JOURNAL OF MORPHOLOGY 213:349-364 (1992)

Anatomy of Synganglia, Including Their Neurosecretory Regions, in Unfed, Virgin Female lxodes scapularis Say (Acari: Ixodidae) EWA SZLENDAK AND JAMES H. OLIVER, JR. Institute of Arthropodology and Parasitology, B~ologyDepartment, Georgia Southern Uncuersity, Statesboro, GeorgLa 30460-8056

ABSTRACT The central nervous system of Ixodes scapularis is fused into a single compact synganglion. The esophagus runs through the synganglion and divides it into supraesophageal and subesophageal parts. The supraesophageal portion contains a single protocerebrum with four pairs of glomeruli, paired optic lobes and cheliceral ganglia, and a single stomodeal bridge. The subesophageal portion contains a centrally located network of commissures and connectives, a pair of palpal ganglia, paired olfactory lobes of the first pedal ganglia, four pairs of pedal ganglia, and a single opisthosomal ganglion. A retrocerebral organ complex (ROC) in close vicinity of the digestive tract, as described in some other tick species, apparently is lacking. Perhaps the function of the ROC is performed by the paired, large, ganglion-like bodies that lie anterolaterad to the cheliceral ganglia. The rind, which is formed from the neuronal somata and glial cells, surrounds the central fibrous core or neuropile. Neurosecretory cells (NSC) are distinct among rind cells due to their large size and concentration of cytoplasmic neurosecretions. NSC are present throughout the synganglion, although the subesophageal portion contains larger groups of these cells. Histological serial sections, stained with Meola's (Trans Am Microsc SOC89:66-71, '70) paraldehyde fuchsin (PAF) procedure revealed 24 PAF-stained, putative neurosecretory regions in the synganglion of virgin, unfed females. All of these regions appear to be connected and associated with the nearest ganglion and are correspondingly named. Eighteen PAF-positive regions occur in the synganglion. In addition, PAF-negative (green-stained) cells occupy 6 distinct 1992 Wiley-Liss, Inc. regions in the synganglion of unfed, unmated females. The black-legged tick, Ixodes scapularis (Say), is a competent vector of the Lyme disease spirochete, Borrelia burgdorferi, in the laboratory (Burgdorfer and Gage, '86; Piesman and Sinsky, '88) and has been found naturally infected with it in Georgia (James H. Oliver Jr., personal communication). More detailed information is needed on the anatomy of this vector, especially on the nervous system. There is experimental evidence showing early invasion of the central nervous system of rats by B. burgdorferi (Garcia-Monco et al., 'go), but the relationship between this spirochete and the neural tissue is unclear. Little published information exists on the morphology and histology of the synganglion and peripheral nervous system of prostriate ticks (Samson, '09). More information is availcj 1992 WILEY-LISS. INC.

able on the synganglion of metastriate species: Dermacentor andersoni Stiles (Douglas, '431, Haemaphysalis flava Neumann (Saito, '601, Dermacentorpictus Herm. (Ioffe, '64a,b), Boophilus calcaratus Birula ( = annulatus (Say)), Hyalomma anatolicum Koch, Hyalomma detritum Schulze (Tsvileneva, '641, Boophilus microplus (Canestrini) (Binnington and Tatchell, '731, Dermacentor variabilis Say (Obenchain, '74; Coons et al., '74), and Amblyomma americanum (L.) (Coons et al., '74; El Shoura, '89). There are also several reports on the synganglion of the argasid Ewa Szlendak's current address is Department of Applied Entomology, Warsaw Agricultural University, 02-766 Warsaw, Poland. Address reprint requests to Dr. James H. Oliver, Jr., Institute of Arthropodology and Parasitology, Georgia Southern University, Landrum Box 8056, Statesboro, GA 30460-8056.

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E. SZLENDAK AND J.H. OLIVER. JR.

of five animals. One group was sectioned horizontally, another sagittally, and a third transversely. Sections were stained with Meola's ('70) paraldehyde-fuchsin (PAF'). Sections revealed the anatomy of the synganglion and the presence and localization of PAF'-stained groups of cells (PAF-positive and PAF-negative). The number of cells per group was recorded as well as their relative staining density, which was expressed as a value from 1 (light) to 4 (dark). Measurements of cell and nuclear dimensions were made from 30 cells from each neurosecretory region with a BIOQUANTc* Image Analysis System IV (R & M Biometrics, Inc., '88) on an IBM microcomputer. Scheffe's F-test (STAT VIEW 512) following ANOVA was used to compare the dimensions of these cells. Photographs were made with a Wild&M-20 microscope using phase-contrast optics and a Nikon automatic exposure camera with Kodakm 2415 Technical Pan film (ASA 50). The entire synganglion and the main nerve trunks were also examined with an IS1 Super-I1 scannning electron microscope (SEM). Specimens were dissected in Shen's saline and were prepared for SEM examination by removing the dorsal cuticle and other tissues surrounding the synganglion, fixing in aqueMATERIALS AND METHODS ous Bouin's, dehydrating in a graded ethanol Unfed, virgin females of 1. scapularis Say series, and drying in a critical-point dryer, used in this study were from laboratory colo- prior to mounting and coating with gold. nies at Georgia Southern University that origRESULTS inated from Bulloch County, Georgia. The General histology of synganglion colonies were established about ten years ago The synganglion is situated ventromedibut have been prevented from becoming inbred by additions of individuals from nature ally in the idiosoma, mediad of legs I1 coxae. on several occasions. When not feeding, ticks It is subspherical, pyriform (Figs. 1, 3B-D) were kept in glass vials with nylon-mesh caps and somewhat flattened dorsoventrally. In a t 25°C and at 97.5% relative humidity within unfed females it is 307-318 pm long, 278desiccators containing saturated solutions of 287 pm wide, and 182-183 pm deep (Table K2S04 (Winston and Bates, '60) and on a 1). It lies dorsad of the genital duct and ventrad of the salivary glands and anterior 16%L:D cycle. Virgin, three-month-old, unfed females part of the gut. The entire synganglion is located within were attached dorsum up to the surface of paraffin-filled Petri dishes and dissected un- the sinus of the circulatory system (Figs. 1 , 2 , der Shen's saline (Pound and Oliver, '82). 3B,C). Neural tissue, which is the main comThe dorsal cuticle was removed and dissected ponent of the synganglion, and peripheral ticks were placed in aqueous Bouin's fixative, nerves leaving the synganglion are surheld under vacuum for at least 1 hr and rounded by a thin, acellular neural lamella allowed to fix for 48 h r at 27°C. Dissected (neurilemma) (Fig. 2 ) . Below the neural ticks were dehydrated in an ethanol series, lamella is located a thin layer of glial cells cleared in Histosol, and finally embedded in which forms the perineurium and separates Paraplast Plus". Serial sections, cut a t 5 the neural lamella from the rind. The rind pm, were placed on albuminized glass slides. ( = cortex) (Figs. 2, 3D, 4B) is formed from Fifteen ticks representing the same develop- the neuronal somata (perikarya) and glial mental stage were assigned to three groups cells. The rind surrounds a central syngan-

