0{; Reprinted from the JOURNAL OF PARASITOLOGY Vol. 60, No.4, August 1974 p. 687-698 Made in United States of America Copyright @ 1974 by the American Society of Parasitologists FINE STRUCTUREOF THE CENTRALNERVOUS SYSTEMOF DERMACENTORVARIABILIS (SAY),AMBLYOMMAAMERICANUM (L.), AND ARGAS ARBOREUS KAISER,HOOGSTRAAL, AND KOHLS(IXODOIDEA)*

Lewis B. Coons,t Mohamed A. Roshdy,:I: and Richard C. Axtell§

ABSTRACT: The central nervous system of unfed male Dermacentor variabilis (Say) and female Am- blyomma americanum (L.) (Ixodidae), and female Argas (Persicargas) arboreus Kaiser, Hoogstraal, and Kohls () was studied by light microscopy using epon sections stained with Azure II and PAS, and by transmission electron microscopy. The fused central nervous system is enclosed within a periganglionic blood sinus and penetrated by the esophagus. This system consists of an outer neural lamella containing layers of homogeneous finely granular material and periodically cross-banded collage- nous fibrils, a cortex of perineurium, glial cells, at least 3 neuronal types, and an inner neuropile of nerve fibers (axons and dendrites) partially ensheathed by glial processes. Large intracellular spaces occur in perineural glial cells in A. arboreus, but heavy glycogen deposits are observed in perineurium of D. vari- abilis and A. americanum. The fine structure of the nervous tissue and of a peripheral nerve is com- pared to that of insects and mites as a basis for investigating structural changes in infected by vi- ruses and rickettsias.

The general anatomy and histology of the lular organization and fiber tract intercon- central nervous system have been studied nections in the supraesophageal and subesoph- by Robinson and Davidson (1913), Douglas ageal portions of the nerve mass of several ( 1943), Sonenshine (1970), and Eichenberger ixodid and argasid ticks. Neurosecretory cells (1970). Ioffe (1963), Tsvilleneva (1965), in nervous tissues were demonstrated by Gabe and Eichenberger (loc. cit.) described the cel- (1955), Ioffe (1964, 1965), Dhanda (1967), and Eichenberger (loc. cit.). Received for publication 14 December 1973. However, the fine structure of the central * From Research Projects MR041.09.01-0037 A 6HJ nervous system is less known in acarines than and MF51.524.009-301OB F61, Bureau of Med- in insects. Recently, Eichenberger (loc. cit.) icine and Surgery, Department of Navy, Wash- ington, D. C. The opinions and assertions con- and Chow et aI. (1972) gave brief electron tained herein are the private ones of the authors microscope descriptions of some structures in and are not to be construed as official or as re- the nervous tissue of moubata flecting the views of the Department of Navy or (Murray) and Rhipicephalus sanguineus Latre- of the naval service at large. This work was also ille, respectively, and Coons and Axtell (1971) supported in part by a special grant from the studied the cellular organization in the syn- Office of Naval Research and by Agreement 03-005-01 between the NIAID ( NIH ) and ganglion of the mite Macrocheles muscae- NAMRU-3. domesticae (Scopoli). t Electron Microscope Center, Department of Ento- In studies to increase knowledge of tick mology, Mississippi State University, Drawer internal organs at the electron microscopic EM, Mississippi State, Mississippi 39762. Pub- level, we described the fine structure of Der- lication No. 2747. Mississippi Agricultural and Forestry Experiment Station. macentor variabilis (Say) salivary glands :I:Department of Zoology, Faculty of Science, Ain (Coons and Roshdy, 1973) and of a neuro- Shams University, and Consultant, Medical hemal organ in Argas (Persicargas) arboreus Zoology Department, U. S. Naval Medical Re- Kaiser, Hoogstraal, and Kohls (Roshdy et aI., search Unit Number Three (NAMRU-3), U. S. 1973). The present paper describes the fine Interests Section, c/o Spanish Embassy, Cairo, Arab Republic of Egypt. structure of the central nervous system of Der- § Department of Entomology, North Carolina State macentor variabilis (Say) and Amblyomma University, Raleigh, North Carolina 27607, americanum L. (Ixodidae) and of A. (P.) where this microscopy was conducted with sup- arboreus (Argasidae) and provides basis for port from the Office of Naval Research, Naval Biology Program Contract NOOO14-70-A-0120- further investigations of this organ in pathogen- 001 (R. C. Axtell, principal investigator). infected ticks. 687 688 THE JOURNAL OF PARASITOLOGY,VOL. 60, NO.4, AUGUST 1974 COONS, ROSHDY,AND AXTELL-FINE STRUCTUREOF THE CNS OF VARIOUS TICKS 689

