Physiology of Ticks

Current Themes in Tropical Science

Frederick D. Obenchain, Rachel Galun, Frederick D. Obenchain, Rachel Galun

• Publisher: Pergamon
• Year: 2013
• Print ISBN:
  9780080249377
• eBook ISBN:
  9781483162348
• Topic: Biological Sciences
• Subtopic: Zoology
• Index: Biological Sciences

Physiology of Ticks focuses on the unique (and probably the most vulnerable) features of tick physiology and the physiological aspects of tick interactions with their hosts. The mechanisms used by non-feeding ticks to maintain their water balance are examined, along with the salivary mechanisms used by feeding ixodid ticks for excreting the enormous excess volumes of water and salts taken in during blood sucking. This book is comprised of 13 chapters and begins with a description of the morphology, deposition, and components of the tick cuticle. The discussion then turns to humidity relationships and water balance of ticks, as well as the sensory basis of tick feeding behavior and the immunological basis of host resistance to ticks. Subsequent chapters explore blood digestion in ticks; tick reproduction, with emphasis on sperm development, cytogenetics, oogenesis, and oviposition; effects of insect hormones and their mimics on tick development and reproduction; and the mechanisms of tick pheromones. The final chapter deals with diapause and biological rhythms in ticks. This monograph will be of value to entomologists, physiologists, biologists, and practitioners of tropical science.

CHAPTER 1

The Tick Cuticle

R.H. HACKMAN and B.K. FILSHIE, Division of Entomology, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia

Publisher Summary

This chapter describes the structure, syntheses, and composition of the tick cuticle. The success of ticks as terrestrial arthropods is related directly to the properties of their cuticles. Like other arthropods, ticks have an external skeleton or integument, which is skin, skeleton, and, if necessary, a food reserve. The cuticle is that part of the integument external that is secreted by the epidermis, which is itself supported on an internal basement membrane. The cuticle is not uniform over the entire animal but varies from hard sclerotized plates to soft extensible membranes. To allow movement, rigid plates are connected together by flexible membranes that act as hinges. There are some differences in structure between the cuticle of the alloscutum of the nymph and the female of B. microplus. In the female, maximum cuticle thickness is reached when the tick is about 4.5 mm long. Ticks, like insects, possess dermal glands throughout the epidermal layer. In insects, the dermal glands are supposed to produce the cement layer over the surface of the wax layer and this is correlated with their wide dispersal over the body surface.

CONTENTS

1.1 Introduction

1.2 Structure

1.2.1 Morphology of the Cuticle

1.2.1.1 Epicuticle

1.2.1.2 Procuticle, Exocuticle, Mesocuticle, Endocuticle, and Subcuticle

1.2.1.3 Pore Canals and Wax Canals

1.2.1.4 Muscle Attachments, Glands, and other Integumental Structures

1.2.2 Deposition of the Cuticle

1.2.2.1 Formation of the Epicuticle

1.2.2.2 Formation of the Procuticle

1.2.2.3 Modifications to Cuticle Structure during Feeding

1.3 Cuticular Components

1.3.1 Chitin

1.3.2 Protein

1.3.3 Lipids

1.3.4 Other Components

1.3.5 Sclerotization

1.3.6 Penetration of Acaracides

1.3.7 Water Exchange

1.4 Conclusions

References

1.1 INTRODUCTION

Initial descriptions of the tick cuticle, which date from around the beginning of this century, were based on histochemical studies using the light microscope, but in the last decade further progress has been made in our understanding of the structure, synthesis, and composition of the cuticle by use of newer techniques such as electron microscopy, chromatography, and electrophoresis. Like other arthropods, ticks have an external skeleton or integument which is skin, skeleton, and, if necessary, a food reserve. The cuticle is that part of the integument external to and secreted by the epidermis (the epidermal cell layer), which is itself supported on an internal basement membrane. In insects there is evidence which suggests that some cuticular proteins may not be synthesized by the epidermal cells but are transported across them from the haemolymph, having been synthesized elsewhere. However, tissue culture experiments have established that epidermal cells secrete cuticle. Cuticle is a heterogeneous, non-cellular membrane, which is not only the external covering but extends into the fore- and hind-guts and lines the ducts of dermal glands and the tracheal system.

The cuticle is not uniform over the entire animal but varies from hard sclerotized plates to soft extensible membranes. To allow movement, rigid plates are connected together by flexible (intersegmental) membranes, which act as hinges. Ixodid ticks are referred to as hard ticks because of their sclerotized (hardened) capitulum, scutum, appendages, and other small areas. On the other hand, argasid ticks are the leathery or soft ticks because sclerotized areas are relatively small. The cuticle, being an exteneral skeleton, determines the maximum size and the shape that a particular developmental stage can attain, and further growth is possible only by moulting, i.e. shedding the cuticle and replacing it by a larger one. Ixodid ticks undergo two moults in their progress from larvae to nymphs to adults. Argasid ticks undergo three or more moults. Some species have two nymphal instars, others up to six or more. Respiration in nymphal and adult ticks is by way of tracheae which terminate in the spiracular plates on the sides of the body. Larval ticks, except for those of some argasid species, do not appear to have spiracles, and it is assumed that gaseous exchange occurs through the intact cuticle.