ticks Argas (Persicargas) persicus (Oken) (Robinson and Davidson, '13; Eisen et al., '73), Ornithodoros (0.)moubata Murray (Eichenberger, '701, 0. (Alectorobius)kelleyi Cooley (Sonenshine, '70), Argas (Persicargas) arboreus Kaiser (Coons e t al., '74; Roshdy and Marzouk, '84), Ornithodoros (Pavlovskyella) parkeri Cooley (Pound and Oliver, ,821, and 0. (P.) erraticus Neumann (El Shoura, '86). Some authors have focused attention on the peripheral nervous system (Saito, '60; Obenchain and Oliver, '761, and others on the ultrastructure of tick synganglia (Coons et al., '74; Binnington, '83; El Shoura, '86, '89). In some articles the information is incomplete and usually lacks data on globuli cells or the retrocerebral organ complex (ROC). Moreover, since only a few reports on synganglia of Ixodes (Prostriata) exist, it is of interest to examine the synganglion of I. scapularis and compare the results with those from argasid and metastriate species. This paper presents, in addition, data concerning the localization, distribution, and appearance of groups of cells stained either positively or negatively with the PAF procedure. These cells occur in the rind of the synganglion and are probably neurosecretory andlor neurosecretion storage sites.

35 1

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

TABLE 1. Dimensions of synganglia of unfed, unmated females of I. scapularis from transuerse, horizontal, and sagittal perspectives Synganglia dimensions' Length Width Depth

Horizontal (mm) 307.2 f 2.2 278.0 f 1.9 -

Transverse hm)

Sagittal (m)

-

318.0 t 3.1

287.4 f 2.8 183.1 & 1.8

182.3 f 2.2

-

'Dimensions are means -C standard errors, calculated from horizontal, transverse, and sagittal sections of 10 specimens.

glial fibrous core (neuropile). The neuropile (axons and dendrites of the neuronal somata in the rind) forms the network of neuropilar ganglia that is interconnected by a system of connectives and commissures (Figs. 2,3B-D, 4B). The paired optic, cheliceral, palpal, pedal (I-IV), and opisthosomal nerves, as well as a single esophageal nerve, extend from the neuropilar ganglia. The esophagus (Figs. 2, 4B) runs through the synganglion and divides its compact mass of n e u r a l tissue into supraesophageal (preesophageal) and subesophageal (postesophageal) parts (Fig. 4B). The esophagus enters the synganglion anteroventrally and emerges dorsomedially anterior to the proventriculus (Figs. I, 2 ) . The supraesophageal portion of the synganglion contains the protocerebrum, paired optic lobes, cheliceral ganglia, and a single stomodeal bridge. The subesophageal portion of t h e synganglion contains a centrally located network of commissures and connectives, a pair of palpal ganglia, a pair of olfactory lobes, four pairs of pedal ganglia, and a single opisthosomal ganglion. Supraesophageal ganglion The protocerebrum is a single neuropilar mass which occupies the dorsomedial part of the supraesophageal ganglion (Figs. 3A, 4A,B,

Fig. 1. Dorsal view of 1.scapularis synganglion showing the most prominent nerve trunks that innervate legs and opisthosoma. Note presence of esophagus with part of proventriculus (PR), broken sinus membrane (white arrow), and grape-like bodies (black arrow) of unknown function surrounding the synganglion. Anterior of organism is at top of figure. Scale bar = 80 pm. Fig. 2. Sagittal section of the dorsal part of synganglion and part of digestive tract showing system of commissures and connectives (CC), sinus membrane (SM), neural lamella (NL),rind (R),neuropile (NR),neurosecretory cells from dorsomedial opisthosomal region (star), esophagus (El, and proventriculiis (PR). Top of figure is angled toward posterior. Left side is dorsal. Scale bar = 50 pm.