MATERIALS AND METHODS the cortical region of each tick species cor- Ticks investigated were unfed male D. variabilis respond to the motor or association-motor and virgin female A. americanum collected from neurons, neurosecretory neurons, and globuli nature near Raleigh, North Carolina, and unfed cells, respectively, described by previous au- virgin female A. (P.) arboreus from a laboratory thors. The globuli cells with characteristic, colony originating from the type locality of this species near Cairo and maintained in the NAMRU-3 intensely staining nuclei, are packed in a paired Zoology Laboratories. mass lying in the first pedal ganglia (Fig. 2). For electron microscopy, materials were fixed, Glial cell bodies and processes are often ob- dehydrated, embedded, and sectioned as previously served between the neurons and in the neuro- described (Coons and Roshdy, 1973). Epon-em- pile. bedded sections, 0.5 I-' thick, were stained with Azure II as a general survey stain and with Transmissioneledron microscopy(Figs. 5-22) periodic acid-Schiff ( PAS) reagent to demonstrate carbohydrates. Neural lamella: The ultrastructure of the neural lamella is the same in each species RESULTS examined (Figs. 5-8). This external con- Lightmicroscopy(Figs. 1-4) nective tissue sheath (2 to 5 f-tthick) consists The central nervous system of each species of repeated layers of homogeneous, finely consists of a fused ganglionic mass with periph- granular, disorganized material. The outer- eral nerves extending to various body organs. most layer is thicker in the two ixodid than in This mass is traversed by the esophagus and the argasid species. The areas between these thus divided into supraesophageal and sub- layers are occupied by a feltwork of col- esophageal portions which fuse laterally. lagenous fibrils showing periodic cross-banding As seen in Azure II-stained epon sections (Fig. 6) and embedded in an amorphous (Figs. 2-4), the nervous tissue is surrounded matrix. No organelles were observed in the by an extracellular neural lamella and consists neural lamella. of two zones. The outer zone, or cortex, is Axons containing neurosecretory vesicles comprised of perineurium, glial cells, and neu- were frequently observed in the neural lamella ronal cell bodies. The inner zone, or neuropile, (Figs. 5, 7, 8). Examination of serial sections consists of nerve fibers (axons and dendrites) showed these axons ending near the neural and glial cells and contains the esophageal lamella surface with no evidence of axonal canal. The nervous mass is located in a thin- extensions into the periganglionic sinus. walled periganglionic blood sinus connected Perineurium: Beneath the neural lamella to the heart by the aorta (Roshdy et aI., 1973). lies a complex series of glial cell layers forming In D. variabilis and A. americanum, the the perineurium (Figs. 8-10). Cells bordering perineurium is adjacent to the neural lamella the neural lamella have more irregular mem- and some cells contain heavy deposits of PAS- branes enclosing narrow intercellular (or extra- positive material, possibly glycogen (Fig. I). cellular) spaces, axons containing neurosecre- However, ill A. (P.) arboreus, the perineural tory vesicles, and tracheal elements. Junctional cells form large intracellular spaces or "vacu- specializations were not observed between ad- oles" demarcated by a network of cell mem- jacent cell membranes. In both D. variabilis branes (Fig. 2). Few glycogen deposits were and A. americanum, perineaural cells con- observed in the cell cytoplasm. taining heavy glycogen deposits (identified by Three major types of neurons (N 1-3) in characteristic rosette appearance) form an ap-

