Biology of the Arthropod Cuticle

Water balance in land arthropods

▪ Abstract Terrestrial arthropods survive subzero temperatures by becoming either freeze tolerant (survive body fluid freezing) or freeze avoiding (prevent body fluid freezing). Protein ice nucleators (PINs), which limit supercooling and induce freezing, and antifreeze proteins (AFPs), which function to prevent freezing, can have roles in both freeze tolerance and avoidance. Many freeze-tolerant insects produce hemolymph PINs, which induce freezing at high subzero temperatures thereby inhibiting lethal intracellular freezing. Some freeze-tolerant species have AFPs that function as cryoprotectants to prevent freeze damage. Although the mechanism of this cryoprotection is not known, it may involve recrystallization inhibition and perhaps stabilization of the cell membrane. Freeze-avoiding species must prevent inoculative freezing initiated by external ice across the cuticle and extend supercooling abilities. Some insects remove PINs in the winter to promote supercooling, whereas others have selected against...

Antifreeze and ice nucleator proteins in terrestrial arthropods.

Publisher Summary This chapter presents recent progress in the study of the midgut, particularly with reference to ion and water transport. However, the midgut does not function in isolation, and so the attempt is to fit the role of the midgut into the context of the operation of the gut as a whole. The chapter considers the cockroach as an example of a classic generalist and looks at scavenging dipteran larvae, mainly aquatic. Midgut function is described in various types of insects such as solid/plant feeders (phytophagous insects), solid/animal feeders (carnivores), liquid/animal feeders (bloodsuckers), and liquid/plant feeders (sap and nectar feeders). The chapter also discusses the process of digestion of cellulose in insects. There are four categories of evidence that an insect can digest cellulose. It could survive on a diet of pure synthetic cellulose; it could be shown to incorporate and metabolize label from radioactive cellulose in the food; it could be shown to break down synthetic crystalline cellulose; or it could be shown that there is less cellulose in the excreta than was consumed in the food.

Insect Midgut Function

The intranuclear spindle of yeast has an electron-opaque body at each pole These spindle plaques lie on the nuclear envelope During mitosis the spindle elongates while the nuclear membranes remain intact After equatorial constriction there are two daughted nuclei, each with one spindle plaque The spindle plaque then duplicates so that two side-by-side plaques are produced These move rapidly apart and rotate so that they bracket a stable 08 µm spindle Later, during mitosis, this spindle elongates, etc Yeast cells placed on sporulation medium soon enter meiosis After 4 hr the spindle plaques of the more mature cells duplicate, producing a stable side-by-side arrangement Subsequently the plaques move apart to bracket a 08 µm spindle which immediately starts to elongate When this meiosis I spindle reaches its maximum length of 3–5 µm, each of the plaques at the poles of the spindle duplicates and the resulting side-by-side plaques increase in size The nucleus does not divide The large side-by-side plaques separate and bracket a short spindle of about 1 µm which elongates gradually to 2 or 3 µm Thus there are two spindles within one nucleus at meiosis II To the side of each of the four plaques a bulge forms on the nucleus The four bulges enlarge while the original nucleus shrinks These four developing ascospore nuclei are partially surrounded by cytoplasm and by a prospore wall which originates from the cytoplasmic side of the spindle plaque Eventually the spore nuclei pinch off and the spore wall closes In some of the larger yeast cells this development is completed after 8 hr on sporulation medium

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Spindles, spindle plaques, and meiosis in the yeast saccharomyces cerevisiae (hansen)