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Figure 3

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

353

5C,D, 6A). It lies directly anterior to the to the cheliceral ganglia (Fig. 5 C ) . Paired esophagus, between the cheliceral and palpal optic nerves extend from the anterior parts ganglia and dorsad of the stomodeal bridge. of the optic lobes and presumably innervate The anterodorsal, posterior, and ventral the photoreceptor areas. glomeruli are of varying density and occur in Paired, cheliceral ganglia mainly occupy the anterolateral parts of the supraesophthe protocerebrum. Three pairs of small, anterodorsal glomer- ageal ganglion (Figs. 3B,C, 4A,B, 5A,B, 6A,B). uli are positioned a t the anterodorsal surface They are located dorsomediad of the palpal of the protocerebrum. They are situated along ganglia, dorsad of the stomodeal bridge, and a semicircular structure forming a “scal- are laterally adjacent to the optic lobes and loped” border (Figs. 3A, 5C, 6A), such that protocerebrum. Paired cheliceral nerves arise the middle pair of anterodorsal glomeruli is from the cheliceral ganglia and leave the located posteriorly in comparison with other, synganglion anterolaterally. The stomodeal bridge (neuropilar bridge) more anterolaterally situated pairs. Histologically, the anterodorsal glomeruli look less is located in the anteroventral part of the dense than the other two groups of protoce- supraesophageal ganglion (Figs. 3C, 4B, rebral glomeruli. 5A-C, 6B). It is adjacent anteriorly to the The second, paired neuropilar areas of esophagus and medially to the palpal ganglia. higher density than the surrounding neuro- Anteriorly it is surrounded by the rind. The pile are found in the posterodorsal part of the single esophageal nerve extends from the anprotocerebrum, posterior to the anterior teroventral region of the stomodeal bridge to glomeruli, close to the anterolateral parts of the esophagus and pharynx. the esophagus (Figs. 3A, 6A). These large, posterior glomeruli are more widely sepa- Subesophageal ganglion Paired palpal nerves enter the ventrolatrated from each other and are more darkly eral regions of the anterior part of the synstained than anterodorsal glomeruli. The ventral glomeruli are also paired, cylin- ganglion and join posterolaterally with the drically shaped, bilobed, and elongated dor- paired neuropilar masses of pedal ganglia. soventrally (Figs. 3C, 5B,C, 6B). In sections, Palpal ganglia lie ventrolaterally adjacent to they appear much more dense and compact the cheliceral ganglia and create the anterothan other glomerular groups. Ventral glo- lateral margin of the supraesophageal ganmeruli lie in the ventrolateral regions of the glion (Figs. 4A, 5A,B, 6B). Although the palprotocerebrum, dorsad to the stomodeal pal nerves and palpal ganglia are located in bridge, and near the border of the protocere- the supraesophageal region, the palpal commissure lies below the esophagus in the subbrum and cheliceral ganglia, The paired, small optic lobes are located in esophageal region. Thus, because of the palthe anterior part of the supraesophageal gan- pal commissures’ location, the palpal ganglia glion. They lie in the concavity of the antero- and nerves are classified as a part of the dorsal region of the protocerebrum, mediad subesophageal neuropilar mass. The central region of the subesophageal ganglion is composed of a system of commissures and connectives (Figs. 3B-D, 4B, 6AFig. 3. Horizontal sections through synganglion of C). At the margin of the network of commisvirgin, unfed female I . scapularzs. Top of figures is antesures and connectives are located four pairs rior of organism. A: Horizontal section through the proof large, pedal ganglia (Figs. 3B,C, 4A, 6A,B), tocerebrum showing anterodorsal glomeruli (stars) located along a semicircular line and posterior glomeruli which individually give rise to pedal nerves (PSI adjacent and anteriad to the esophagus (Ej.B: that innervate the four pairs of legs. Pedal Horizontal section through middorsal part of synganglion showing a sinus of the circulatory system (SC), nerves from the first pedal ganglia extend cheliceral ganglia (CG), pedal ganglia (Pl-P4), and sys- anterolaterally. Pedal nerves which innertem of commissures and connectives (CC). Note presence vate the second and third pairs of legs exit of one of three lobes in opisthosomal ganglion (arrow- laterally from the midlateral part of the synhead). C: Horizontal section through midventral part of synganglion showing a sinus of the circulatory system ganglion, whereas the fourth pair of pedal (SC), stomodeal bridge (SB), ventral glomeruli (arrows), nerves leaves from the posterolateral part of cheliceral (CG), and pedal ganglia (Pl-P4), and the sys- the subesophageal ganglion in a posterolattem of commissures and connectives (CC).D: Horizontal eral direction. section through the ventral part of the synganglion showThe paired olfactory lobes lie ventromediing olfactory lobes (stars) with glomeruli connected by the fibrous tract with globuli cells (arrowhead). Scale ally, very close to each other, beneath proxibars = 100 km. mal aspects of the pedal I ganglia (Figs. 3D,

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Figure 4

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

4B, 5D, 6C). They are connected by nerve fibers with the first pair of pedal ganglia. The olfactory lobes are filled with numerous glomeruli (Fig. 6C), which have the same, finegrained structure as the ventral glomeruli. Broad fibrous tracts join the olfactory lobes with paired groups of globuli cells (Figs. 3D, 5D, 6C). The globuli cells lie ventrad to the palpal ganglia and are adjacent anterolaterally to the first pedal ganglia. The nuclei of globular cells are small (3 pm), stain deep green in the PAF procedure, and are darker than the nuclei of other cells in the rind. By comparison, the diameter of the nuclei in the rind cells is approximately 5 pm. The unpaired, opisthosomal ganglion has four distinct lobes, although only three can be seen simultaneously in a plane (Figs. 3B, 5E, 6A,B), and is located in the posteromedial part of the subesophageal ganglion (Figs. 3B, 4A,B, 5E, 6A,B). Two pairs of nerves extend from it and leave the synganglion posterolaterally .

Neurosecretory regions and neurosecretory cells Neurosecretory cells are present throughout the synganglion, although the subesophageal ganglion contains the larger number of neurosecretory regions as well as neurosecretory cells (Fig. 6). Neurosecretory cells are distinct among other rind cells because of their large size and the concentration of neurosecretions in the cytoplasm (Figs. 2, 4B). Their secretions are sometimes homogeneous but are more often granular and densely distributed. The nuclei of most neurosecretory cells have the same approximate diameters (ca. 5 pm) as non-secretory neuronal nuclei. The nuclei are larger, reaching almost 8 pm (Table 2) only in prominent,

Fig. 4. Sagittal sections through the synganglion of virgin, unfed female I . scapularis. Anterior of synganglion and organism is to left of figure, dorsal at top. A: Parasagittal section showing ganglion-like body (small asterisk to the left of CG), protocerebrum (PC),cheliceral ganglion (CG), palpal ganglion (PG), pedal ganglia (PlP4), and opisthosomal ganglion (OG). B: Mid-sagittal section through synganglion showing protocerebrum (PC), cheliceral ganglion (CG), opisthosomal ganglion (OG), stomodeal bridge (SB), and olfactory lobes (large asterisk) with olfactory glomeruli. Note presence of two neurosecretory regions anteromedial stomodeal (8) (single arrowhead) and dorsomedial opisthosomal(12) (double arrowheads) and esophagus (E) that runs through the synganglion and divides its mass into supraesophageal and subesophageal portions. Scale bars = 50 pm.