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FIGURES1-4. Photomicrographs of cross sections of the tick central nervous system. 1. Dermacentor variabilis showing heavy deposits of glycogen (Gl) in the perineural glial cells (Pn) and processes ( PAS reaction). 2. Argas (Persicargas) arboreus at the level of first pedal nerve (PN,) showing types 1,2, and 3 neurons (N 1- N3) and intracellular spaces (IS) in the perineurium (Pn) (Azure II stain). 3. Am- blyomma americanum showing compact neurons (N) in the cortex and absence of intracellular spaces in the perineurium (Pn) (Azure II stain). 4. Argas (Persicargas) arboreus showing type 1 (N,) and type 2 (N2) neurons and glial cell bodies (GC) in a ganglion. Es, esophagus; Np, neuropile; NL, neu- rallamella; PgS, periganglionic blood sinus; W, wall of blood sinus (Azure II stain). 690 THE JOURNAL OF PARASITOLOGY,VOL. 60, NO.4, AUGUST 1974

.-- COONS, ROSHDY, AND AXTELL-FINE STRUCTUREOF THE CNS OF VARIOUS TICKS 691

parently continuous layer surrounding neuronal similar to those described for the mite Macro- cell bodies in the cortex. However, in A. (P.) cheles muscaedomesticae (Coons and Axtell, arboreus, glial cells in the perineurium form 1971), were observed in the nervous tissues an extensive system of tortuous extensions of both ixodids and the argasid tick. containing large intracellular spaces, or empty, Type 2 neurons are neurosecretory cells membrane-bound vacuoles (Figs. 8, 10). The (Figs. 15-17) distributed among other neurons. velY thin cytoplasm surrounding these spaces These cells, 5 to 15 p. in diameter, differ from contains mitochondria, microtubules, free ribo- type 1 neurons by having a much greater somes, and small vacuoles (Fig. 10, insert). cytoplasmic volume. The cytoplasm contains Contiguous surface membranes of adjacent cells numerous membrane-bound electron-dense neu- enclose narrow intercellular spaces and lack rosecretory vesicles apparently manufactured in junctional specializations. the Golgi region (Fig. 16), mitochondria, Glial cells: Other glial cells are found rough endoplasmic reticulum, and lysosomelike between neuronal cell bodies in the cortex and bodies. at the neuropile periphery. Glial cells investing Type 3 neurons, grouped in a paired mass neurons (Figs. 11-18) have several glial sheaths in the first pedal ganglia of both ixodid and which may extend into deep invaginations on argasid ticks (Figs. 2, 13, 14), are uniform the surface membranes of the perikaryia (Fig. in size (ca. 6 p.) and ultrastructure. Nuclei 17) . Glial cell bodies contain smooth endo- containing large, electron-dense clumps of plasmic reticulum, mitochondria, microtubules, chromatin occupy most of the cell body. The and glycogen deposits. Extracellular spaces cytoplasm contains a few mitochondria, rough filled with amorphous electron-dense material endoplasmic reticulum, and free ribosomes. are frequently observed between glial cells in Glial ensheathment of these closely packed the cortex of ixodid ticks but not the argasid neurons is much narrower than that of other tick (Fig. 18). neuronal types and sometimes absent between Neurons: This study is not concerned with contiguous cell membranes (Fig. 13). identifying functional types of neurons, which Neuropile. Glial cells: Within the neuropile, is better accomplished by the light microscope. glial cells originate from two different sources, At a descriptive level, we attempt to divide from extensions of the cortex glial cell layers neurons in the cortical region into three major (Figs. 11, 19) and from spiderlike glial cells types of neuronal cells bodies which are con- originating in the neuropile (Fig. 20). These sistently present in the tick species investigated. latter cells partially ensheath nerve fibers and Type 1 neurons (6 to 9 p.), commonly dis- tributed in all ganglionic centers, are char- surround tracheal elements by an extensive acterized by a large nucleus and a relatively system of glial processes several microns in small cytoplasmic volume. The perikaryia con- length. The cytoplasm contains numerous tain free ribosomes, rough endoplasmic retic- mitochondria, microtubules, free ribosomes, a ulum, mitochondria, a Golgi region, and lyso- small amount of rough and smooth endoplasmic somelike bodies. Occasional "dark" neurons, reticulum and scattered glycogen deposits. In

