the arcuate body. Layers of different textures (A-E) and posterior visual commissure (PVC). Seriesof fibres from anterior visual centres enter the arcuate body (arrow heads) especially those linked to the medullae of anterior median reti-nae. Plumbic hematoxylin, green filter (scale bar, 50 um ). 28 - Thick bundles of fibres originating from cortex neurons located closely tothe arcuate body (AB) and plunging into the midbrain. Phosphotungstic hematoxylin, green filter (scale bar, 100 pm ). 29 - Posterior ven-tral lobe of the arcuate body. Note the fine dense texture of the two distinct layers (F-G). PVC, posterior visual commissure. Plumbichematoxylin, green filter (scale bar, 50 pm ). 30 - Detail of Fig. 29. In F and G layers, numerous aligned fibrils cross the posterior ventrallobe of the arcuate body; their fine ramifications tangled in various orientations (scale bar, 25 pm ). 31 - Fibres issued from the medullaof an anterior median retina (AM S2) run along the brain cortex and reach the arcuate body (AB, arrow). Other afferent/efferent fibresshow an opposite pathway. Plumbic hematoxylin, green filter (scale bar, 50 pm ). 32 - Formation of fibre bundles from cortex neuronslocated anteriorly to the arcuate body. NS, neurosecretory cell. Phosphotungstic hematoxylin, green filter (scale bar, 50 pm).

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PROTOCEREBRAL OPTIC CENTRES IN LYCOSA TARENTULA (ARANEAE) 215

prise some fibres from ipsilateral second order PL neu-rons and, conversely, PL laminae receive some fibresfrom ipsilateral PM second order neurons. Laminae ofthe AL retinae are devoid of fibres from either PM or PLsecond order globuli. On the other hand, second orderAL globuli are connected with PL medullae. Anterior lat-eral and posterior lateral and median retinae thus carryafferent and efferent messages through synapses in thelaminae or medullae. It should be noted that the im-pulse to initiate or interrupt spontaneous locomotor ac-tivity of L. tarentula originates mostly in the anterior lat-eral retinae (Ortega-Escobar, 2002b).

All wide interneurons joining AL SI, PL SI and PM SIfollow the same path, in the axis of the optic lobes. Sim-ilar fibres, similarly localized, were described by Straus-feld & Barth (1993) in Cupiennius salei (Ctenidae); thecourse of some of these centrifugal fibres was followedup to the laminae of indirect retinae. The large somataof these neurons have been found along with the thirdorder globuli which serve the posterior retinae. Connex-ions of these huge fibres with axons of neurosecretorycells transiting in the same region suggest that they areinvolved in the control of the circadian rhythm both oflocomotor and visual activities of L. tarentula. On theother hand, wide fibres also enter the lateral end of the"mushroom body" head (see Fig. 17); they might there-fore transmit visual information through the latter.

Most large sized neurons, the somata of which are inthe cortex anterior to or bordering the arcuate body,send their axons toward the midbrain (Figs 28, 32) andmake no connexion with visual centres. Nevertheless,some of these descending neurons are connected to the"mushroom body" (Figs 20, 21); they are likely to relayvisual information to motor centres of the suboe-sophageal ganglion. This hypothesis is supported by theobservations of Gronenberg (1990) on mechanosensitiveinterneurons of the nervous system of Cupiennius salei.

AM synaptic zones

Anterior median retinae comprise two types of pho-toreceptors: the medio-ventral ones are able to analysethe polarization plane of the sky light (Kovoor et al.,1993; Ortega-Escobar & Muñoz-Cuevas, 1999; Dacke etal., 2001). A double structure can be detected in AM SIand AM S2, in which wide (A) and thin (B) fibres arepresent. Cell bodies of A neurons are located posterior-ly, and those of B neurons anteriorly to the medullae.Afferent branches of B cells seem (we were unable toactually visualize the connexion) to reach the "mush-room bodies", while those of A cells enter the arcuatebody; information relative to polarized light would thenbe treated in the "mushroom body".

A double structure of the synaptic masses of the ante-rior median retinae has not been mentioned for thectenid C. salei (Strausfeld et al, 1993). This mostly noc-turnal spider spends the whole day hidden in retreatson the plants on which it hunts, moults or courts a part-ner. On the other hand, L. tarentula is active day and

night. During the daytime, it sits inside its terrier, nearthe opening, in wait for a prey; it may also run out for awhile. At night, it spends hours outside its burrow (Or-tega et al, 1992; Ortega-Escobar, 2002b). This spider us-es its sensitivity to the polarization plane of light,among other stimuli, to return to its nest site during theday; moreover, memorization of the nycthemeral varia-tions of the polarization plane provides the spider witha measure of time. A similar structure has been de-scribed in the anterior median retinae of an agelenid,Agelena gracilens, by Schröer (1974, 1975, 1976) whodetected its function of analysing polarized light. Thisspider inhabits a silken tubular retreat opening on to awide silk sheet; it sits and waits for prey which will becaught on the sheet. In both cases, light is perceivedfrom the opening of a tube where the animal remainsmotionless with its prosoma oriented to the sky.

Oxyopidae are hunting spiders which are more orless diurnal. We have studied the structure and physiol-ogy of the visual system in several species belonging totwo genera: Peucetia and Oxyopes (Kovoor & Muñoz--Cuevas, 1996/97; Muñoz-Cuevas et al, 1998). Oxyopeslineatus represents an evolutionary line in which visualchannels of communication have become predominantand are used in most behavioural pathways. The verysmall anterior median retinae present a double structuresimilar to that found in the AM retinae of L. tarentula; itcan be inferred from this similarity that Oxyopes lineatusanalyses polarized light. The optic lobes of these spi-ders are also remarkable in their size and complexity.The "mushroom bodies", which succeed the indivisiblemasses of indirect eye medullae, are especially largeand lobulated; on the contrary, the arcuate body seemsrelatively small (unpublished observations).

Volumetric relationship between "mushroom bodies" andarcuate body

The inverse relative volumes of "mushroom bodies"and the arcuate body, first indicated by Hanström(1928) and highlighted by Babu (1965, 1985) haveevoked different interpretations partially refuted byWeltzien & Barth (1991). In the case of Phidippusregius, salticid spiders whose exceptional visual perfor-mances guide their behaviour, Weltzien's observationsand measurements of the optic lobes showed the vol-ume of the "mushroom bodies" to be particularly largecompared to the arcuate body; this Author also statedthat all the retinae of P. regius were in connexion withthe "mushroom bodies" (Weltzien, 1988, unpubl. diss.,Univ. of Frankfurt-am-Main). On the other hand, in C.salei, a nocturnal species, the arcuate body, very largecompared to the "mushroom bodies", is the third neu-ropil for the anterior median retinae (Strausfeld et al,1993). The evolution of certain lineages of spiderswhich are adapted to illuminated habitats seems to in-volve an extensive development of the "mushroombodies" and a reduction of the arcuate body to other in-tegrative functions; this neuropil might be considered to

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be the "primordial" visual centre, conserved in seden-tary species living in shady or dark habitats.

The diurnal and nocturnal functioning of the visualsystem, corresponding to various visually guided activi-ties of L. tarentula, matches the large size and complex-ity of the "mushroom bodies". Nevertheless the arcuatebody is also well developed and complex in structure.The integrative neuropils of the brain thus reflect the in-termediary evolutionary characteristics of the structureand behaviour of this species. Both sedentary andvagabond, it makes use of visual, as well as vibratory,and tactochemical stimuli for hunting prey, and forcourtship and mating.

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