355

presumably active cells filled with neurosecretion. Eighteen PAF-positive neurosecretory regions were found in the rind surrounding the neuropile in unfed, virgin females. All these regions appear to be connected and associated with the nearest ganglia and are named according to the location and ganglion with which they are in contact. Two unpaired neurosecretory regions lie in close association to the protocerebrum: the posterior protocerebral region (1) and the anterior protocerebral region (2). The posterior protocerebral region is situated in the posterodorsal part of the protocerebrum, posteromediad of the posterior glomeruli, anteriorly adjacent to the esophagus (Fig. 6A). This region consists of a pair of large, globular cells (ca. 19.2 pm x 14.9 pm) that possess large, centrally located spherical nuclei ca. 7.9 pm x 6.5 pm, and generally stain very light pink. The cytoplasm contains a few, light staining, fine-grained granules which are equally dispersed. In some cells, clusters of neurosecretory materials occur close to the cell membrane. These clusters stain blue with the PAF procedure, and appear to be similar neurosecretory material as that observed in other neurosecretory cells, in the neuropilar commissures and connectives, and some boundary areas of the ganglia. The second protocerebral neurosecretory region (= the anterior protocerebral region) occupies a large area in the anterodorsal rind mediad of the anterodorsal glomeruli and anterior to the optic lobes (Fig. 6A). Neurosecretory cells in this area (4-5 pairs) are usually irregularly dispersed. Their orange-pink cytoplasm is filled with various sized clusters of violet granules and blue secretory material, which look like chains of droplets. The latter are also observed in some parts of the neuropilar core. Dense accumulations of neurosecretory material usually occupy the margins of the cells or sometimes only appear in separated areas in the cytoplasm; the greater cytoplasmic volume lacks any neurosecretory substances. Cells from this region are oval (ca. 13.2 pm x 8.3 pm) and possess acentric round nuclei (6.2 pm x 5.0 pm). Neurosecretory cells of cheliceral ganglia form three groups: anterodorsal cheliceral (3), ventral cheliceral (4)and dorsolateral cheliceral (5). The paired anterodorsal cheliceral regions (3) lie anteriad of the chelicera1ganglia and anteroventrad of the protocerebrum (Fig. 6A). Each of these regions

356

E. SZLENDAK AND J.H. OLIVER, J R

Figure 5

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

consists of one or two large, globular cells (ca. 14.3 yrn x 12.1 km). These cells contain very fine, light staining granules dispersed equally throughout the entire cytoplasm. Sometimes the granules and other neurosecretory material create small, polygonal, violet aggregates in the yellow-pink cytoplasm. Nuclei of these cells are spherical and have moderate dimensions (ca. 6.2 pm x ,5.0 pm). The second cheliceral group, the ventral cheliceral region (4), is paired, with each part consisting of four or five cells that lie a t the ventrolateral ventral glomerular extremities, between the margin of the cheliceral ganglia and ventral glomeruli (Fig. 6B). Cells from these regions contain abundant amounts of PAF-positive stained materials. The pink cytoplasm of these oval cells (10.9 pm x 7.4 km) is loaded with densely packed, dark staining longitudinally oriented chains of coarse neurosecretory granules and blue staining neurosecretory “droplets” oriented in the same direction. This orientation in the cytoplasm appears as a striated pattern, whereas the rest of the cytoplasm lacks dark staining substances. Paired, large, ganglion-like bodies lie anterolaterad of the cheliceral ganglia (Figs. 4A, 6A). Fibrous tracts proceeding from the cheliceral ganglia join the dorsal part of a ganglion-like body. Neuronal fibers of ganglionlike bodies are filled with small grains and clumps of PAF-positive granules. In addition to the elementary granules, neuronal fibers also contain a n accumulation of other neurosecretory substances in the form of large blue stained “droplets.” Some neurosecretory substances present in ganglion-like bodies appear to have originated from the neurosecre-

357

tory cells which are located in the dorsolateral cheliceral region (5). These paired, neurosecretory areas lie in the dorsolateral extremity of the rind, adjacent to the anterolateral margin of the cheliceral ganglia (Fig. 6B). Only two elongated neurosecretory cells (12.8 pm x 8.3 pm) usually occur in these regions. Their nuclei are globular (5.7 pm x 4.8 km) and lie on one side of the cells. All cells contain violet-staining neurosecretory material, which appears granular and densely packed. Two paired regions are closely associated with palpal ganglia: dorsolateral palpal (6) and anteroventral palpal (7). Dorsolateral palpal (6) cells lie dorsad of the posterolateral margin of palpal ganglia, posteriad and ventrad of the dorsolateral cheliceral regions (Fig. 6B). As with the dorsolateral cheliceral regions (5), these consist of two cells in each area and have a similar appearance and shape (Table 2). Anteroventral palpal (7) cells are globular (11.3 +m x 10.7 km) with central, moderately sized nuclei (5.0 pm x 4.2 pm). Their cytoplasm is light pink and contains a few interspersed aggregations of neurosecretory material (violet granules and blue “droplets”), although most of the cytoplasmic volume is homogeneous and lacks secretory materials. Each of these regions contains 3-4 cells and lies in the anteroventral margin of the palpal ganglia. The anteromediad stomodeal region (8) is one of the most prominent of the paired regions in the I. scapularis synganglion and lies closely anteriad to the stomodeal bridge (Fig. 4B). These regions contain 5-6 large cells (16.8 pm x 12.4 pm) with centrally located, globular nuclei (5.7 pm x 5.2 +m). Cells from these areas contain fine to medium-sized, violet granules dispersed irreguFig. 5. Transverse sections through the synganglion larly in the cytoplasm. Heavy aggregates of ofvirgin, unfed female I. scupularzs. Dorsal part of organdensely stained granules occur in the vicinity ism is at top. A Transverse section through the anterior part of synganglion showing cheliceral ganglia (CG), pal- of unstained areas and clumps of blue pal ganglion (small asterisk toward left of center), sto“droplets” at the cell periphery. moded bridge (SB), and esophagus (El. B: Transverse The single anteroventral commissure and section showing ventral glomeruli (small asterisks) and connective region (91, undescribed in other the same elements as in Figure 5A. C: Transverse section tick species, is located in the anteroventral showing protocerebrum with anterodorsal glomeruli (small asterisks), optic lobes (OP), ventral glomeruli part of the subesophageal ganglion adjacent (stars), stomodeal bridge (SB), and esophagus (El. D: to the entrance of the esophagus (Fig. 6C). Transverse section on the level of the pedal I ganglia, Paired, oval cells (10.8 pm x 8.4 km) in this showing posterior part of protocerebrum (PC), pedal I ganglia (Pl), olfactory lobes (small asterisks) with glo- region are partially filled with coarse granmeruli and globuli cells (small arrows just to outside of ules. The large aggregates of granules are asterisks). E: Transverse section through the posterior irregularly dispersed in the cytoplasm and part of synganglion showing lobes of opisthosomal gansurrounded by unstained areas. Blue secreglion (small asterisks) and pedal ganglia (P4). Scale tory material forms large clusters at the cell bars = 50 bm.

ganglion-like body protocerebrum anterodorsal glomeruli cheliceral ganglion posterior glomeruli pedal I ganglion esophagus pedal II ganglion commissures 81connec:tives pedal 111 ganglion

pedal IV ganglion opisthosomal gangfion

ventral glomeruli cheliceral ganglion stomodeal bridge palpal ganglion pedal Iganglion . esophagus pedal I1ganglion commissures & connectives pedal 111 ganglion pedal IV ganglion '

opisthosomal ganglion

esophagus olfactory lobes with glomeruli globuli cells commissures & connectives

Figure 6

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

359

TABLE 2. Measurements of cell parameters in neurosecretory, PAF-positive regions in synganglia of unfed, unmated female I. scapularis' Region