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FIGURES5-9. Transmission electron micrographs of neural lamella and perineurium in the tick central nervous system. 5. Neural lamella of Amblyomma americanum showing repeated layers of homogeneous material (arrows) and collagen fibrils (CF) embedded in amorphous matrix. An axon (Ax) containing neurosecretory vesicles (NV) is found within the neural lamella. PgS, periganglionic blood sinus. X 15,000.6. High magnification view of collagen fibrils in the neural lamella of Amblyomma ameri- canum showing cross-banding periodicity. Asterisk indicates amorphous matrix. X 40,000. 7. Neural lamella and perineurium (Pn) of Dermacentor variabilis. Arrow indicates perineural cells bordering neu- rallamella. Nu, nucleus; M, Mitochondrion; rest as in Fig. 5. X 15,000. 8. Neural lamella and peri- neurium (Pn) of Argas (Persicargas) arboreus. A large intracellular space (IS) is shown in a perineural cell whose cytoplasm is indicated by asterisk. GC, glial cell; ER, rough endoplasmic reticulum; rest as in Figs. 5 and 7. X 9,700. 9. Perineurium of Amblyomma americanum showing perineural cell containing large glycogen deposits (GI). Double arrows indicate border of perineurium and neural lamella. Single arrows indicate synaptic area in neuropile (Np). Tr, trachea; rest as in Figs. 5, 7, 8. X 7,500.

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II 'I COONS, ROSHDY, AND AXTELL-FINE STRUCTUREOF THE CNS OF VARIOUS TICKS 693

the ixodid ticks, extracellular spaces similar in different organs. Sensory and motor axons are structure and probably contiguous with those mixed in these nerves (Tsvileneva, 1965). in the cortex are found in the glial cells and Enclosed within a limited hemolymph space their extensions. extending from the periganglionic sinus (Fig. Nerve fibers: The neuropile, the most com- 2), each nerve is ensheathed by a thin neural plex region of tick nervous tissue, is almost lamellar extension. The neural lamella of the filled by nerve fibers (axons and dendrites), nerve, 0.6 fL thick, shows little organization 0.13 to 5.0 fL in diameter, which are partially and contains no demonstrable collagenous ensheathed by thin glial cell processes (Figs. fibrils (Fig. 22). Light areas in the lamella 19-21). Unsheathed nerve fiber (Fig. 20) appear to contain fine fibrillar structures. contacts are of three forms: longitudinal, cross, Lamellar invaginations extend deeply through- and end-knob. Synaptic areas appearing as out the nerve and ensheath individual nerve dark, thickened membranes are also found fibers, 0.26 to 6 fL in diameter. between contiguous unsheathed nerve fibers Glial cell bodies below the neural lamella (Fig. 9). contain sparse mitochondria, membranous cis- The nerve fibers contain numerous neuro- ternae, and numerous micro tubules oriented tubules, peripheral mitochondria, free ribo- almost parallel to the axons. Thin glial sheets somes, a few scattered rough and smooth with cytoplasmic traces surround individual endoplasmic reticulum, and random glycogen axons to form mesaxons. These mesaxons are deposits. Axons containing neurosecretory ves- separated from the axonal surface membranes icles identical to those in neurosecretory cell by thin lamellar involutions originating from bodies are common in the neuropile. the outer neural lamella. Esophageal canal: The esphagus, penetrating The nerve fiber cytoplasm, or "axoplasm," the neuropile through an esophageal canal contains numerous neurotubules, a few mito- (Fig. 21), divides the nervous mass into chondria, and smooth endoplasmic reticulum. supraesophageal and subesophageal portions. Some axons contain neurosecretory vesicles. The esophageal canal is lined by a thin (0.3 Smaller nerve fibers are usually grouped. fL) layer of amorphous material containing They have few glial cell extensions and are little fibrillar organization and apparently con- separated from each other by thin lamellar tinuous with the outermost layer of the neural material. lamella (Fig. 3). This layer is underlined by DISCUSSIONAND CONCLUSIONS numerous glial cells similar to those in the perineurium. The general organization of the The tick nervous system is an important site esophageal canal resembles that of the mite for spirochete growth and multiplication (Burg- Macrocheles muscaedomesticae (Coons and doner, 1951) and viruses and rickettsias also Axtell, 1971). develop in this system (:Rehacek, 1965). Ob- Peripheral nerves: Peripheral nerves arising servations on the nervous system structure in from the various ganglionic centers extend to uninfected ticks should be followed by com-