The cuticle is divided into two layers, a thin outer epicuticle and a thicker inner procuticle. The epicuticle is complex and contains several layers, viz. lipid, polyphenol, and cuticulin; in argasid ticks a cement layer overlies the lipid layer. The procuticle is formed of protein and chitin. Its outer region may be sclerotized to form an exocuticle; in which case the remaining inner region is known as the endocuticle. In some cuticles the outer part of the endocuticle may show different staining properties from the inner part. The outer part is then referred to as mesocuticle. Mesocuticle is described as a stabilized but non-sclerotized region. Ticks, in common with other cheli-cerates, possess an endosternite or internal skeletal support for some prosomal muscles. Contrary to earlier claims, Cutler and Richards (1974) report that the endosternite does not contain chitin; its chemical composition is unknown. Water loss in ticks is almost exclusively through the cuticle (Lees, 1946), and the lipid layer of the epicuticle plays a major role in regulating the movement of water.

Setae or bristles are present on the cuticle and each is set in a socket on top of a canal leading from an epidermal cell. Pore canals extend through the procuticle from the epidermis up to the epicuticle. A number of sense organs and dermal glands open onto the surface of the cuticle. Some setae are thought to be temperature sensors and others tactile sensors. Figure 1.1 is a diagrammatic representation of the arthropod integument, Fig. 1.2 an electron micrograph of a transverse section of the integument from the dorsal surface of the alloscutum of the nymph of Boophilus microplus. For recent reviews on the arthropod cuticle, reference can be made to Hackman (1971, 1974a) and Neville (1975). References to the early work on tick cuticle are to be found in Richards (1951).

FlG. 1.1 A diagrammatic representation of the arthropod integument. (From Hackman, 1971.)

FIG. 1.2 Transverse section of the cuticle of the alloscutum of an engorged nymph. Inner endocuticle (inner) is distinctly lamellate. Outer endocuticle (outer) is not lamellate but bundles of microfibrils (arrows) tend to occur in the same orientation at regular spacings through the thickness of the outer endocuticle, with a period approximately the same as the lamellar spacing of the inner endocuticle. Epicuticle, epi; pore canals, pc; “wax” layer, w. Scale = 2.5μm.

1.2 STRUCTURE

Our present knowledge of tick cuticular structure comes from earlier studies with the light microscrope combined with a few recent descriptions of the fine structure from transmission and scanning electron microscope studies. By comparison with insect cuticle, published information on the fine structure of the tick integument is sparse indeed. These few studies indicate that many parallels may be drawn between insect and tick cuticles. However, several differences have been discovered, particularly in the structure and development of the cuticle of the alloscutum of female ixodid ticks. This cuticle has an enormous capacity for growth, expansion, and stretching during engorgement of blood in the adult instar. The following description of the structure of the cuticle combines a review of published information augmented by some new descriptions and micrographs of the cuticle of the adult female cattle tick, B. microplus.

1.2.1 Morphology of the Cuticle

In common with similar descriptions of insect cuticle, difficulties exist in correlating the interpretation of structures seen in the light microscope with those seen at higher resolution in the electron microscope. These difficulties result from the different methods of preparation and examination which are used with the two techniques and the different ways in which images are formed. For instance, different staining methods applied to paraffin sections have allowed the procuticle to be subdivided into a number of layers– exocuticle, mesocuticle, endocuticle, and subcuticle–but with electron stains these layers are difficult or even impossible to distinguish from one another. The wax and cement layers, both largely soluble in lipid solvents, are not preserved by the normal methods used to prepare cuticle for transmission electron microscopy. Because thin (50–100 nm) sections must be used for...

Table of contents

• Cover image
• Title page
• Table of Contents
• Other Pergamon publications of interest
• Copyright
• FOREWORD
• PREFACE
• Chapter 1: The Tick Cuticle
• Chapter 2: Humidity Relationships and Water Balance of Ticks
• Chapter 3: The Sensory Basis of Tick Feeding Behaviour
• Chapter 4: Tick Attachment and Feeding: Role of the Mouthparts, Feeding Apparatus, Salivary Gland Secretions and the Host Response
• Chapter 5: Immunological Basis of Host Resistance to Ticks
• Chapter 6: Blood Digestion in Ticks
• Chapter 7: Ion and Water Balance in Feeding Ticks: Mechanisms of Tick Excretion
• Chapter 8: Tick Reproduction: Sperm Development and Cytogenetics
• Chapter 9: Tick Reproduction: Oogenesis and Oviposition
• Chapter 10: Structure and Function of the Circulatory, Nervous, and Neuroendocrine Systems of Ticks
• Chapter 11: Endocrine Mechanisms in Ticks: Effects of Insect Hormones and their Mimics on Development and Reproduction
• Chapter 12: Tick Pheromone Mechanisms
• Chapter 13: Diapause and Biological Rhythms in Ticks
• Index