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18

Neurosecretory ~

No. of cells

Color of

NSC length NSC width

NSC nucleus NSC nucleus length width

~

6.5 f .12' 2.2 t . l Z a b 1.3 f .08" 19.2 t ,349 14.9 f .32d 7.9 t .17' Posterior protocerebral 5.0 ? .lob Anterior protocerebral 8.2 f .36g 2.2 i .13b"d 13.2 f .25cd 8.3 t .20" 6.2 2 .lod Anterodorsal cheliceral 1.8 .20a 1.4 f .loab 14.3 t .2Sde 12.1 f .18bc 6.2 t .12cd 5.0 f .llb Ventral cheliceral 4.1 t . 1 8 e d 3.4 f .llef 10.9 f .24ab 7.4 f .23" 5.0 t .12ab 4.2 f .13ab 4.8 f .07ab Dorsolateral cheliceral 2.1 f .14ab 3.1 f .loef 12.8 f .28bcd 8.3 f .17" 5.7 f .lObCd 1.9 c .18ab 2.9 f .12de 12.8 f .26bcd 8.5 t .16" 5.7 f .12b"d 4.7 2 . l o a h Dorsolateral palpal Anteroventral palpal 3.4 f .14bC2.0 f .1ZabL 11.3 i .21abc 10.7 f .2fjb 5.0 t .OSab 4.2 f .09"b Anteromedial stomodeal 5.6 f .24de 3.8 f .Ogf 16.8 t .24' 12.4 f .23bc 5.7 f .lObCd 5.2 f .Ogb Anteroventral commissure 4.0 c .09" & connective 2.2 f .17sb 3.2 f .13"' 10.8 f .Eab 8.4 t .20a 4.8 f .lla Middorsal esophageal 6.2 f .28ef 3.2 -t .14"' 12.5 f .22bcd 8.9 t .20a 6.1 f .14cd 4.8 f .loab 11.5 f .22abc 8.0 c .19" 5.4 f .12abcd 4.3 c .10"b Ventrolateral pedal I 1.2 f .12a 3.3 f .11" 6.1 f .lob Posteroventral pedal I 1.7 f .13a 3.5 f .12ef 16.1 c .32er 11.6 f .2lbC 7.6 i .16' Ventral pedal I 2.0 .20ab 2.3 i .12d 11.0 f .lgab 8.3 f .lSa 5.1 f .Ogab 4.4 f .O€Ph 7.8 t .17a 5.1 f .Ogab 4.2 t .07ab Ventral pedal I1 1.8 f .20a 2.2 t .17cd 10.5 f .23" 4.4 f .09ab Ventral pedal 111 1.7 f .lSa 2.2 f .08bcd 11.0 f .27ab 8.1 f .26" 5.5 f .llabcd 4.2 f .loab Ventral pedal IV 1.8 f .17a 2.1 f .14abcd11.2 f .26abc 8.2 f 2.1" 5.1 f . O W 6.5 t .1SC Dorsomedial opisthosomal 5.0 f .20de 3.9 2 .lof 17.6 f .46fg 12.8 t .36' 7.5 f .14a Ventromedial opisthosomal 7.8 f .26fg 1.3 f .loab 13.2 f .34cd 10.8 f .21b 5.3 i .12abc 4.9 c . X h

'Values are means i standard errors. Mean numbers of cells per region calculated from 15specimens for each region. Estimates of color (light to dark = 1-4) and dimensions of cells and nuclei calculated from measurements of 30 cells from each neurosecretory region. 'ANOVA was used to compare means for each dimension. If ANOVA showed a difference,Scheffe's multiple comparison F-Test was used to determine which means were significantly different from each other. Means within a column with the same letter superscripted after are not significantly different; those with different letters are significantly different (P< .05).

periphery. The small nucleus (4.8 pm x 4.0 plasm contain sparse, large spots of blue neupm) is situated near the cell membrane and rosecretory material and clumps of flocculent the neurosecretory material is sparse or ab- appearing granules. The latter form striated sent in its vicinity. structures encircled by unstained areas. The middorsal esophageal (10) group of Paired posteroventral pedal I regions (12) neurosecretory cells lies posterior to the exit lie ventrolaterally, close to the ventral extremof the esophagus from the synganglion (Fig. ities of the rind, dorsad to the olfactory lobes, 6A), adjacent to the ventral margin of the and at the border of the pedal I and I1 ganglia esophageal canal. These cells are somewhat (Fig. 6C). Large cells (16.1 pm x 11.6 pm) pyriform (12.5 pm x 8.9 pm) with narrow from these regions have acentric nuclei that ends extending toward the esophageal canal. are somewhat larger (7.6 pm x 6.1 pm) than This region is represented by 6-8 cells with in other neurosecretory regions connected acentric, oval nuclei (6.1 pm x 4.8 pm), dark- with the palpal ganglia (Table 2). Cells are pink cytoplasm with fine to coarse, violet usually densely filled with dark-staining grangranules regularly dispersed in the cyto- ules and blue secretions which, when present, plasm, and a layer of blue secretory material occupy the cell margin. Some cells from this at the cell periphery. region possess lesser amounts of secretion, so Single PAF-positive cells lying among glob- that part of the cytoplasm in these cells lacks uli cells belong to the ventrolateral pedal I PAF-positive material. region (11) (Fig. 6C). These pyriform cells Four other, paired ventral regions are also (11.5 pm x 7.8 pm) with acentric nuclei (5.4 associated with pedal ganglia: ventral pedal I pm x 4.3 pm) and opalescent, pink cyto- (131, I1 (141, I11 (151, IV (16) (Fig. 6 0 . They contain single cells which lie midventrally beneath each of the eight pedal ganglia. Although they generally stain lighter, they have Fig. 6. Drawings based on horizontal sections of a synganglion, showing the location of synganglial neuro- approximately the same cell dimensions (Tapilar ganglia and paraldehyde-fuchsin positive (black, ble 2) and the same cytoplasmic distribution numbered circles, 1-18) and negative (white, numbered of neurosecretory material as the cells of the circles, 14) regions. A: Diagram of section near the ventrolateral pedal I regions. dorsal margin of the synganglion. B: Diagram of section Two single, medial PAF-positive regions, near the middle of the synganglion. C:Diagram of section the dorsomedial opisthosomal (17) and the near the ventral margin of the synganglion.