1

f COONS, ROSHDY, AND AXTELL-FINE STRUCTUREOF THE CNS OF VARIOUS TICKS 695 parative studies of infected ticks in the hope intracellular spaces or "vacuoles" observed in of discovering weak lines in the pathogen-tick the perineural cells are characteristic of A. association that may be utilized for biological (P.) arboreus. These spaces may be a common control. feature of argasids, as they were also observed The general organization of the central in A. (P.) persicus (our unpublished obser- nervous system in the unfed adult D. variabilis vations) and O. moubata (Eichenberger, 1970, and A. americanum (Ixodidae) and A. (P.) fig. 7). Similar but mainly extracellular spaces arboreus (Argasidae) conforms to that de- forming a "glial lacunar system" in the peri- scribed for other tick species by using the neurium of several insects were described by light microscope (Ioffe, 1963; Tsvileneva, Wigglesworth (1960), Smith and Treherne 1965; Eichenberger, 1970), and to that of (1963), Smith (1968), and others. These insects by using the electron microscope (Hess, spaces are known to play an important trophic 1958; Wigglesworth, 1960; Smith and Tre- role in nervous tissues. Therefore, it is reason- herne, 1963; Smith, 1968) and to the mite able to assume that the intracellular spaces in Macrocheles (Coons and Axtell, 1971). the argasid nervous system are analogous to The composite fine structure of the neural the insect glial lacunar system, and that soluble lamella in the three tick species suggests a nutrients may be stored or passed through these closer structural similarity to insects than mites. vacuolar spaces to underlying neurons. How- Histochemical and electron microscopic in- ever, the ixodid ticks possess no such intra- vestigations of the insect neural lamella have cellular spaces but instead have large glycogen demonstrated the collagenous nature of the deposits in the perineural glial cells. It is un- periodically cross-banded fibrils which are em- certain whether this difference in perineurium bedded in a mucopolysaccharide matrix (Ash- structure in argasid and ixodid ticks is related hurst, 1968; Treherne and Pichon, 1972). Like to the remarkable difference in feeding habits insects the outer, homogeneous, finely granu- of both families, a factor possibly influencing lar layer in the neural lamella appears to be nutritional requirements and reserves in the repeatedly deposited between the fibrillar nervous tissues. layers. In the mite Macrocheles, the neural The presence of three types of neurons in lamella is a single homogeneous layer lacking the nervous tissues of D. variabilis, A. ameri- fibrillar structures (Coons and Axtell, 1971). canum, and A. arboreus conforms with light Tick and insect neural lamella possibly microscopic observations in other tick species function similarly. In insects, they are assumed (Ioffe, 1963; Tsvileneva, 1965; Eichenberger, to provide support to underlying tissues, resist 1970). Type 1 neurons correspond to motor positive hydrostatic pressure, and allow relative or association-motor neurons, type 2 to neuro- permeability to nutrients and ionic exchange secretory cells, and type 3 to "olfactory" globuli (Smith and Treherne, 1963; Ashhurst, 1968; cells in the first pedal ganglia, as described Ashhurst and Costin, 1971; Treherne and by these authors. Chow et al. (1972), working Pichon, 1972). on the Rhipicephalus sanguineus brain, clas~i- We find a notable difference between the tied neurons into motor and ganglionic cells, ixodid and argasid ticks in the structural organi- the latter including neurosecretory, spherical zation of the perineural glial cells. The large nucleus, and irregular nucleus neurons. The

~ FIGURES14-18. Transmission electron micrographs of neurons in the tick central nervous system. 14. Type 3 neurons in Dermacentor variabilis; lettering as in Fig. 13. X 11,600. 15. Type 2 neurons in Am- blyomma americanum. GC, glial cell; M, mitochondrion; Nu, nucleus; NB, Nissl bodies; NF, nerve fibers; NV, neurosecretory vesicles. X 4,500. 16. Golgi region (G) in type 2 neurons of Amblyomma americanum showing electron lucent and dense neurosecretory vesicles (NV). Gl, glycogen; GC, glial cell. X 15,000. 17. Type 2 neurons in Argas (Persicargas) arboreus. A glial cell (GC) process containing smooth endo- plasmic reticulum (SER) invaginates into the perikaryion of type 2 neuron. Asterisk indicates glial sheets. ER, rough endoplasmic reticulum; LB, lysosomelike bodies; rest as in Fig. 15. X 8,700. 18. Extracellular material (asterisk) between glial cells (GC) ensheathing type 1 neurons in Amblyomma americanum. Ce, centriole; Ri, free ribosomes; rest as in Figs. 16, 17. X 15,400.