An electron microscopical study has been made of the rectal complex. The perinephric membrane is a complicated structure which, in the posterior region, comprises an inner and an outer sheath separated by a space containing tracheolar end cells. The outer sheath is formed of a single layer of cells covered by an external basement membrane. The inner sheath is a multi-laminate structure made up of many thin, cellular layers which in places are reduced to closely apposed plasma membranes. Anteriorly the cellular layers are reduced in number, but each layer is of greater thickness; they finally terminate where the perinephric membrane is applied to the intestine. Posteriorly the inner sheath makes contact with the rectal epithelium. An earlier description identified three spaces within the rectal complex: the perirectal, subepithelial and peritubular spaces. The first two are true intercellular spaces, bounded by basement membranes, but the so-called peritubular space is occupied by necrotic cells. The inner sheath of the perinephric membrane is interrupted by the leptophragmata. Each leptophragma is bounded by a prominent electron-dense ring into which the laminae of the inner sheath are inserted. The outer sheath forms a blister over the leptophragma and is completely noncellular in this region. At the base of the blister a basement membrane covers the leptophragma itself, and the body of the leptophragma cell projects into the lumen of the tubule, with a thin layer of cytoplasm lying beneath the basement membrane. Both this layer and the cell body itself bear microvilli. The cell has a normal complement of mitochondria, but these do not invade the microvilli. In this last respect the ordinary tubule cells differ from the leptophragma cells in that most of their microvilli contain mitochondria, with connexions between the outer mitochondrial membrane and the plasma membrane. The tubule cells have a poorly developed endoplasmic reticulum but are filled with numerous small granules; basal infoldings are restricted to those parts of the cell which face the perirectal space. The permeability of the perinephric membrane has been re-investigated and it is shown that the membrane is more permeable to water and solutes at the anterior end, as might be expected if the inner sheath were the main barrier. Using preparations isolated in small volumes of haemolymph or other external media it has been shown that the rectal complex takes up potassium against a gradient of concentration. The lumen of the perirectal tubule is some 50 mV positive with respect to the external medium, so the uptake of potassium must be active. The leptophragma is freely permeable to chloride and this ion appears to enter the tubule passively. A model of the mechanism of the rectal complex is proposed, whose main feature is that the high osmolarity of the fluids within the rectal complex is brought about by the inward secretion of potassium chloride, unaccompanied by water, at the leptophragmata. This should result in a fall in the osmolarity of the external medium. A substantial fall has been observed on occasion, but in most experiments a fall is barely detectable. It is believed that the impermeability of the leptophragmata to water is rapidly lost in a deteriorating preparation.

Further Studies on the Rectal Complex of the Mealworm Tenebrio molitor, L. (Coleoptera, Tenebrionidae)

The centriolar apparatus in the flagellate genera Trichonympha and Pseudotrichonympha is located at the anterior end of the cell, in the rostrum. It forms part of a complex system of structures which includes the rostral tube, inner and outer caps, and the rostral flagella. The fine structure of these organelles is described in detail on the basis of an electron-microscope study of sectioned and negatively stained material. In Trichonympha the rostral tube is a hollow cylinder, made of a cross-striated protein with a periodicity of about 450 A. This is organized into radially arranged lamellae, which continue posteriorly as the parabasal filaments. The tube is continuous anteriorly with two finely striated crescentic bodies, which correspond to the so-called short centrioles of some previous workers. There is no evidence that they are centriolar in function. In the interphase cell the centriolar apparatus consists principally of a long centriolar rod, of complex fine structure, lying in the anterior end of the rostral tube. There is no evidence of typical centriolar structure in this, but at division an aster forms around one end of it. Surmounting the apex of the rostral tube is a dense, finely fibrous body, the inner cap. Lying within this there is a typical centriole (similar in structure to a basal body), and also the basal body of one flagellum, which appears to be distinct from all the rest. The functions of these two structures are not known. The margin of the inner cap connects with the complex system of delicate fibres which links the basal bodies of the rostral flagella. The function of the fibres, and possibly also of the inner cap, may be to coordinate the activities of the rostral flagella. The outer cap is composed mainly of tubules, 250 A in diameter, but shows variations in structure in different species. The structures in Pseudotrichongmpha which presumably serve similar functions are in many respects very differently organized. The rostral tube is more complex, with distinct inner and outer walls of different fine structure. There are also complex inner and outer surface layers. A striking feature is that although the various components of the tube are quite different in structure, they display a common periodicity in their organization. The centriolar apparatus appears to consist principally of two dense bands running along the inner wall of the tube, connecting anteriorly to an extended layer of centriolar material to which spindle fibres are attached in radially polarized fashion throughout interphase. There is no centriolar rod or typical centriole, such as is found in Trichonympha. Very elaborate systems of fibres are associated with the inner cap and the anterior end of the rostral tube. The two genera are compared, and the findings related to knowledge of centriolar structures in other types of cell. Possible evolutionary explanations for the complexity and variation in fine structure in these flagellates are considered.

A. V. Grimstone

Papers

Further Studies on the Rectal Complex of the Mealworm Tenebrio molitor, L. (Coleoptera, Tenebrionidae)

The Fine Structure of the Centriolar Apparatus and Associated Structures in the Complex Flagellates Trichonympha and Pseudotrichonympha