360

E. SZLENDAK AND J.H. OLIVER, J R

ventromedial opisthosomal (181, are connected with the opisthosomal ganglion. The dorsomedial opisthosomal region (17) consists of five of the most prominent neurosecretory cells in the entire synganglion (Figs. 2, 4B, 6A). Although these large cells (17.6 ym x 12.8 pm) are smaller than the neurosecretory cells from the posterior protocerebral region (Table 21, they stain the darkest and are densely filled with violet, fine to mediumsized granules, consistently distributed in whole, opalescent, pink cytoplasm. A narrow layer of blue secretion occupies the cell periphery. This neurosecretory region is located middorsal of the opisthosomal ganglion. The ventromedial opisthosomal region (18) lies midventrally to the opisthosomal ganglion (Fig. 6B). This region is represented by eight oval (13.1pm x 10.8 pm), lightly staining cells which possess moderate sized nuclei (5.3 ym x 4.9 ym) and light pink cytoplasm. The latter contains fine, irregularly dispersed granules and, rarely, coarse grains that cluster a t the cell periphery. In addition to the PAF-positive cells, PAFnegative (green-stained) cells occur in unfed, unmated female 1. scapularis. These cells are usually globular or oval and occupy six distinct regions in the synganglion (Table 3). One pair of grey-green cells (PAF-negative region 1)the anterolateral stomodeal lies in the anteroventral part of the rind close to the stomodeal bridge just below the cells from the PAF-positive anteromedial stomodeal region (8) (Fig. 6B). A second pair of singular, light green-staining cells (PAF-negative region a), the anterolateral pedal I, occupy the rind in the dorsolateral part of the subesophageal ganglion, anteriad to the pedal I ganglia at the border of the subesophageal and supraesophageal ganglia (Fig. 6B). Two other

pairs of green-staining cells (PAF-negative region 3), the ventral pedal, are located in the ventral part of the subesophageal ganglion. They lie midlaterally beneath the pedal I1 and pedal IV neuropilar ganglia (Fig. 6C). The main PAF-negative region (4), medial opisthosomal, which contains more than twelve large cells, occupies the rind area very near the opisthosomal ganglion (Fig. 6B). Green cells appear along the posterodorsal, posterolateral, and posteroventral margin of the subesophageal ganglion. The concentration of these cells is between the opisthosomal nerve trunks along the line between the points where the nerve trunks exit the synganglion. This region seems to be connected with another paired PAF-negative region (51, the lateral opisthosomal, which consists of two prominent cells occupying the posterolatera1 part of the rind near the point where the opisthosomal nerve trunks exit (Fig. 6B). PAF-negative cells from all of the rind regions mentioned above are large (Table 3); the largest cells are 20.6 pm x 15.3 ym with nearly central, oval nuclei (10.0 ym x 7.4 pm). The PAF-negative cytoplasm stains different shades of green and contains numerous darker, grey, fine-grained granules. Another group of PAF-negative cells stain a malachite color with the PAF-procedure. This region (61, the dorsolateral esophageal, lies in the paired regions posterolateral to the posterior protocerebral region (l),and is adjacent laterally to the esophagus (Fig. 6A). Each region consists of 3-4 pyriform, moderate-sized cells (13.4 ym x 9.2 pm) with oval, acentric nuclei (5.9 pm x 4.4 pm). The malachite-stained cytoplasm of these cells contains a few equally distributed fine, darkgrey staining granules.

TABLE 3. Measurements of cell parameters in neurosecretory, PAFnegatiue regions in synganglia of unfed. unmated female I. scapularis' Region number

Neurosecretory region'

1

Anterolateral stomodeal Anterolatcral pedal I Ventral pedal Medial opisthosomal Lateral opisthosomal Dorsolatcral esophageal

No. of cells per region

NSC length

NSC width

(pm)

(pm)

1.4 i .13% 1.1 i .09" 2.1 i .17* 12.7 t 51' 1.9 ? .13" 3.4 ? .13b

19.3 t .37' 13.2 i .19" 15.3 f .23ab 17.1 i 55" 20.6 -t .25' 13.4 t .40a

14.2 i .38cd 12.1 i .24h 12.8 i .45" 12.9 i .35hc 15.3 t .2gd 9.2 t .24"

NSC nucleus length (pm)

NSC nucleus width (pm)

~

2 3 4 5 6

9.1 i .55c 6.4 i .13"b 6.5 r+_ .16ab 7.2 t .23b 10.0 i .33' 5.9 t .lo"

7.9 i .23" 5.5 2 .14'1 6.2 lr .16Ii 5.6 lr .15h 7.4 i .16' 4.4 ? .lo"

'Values are means 2 standard errors. Mean number of cells per region calculated from 15 specimens for each region. Cell and nuclear dimensions calculated from measurements of 30 cells from each neurosecretory region. 'ANOVA was used to compare means for each dimension. If ANOVA showed a difference,Scheffe's multiple comparison F-Test was used to determine which means were significantly different from each other. Means with the same letter superscripted after are not significantly different;those with different letters are significantly different (P < 0.05).

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

DISCUSSION

The basic arrangement of the synganglion in prostriate species is the same as in argasid and metastriate species. The synganglion shows fusion of supraesophageal and subesophageal ganglia into an integrated, single, central mass with a small protocerebral part of unknown segmentation and a larger tritocerebral part. The deutocerebrum has either been lost or is completely fused with other parts of the brain. This indicates specialization in ticks, since the deutocerebrum is characteristic of primitive Chelicerata. The main differences between I. scapularis and various other tick species include specific patterns of dorsal glomeruli, presence of optic lobes, lack of a retrocerebral organ complex, and relegation of palpal ganglia to the subesophageal portion of the synganglion (as implied by localization of palpal ganglial commissures). Different species of ticks have various patterns of dorsal glomeruli, but the number and degree of their development are rather similar in the three main taxa of ticks (Prostriata, Metastriata, Argasidae). Three pairs of anterodorsal protocerebral and one pair of prominent posterior protocerebral compact masses of glomeruli are present in I. scapularis. A similar pattern exists in three metastriate ticks, D. pictus (four pairs of small and one pair of large, compound glomeruli) (Ioffe, '63), in D. uariabilis (two pairs small and one pair of large glomeruli) (Obenchain, '741, and in B. microplus (one pair of small, anterior and one pair of large posterior glomeruli) (Binnington and Tatchell, '73). A variation in distribution and number of dorsal glomeruli exists in two eyeless argasid ticks-e.g., 0. (Paulouskyella)parkeri and 0. (0.)moubata. Ornithodoros (P.)parkeri lack large posterodorsal glomeruli and the protocerebrum in this species has only three pairs of moderately sized anterodorsal glomeruli (Pound and Oliver, '82). This condition (missing posterodorsal glomeruli in eyeless species) does not occur in 0. (0.)moubata (Eichenberger, '701, in which seven dorsal glomeruli are present. Three of them are prominent and occupy the posterodorsal part of the protocerebrum. The function of the dorsal glomeruli in ticks is still unclear. The corpora pedunculata and central body, typical structures occurring in most Arthropoda, are missing from the protocerebrum in ticks, and only glomeruli (= nuclei of Eichenberger, '70) remain.