- 696 THE JOURNAL OF PARASITOLOGY,VOL. 60, NO.4, AUGUST 1974 COONS, ROSHDY,AND AXTELL-FINE STRUCTUREOF THE CNS OF VARIOUS TICKS 697 so-called irregular nucleus neurons illustrated category, nerve fibers are ensheathed by by these authors apparently represent a glial mesaxons from glial extensions. A remarkable cell adjacent to a neurosecretory cell body. difference between insect and tick nerves is The fine structure of the tick neurons agrees the presence of extensive invaginations of the with that described for several insects and a tick neural lamella surrounding individual mite. Unlike mites (Coons and Axtell, 1971), fibers before mesaxon ensheathment. This glial ensheathment of types 1 and 2 neurons is feature is not found in insects. The smaller, complete, and direct contact between adjacent usually grouped nerve fibers of ticks lack in- cell membranes was observed only in type 3 dividual mesaxon ensheathment. The func- neurons. Furthermore, glial extensions invag- tional significance of this orientation in tick inate into the neuronal cell bodies, a feature peripheral nerves is unknown. probably related to the transfer of trophic material (Smith, 1968). ACKNOWLEDGMENT Neurosecretory cells commonly distributed Weare greatly indebted to Dr. Harry in the tick nerve ganglia (loffe, 1964; Dhanda, Hoogstraal for continuous encouragement and 1967; Eichenberger, 1970) contain electron- interest in this work and for critically reading dense vesicles manufactured in the Golgi region the manuscript. and distributed in the cell bodies and axons. It is interesting to mention that fine axons LITERATURE CITED containing neurosecretory vesicles were ob- ASHHURST,D. E. 1968. The connective tissues of served in the neural lamella. These axons may insects. Ann. Rev. Ent. 13: 45-74. release neurosecretory contents directly into -, AND N. M. COSTIN. 1971. Insect muco- substances. II. The mucosubstance of the the hemolymph in the periganglionic sinus. central nervous system. Histochem. J. 3: Other axons containing neurosecretory material 297-310. are also found in the neuropile and peripheral BURGDORFER,W. 1951. Analyses des Infections- nerves. verlaufes bei Ornithodorus moubata (Murray) The overall morphology of the nerve fibers und der natiirlichen Uebertragung von spiro- and glial cells, as well as the contacts between chaeta duttoni. Acta Trop. 8: 193~262. individual axons and dendrites, in the neuro- CHOW, Y. S., S. H. LIN, AND C. H. WANG. 1972. An ultrastructural and electrophysiological pile region is identical in all three tick species study of the brain of the brown dog-tick, and conforms to the neuropile arrangement in Rhipicephalus sanguineus (Latreille). Chinese insects and in Macrocheles (Coons and Axtell, Bioscience 1: 83-92. 1971). The fine structure of the tick esophageal COONS, L. B., AND R. C. AXTELL. 1971. Cellular canal is essentially similar to that of the mite. organization in the synganglion of the mite As in insects (Smith, 1968), tick peripheral Macrocheles muscaedomesticae (Acarina: Mac- rochelidae). An electron microscopic study. nerve fibers are neither myelinated or un- Z. Zellforsch. 119: 309-320. myelinated, but rather fall into a third morpho- -, AND M. A. ROSHDY.1973. Fine structure logical category termed "tunicated." In this of the salivary glands of unfed male Derma-