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Perhaps t h e protocerebral glomeruli are strictly association centers in ixodid brains (Tsvileneva, '64). The dorsal glomeruli may be homologous to the mushroom bodies of other Chelicerata (Ioffe, '64a). Obenchain ('74) also reports that the dorsal glomeruli of D. uariabilis may correspond to the corpora pedunculata of other arthropods, and "that the absence of globuli cells in this part of the synganglion indicates the decreased importance of optic and tactile associative centers in the supraesophageal nerve mass." The regression of these structures may be associated with the parasitic life style of ticks (Eichenberger, '70). The function of dorsal glomeruli in ticks still remains unclear, but it seems to be different from that reported for the corpora pedunculata of arachnids. Certain arachnids in which vision is of minor importance (e.g., pseudoscorpions) lack not only optic ganglia but also corpora pedunculata (Weygoldt, '85), whereas among ticks the species 0. (0.)moubata, which was described as "eyeless" (Eichenberger, '70) or as possessing only photoreceptors (Binnington, '72), has well-developed glomeruli. I. scapularis females possess characteristic, distinct, small lobes, connected with cheliceral ganglia and located mediodorsad in the supraesophageal portion of the synganglion. The location of these lobes corresponds to the location of optic ganglia described for metastriate species which possess external eyes-e.g., Boophilus calcaratus, Hyalomma anatolicum, H. detritum (Tsvileneva, '64), Dermacentorpictus (Ioffe, '64a), B. microplus (Binnington and Tatchell, '731, D. uariabilis (Obenchain, '74), Rhipicephalus sanguineus (Chow and Wang, '741, and the argasid tick Ornithodoros (0.) sauignyi (Eichenberger, '70). Neither optic ganglia nor their rudiments were reported for the "eyeless" argasid species 0. (Paulouskyella)erraticus, 0. (Alueonasus)Lahorensis (Gabe, '551,O. (Paulouskyella) turicata (Cox, '60), 0. (Alectorobius) kelleyi (Sonenshine, '701, 0. (0.)moubata, 0. (Paulouskyella) tartakowskyi (Eichenberger, '70), Argas (Persicargas)persicus (Eisen et al., '731, 0. (Paulouskyella) tholozani (Gabbay and Warburg, '77), 0. iPaulouskyella1 parkeri (Pound and Oliver, '82), and A . (P.)arboreus (Roshdy and Marzouk, '84). In contrast, the precise location of photoreceptors was described in several species previously thought to be eyeless (Binnington, '72)-e.g., the argasids 0. (0.)moubata, 0. (Paulouskyella) gurneyi Warburton, Ar-

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E. SZLENDAK AND J.H. OLIVER. J R

gas (P.)persicus, and A. (Persicargas) robertsi Hoogstraal, Kaiser and Kohls and the metastriate species Haemaphysalis longicornis Neumann, A p o n o m m a a u r u g i n a n s Schulze, and A. hydrosauri (Denny). There is no evidence that the prostriate I. scapularis possesses any photoreceptors, but several individual “optic cells” were found in the closely related Ixodes ricinus (Peter Diehl, personal communication). An “optic nervous system” was also described for I. holocyclus Neumann (Binnington, ’72). We suggest that the small lobes observed in I. scapularis represent optic lobes, and that the size is correlated with reduction of associated centers and reduction of sense organs. Although optic ganglia were not described in some of the “eyeless” argasids, it would be interesting to reexamine those species to determine if they might have greatly reduced optic ganglia. Optic ganglia of ticks are presumably homologous to optic ganglia of lateral eyes of other Chelicerata (Tsvileneva, ’64). Within the Argasidae two pairs of eyes have been described for Ornithodoros (0.) savignyi (Eichenberger, ’701, 0. (Ornament u m ) coriaceus (Koch) and larval Otobius megnini (Duges) (Cooley and Kohls, ’441, although optic ganglia have not been studied for these species. In addition Ornithodoros (P.)gurneyz, although not having well-developed external eyes, has distinctly differentiated areas externally on the cuticle at the site of concentration of photoreceptors (Binnington, ’72). All of these species are associated with hosts which do not inhabit permanent or semi-permanent nests, a relatively rare condition within the Argasidae. This suggests a correlation between the presence of external eyes in this family and vagrant hosts, a correlation possibly independent of phylogeny. It would be interesting to reexamine the condition of the optical apparatus (glomeruli, photoreceptors, possibly reduced external eyes) in other “eyeless” Argasidae associated with vagrant hosts, such as Ornithodoros (Alveonasus) Iahorensis and 0. (A,)canestrinii. This type of correlation is not consistent in the eyeless metastriate Haemaphysalis, Aponomma, and some Anomalohimalaya. Nevertheless, species with eyes in other metastriate genera have well-developed optic ganglia and search for the hosts. The prostriate (Ixodes)Ixodidae are intermediate to the Metastriata and Argasidae regarding nidiculous behavior and I. scapularis has optic lobes,