~ FIGURES19-22. Transmission electron micrographs of neuropile and peripheral nerve in the tick nervous system. 19. Neuropile of Amblyomma americanum. A complex series of glial cell (GC) extensions par- tially ensheath nerve fibers (NF), and surround neurons (arrows) in the cortex. Gl, glycogen; M, mito- chondrion; Nu, nucleus; Nt, neurotubules in nerve fibers; Tr, tracheal elements surrounded by glial cell extensions. X 6,200. 20. Neuropile of Argas (Persicargas) arboreus showing a glial cell (GC) extension between nerve fibers (NF). Nerve fibers are partially ensheathed and show direct longitudinal (LC) and cross (CC) contacts. NV, neurosecretory vesicles; rest as in Fig. 19. X 4,000. 21. Esophageal canal (EsC), containing esophagus, in the neuropile (Np) of Amblyomma americanum. BM, basement mem- brane of esophagus; Cu, cuticle; Gl, glial cell; Lu, lumen of esophagus; MF, muscle fiber; Nu, nucleus of epithelial cell in esophagus; NL, neural lamella lining esophageal canal; Pn, perineurium. X 5,000. 22. Peripheral nerve of Dermacentor variabilis. Arrows indicate invaginations of neural lamella (NL) to sur- round individual nerve fibers (NF). Glial cell (GC), also surrounded by neural lamellar material, forms mesaxons (Ma) around nerve fibers. M, mitochondrion; Mt, microtubules; Nt, neurotubules; Nu, nucleus of glial cell; Nv, neurosecretory vesicles. X 7,500. 698 THE JOURNAL OF PARASITOLOGY,VOL. 60, NO.4, AUGUST 1974

centor variabilis (Say) (Ixodoidea: Ixodidae). (In Russian; English summary.) Med. Para- J. Parasitol. 59: 900-912. zit., Moskva 34: 57-68. DHANDA, V. 19'67. Changes in neurosecretory REH..lCEK,J. 1965. Development of viruses activity at different stages in the adult Hya- and rickettsiae in ticks and mites. Ann. Rev. lomma dromedarii Koch, 1844. Nature 214: Ent. 10: 1-24. 508-509. ROBINSON,L. E., AND J. DAVIDSON. 1913. The DOUGLAS, J. R. 1943. The internal anatomy of anatomy of Argas persicus (Oken). Para- Dermacentor andersoni (Stiles). Univ. Calif. sitology 6: 382-430. Publ. Entomol. 7: 207-272. ROSHDY,M. A., N. M. SHOUKREY,ANDL. B. COONS. EICHENBERGER,G. 1970. Das Zentral-nervensystem 1973. The subgenus Persicargas (Ixodoidea, von Ornithodorus moubata (Murray), Ixodo- Argasidae: Argas) . 17. A neurohemal organ idea: Argasidae, und seine postembryonale in A. (P.) arboreus Kaiser, Hoogstraal, and Entwicklung. Acta Trop. 27: 15-53. (In Kohls. J. Parasitol. 59: 540"-544. English: NAMRU3-T4J9..) SMITH, D. S. 1968. Insect Cells: Their Structure GABE, M. 1955. Donnees histologiques sur la and Function. Oliver and Boyd, Edinburgh, neurosecretion chez les Arachnides. Arch. 372p. Anat. Microsc. Morphol. Exp. 44: 351-383. -, AND J. E. TREHERNE. 1963. Functional HESS, A. 1958. The fine structure of nerve cells aspects of the organization of the insect ner- and fibers, neuroglia and sheaths of the gan- vous system. Adv. Insect Physiol. 1: 401-484. glion chain in the cockroach (Periplaneta SONENSHINE,D. E. 1970'. A contribution to the americana). J. Biophys. Biochem. Cytol. 4: internal anatomy and histology of the 731-742. tick Ornithodoros kelleyi Cooley and Kohls, IOFFE, 1. D. 1963. Der bau des Nervenapparats 1941. II. The reproductive, muscular, respira- von Dermacentor pictus Herm. (, tory, excretory, and nervous system. J. Med. Acarina). (In Russian: German summary.) Entomol. 7: 289-312. Zool. Zh. 42: 1472-1484. (English translation: TREHERNE, J. E., AND Y. PICHON. 1972. Insect TT64-19707. ) blood barrier. Adv. Insect Physiol. 9: 257- -. 1964. Distribution of neurosecretory cells 313. in the central nervous system of Dermacentor TSVILENEVA,V. A. 1965. The nervous structure pictus Herm. ( Acarina, Chelicerata) . ( In of the ixodid synganglion (Acarina, Ixodoidea). Russian.) DoH Akad. Nauk SSR 154: 229- Entomol. Rev. 44: 135-142. 232. (In English: NAMRU3-T326.) WIGGLESWORTH,V. B. 1960. The nutrition of the -. 1965. Seasonal changes of neurosecretion central nervous system of the cockroach, contents in the neurosecretory cells of Derma- Periplaneta americana. The mobilization of centor pictus Rerm. ( Ixodoidea, Acarina) . reserves. J. Exp. BioI. 37: 500-512.