but they are small compared with those of the eye-bearing metastriate species. Ixodes individuals are also intermediate between the Metastriata and Argasidae in several other morphological and physiological characteristics (Oliver, ’89). Olfactory lobes, adjacent ventrally to the pedal I ganglia in ticks, are considered as association centers in the Acarina (Ioffe, ’63; Tsvileneva, ’64). The same type of olfactory lobes with globuli cells that we observed in I. scapularis are described in most of the metastriate and argasid ticks mentioned above. These types of olfactory centers also occur in I. ricinus, H. asiaticum Schulze and Schlottke and 0. canestrinii (Birula) (Ioffe ’64a). Glomeruli that are associated with the first pair of tick legs are very similar to the antennal glomeruli of other arthropods (Tsvileneva, ’64). The characteristic shape of olfactory glomeruli and the first pair of legs, which possess olfactory and moisture receptors (Haller’s organ), suggest that forelegs in ticks function similarly to antennae in insects (Tsvileneva, ’64) as well as in locomotion. Similar glomeruli in ganglia and sensory receptors in the first pair of legs were also described for whip spiders (Weygoldt, ’85). These glomeruli in spiders function as association centers and process sensory inputs. A characteristic feature of the tick central nervous system is a high concentration and fusion of neuropilar ganglionic masses. These modifications are considered to be specializations among Chelicerata (Ioffe, ’63; Eichenberger, ’70). Cheliceral and palpal ganglia originate from the subesophageal part of the synganglion and migrate during embryogenesis to the supraesophageal part (Aeschlimann, ’58). Palpal ganglia are described as associated with the supraesophageal or subesophageal part. Criteria used in assigning them to either part are the location of commissures. Palpal ganglia in H. anatolicum, H. detritum, and B. calcaratus are treated as part of the supraesophageal mass (Tsileneva, ’64), although the author “did not see any cross connections between the palpal nuclei either in histological sections or in methylene blue preparations.” In other species, such as D. pictus (Ioffe, ’63), D. variabilis (Obenchain, ’74), 0. (P.) parkeri (Pound and Oliver, ’82), and A. (P.)arboreus (Roshdy and Marzouk, ’84),palpal ganglia are classified as elements of the subesophageal part of the synganglion. Horizontal synganglial sections of I. scapularis show palpal commissures in

SYNGANGLIAL ANATOMY AND NEUROSECRETORY REGIONS

the subesophageal region, although the palpal ganglionic main body occurs near the supraesophageal part. The close association of the palpal ganglion with the subesophageal part can probably be explained by the lack of a deutocerebrum (found in insects and crustaceans). This is probably compensated for in ticks by the fusion of the subesophageal ganglionic mass, which in the absence of antennae receives most of the sensory input from the legs and palps. The opisthosomal ganglion in Z. scapularis is histologically homogeneous but morphologically appears to represent a cluster of multiple ganglia fused into a single ganglionic mass. Our study reveals the presence of at least four distinct lobes in the opisthosomal part of the synganglion that are easily recognizable in tissue sections cut in various planes (Figs. 3B, 5E, 6A,B). Similar anatomy of opisthosoma1 ganglia is present in 0. (A.)kelkyi (Sonenshine, '70) and in A. (P.)persicus(Eisen et al., '73). 0. (0.)moubata opisthosomal ganglia also consist of four fused neuropilar masses, but they can no longer be distinguished as such because metamerization has been completely lost during embryonic development (Eichenberger, '70). Neurosecretory cells (NSC) are a typical component of endocrine systems in Chelicerata. These cells synthesize neurohormones and transport and release them a t the endings of the axons. There are four major PAFpositive regions (Nos. 1, 1 2 , 8 , and 17) in the synganglia of unfed I. scapularis females. The smallest groups of NSC are connected with four pedal ganglia (Nos. 13, 14, 15, and 16)(Table 2). All groups of cells, regardless of their cytoplasmic dimensions, appear to be active in the production or storage of secretion (Table 2). It is difficult to interpret the quantitative variations in neurosecretory products stored in t h e somata, because heavily filled cells can be in a period of intense secretory activity accompanied by a hormonal release into the blood, but they may also be in a storage phase. Material produced in these cells may be transported to a storage organ that can be located near or far away from the site of its production. Neurosecretory material may be transported to a retrocerebral organ complex (ROC), or similar organ, where it is stored and where new secretions from different neurosecretory regions may be added before release into the hemolymph. A retrocerebral organ near the esophagus and adjacent to the synganglion in

363

metastriate and argasid ticks was described by Gabe ('551, Eichenberger ('70), Obenchain and Oliver ('751, Binnington ('831, Pound and Oliver ('82, '84), Roshdy e t al. ('731, Roshdy and Marzouk ('821, and Marzouk et al. ('85). Some of these authors suggest that the ROC is structurally (Roshdy et al., '73; Roshdy and Marzouk, '82) or functionally (Gabe, '55; Eichenberger, '70) homologous to insect corpora cardiaca. Unfortunately, no information about the chemical nature of the compounds stored and released by this organ was presented. Our examination did not reveal an ROC inZ. scapularis (Fig. 2). Perhaps the function of this organ in Z. scapularis is assumed by the paired, large, ganglion-like bodies lying anterolaterad to the cheliceral ganglia (Figs. 4A, 6A). No ROC or other neurosecretory storage organs were reported in representatives of two subfamilies of Ixodidae (Z. ricinus and H . asiaticum) and Argasidae 0. (Alueonasus) canestrinii, 0.(Alueonasus) lahorensis (Ioffe, '64a). ACKNOWLEDGMENTS

We thank Dr. S. McKeever, E.M. Dotson, M.S. Joiner, H.J. Hutcheson, Institute of Arthropodology and Parasitology, Department of Biology, Georgia Southern University for assistance and help in our research. Thanks also to Dr. J.M. Pound, Knipling-Bushland U S . Livestock Insects Research Lab., Kerrville, Texas, and Dr. J.S.H. Klompen, Institute of Arthropodology and Parasitology, Department of Biology, Georgia Southern University for critical reading of the manuscript. Prof. Peter A. Diehl, Institute de Zoologie, Universite de Neuchgtel, Chantemerle 22, 2007 Neuchgtel, Switzerland, kindly informed us of his unpublished observations of "optic cells" in lxodes ricinus. This study was partially supported by the National Institute of Allergy and Infectious Diseases grants 09556 and 24899. LITERATURE CITED Aeschlimann, A. (1958) Development embryonnaire d' Ornithodoros moubata (Murray) e t transmission transovarienne de Borrelia duttonz. Acta Trop. 1515-124. Binnington, K.C. (1972) The distribution and morphology o f probable photoreceptors in eight species of ticks (Ixodoidea). Z. Parasitenkd 40:321-332. Binnington, K.C. (1983) Ultrastructural identification o f neurohemal sites in a tick: Evidence that dorsal complex may be a true endocrine gland. Tissue Cell 15.317327. Binnington, K.C., and R.J. Tatchell (1973) The nervous system and iieurosecretory cells o f Boophilus mzcroplus (Acarina: Ixodidae). Z. Wiss. Zool. 185.193206.

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