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The Structure and Development of Wax Glands of Pseudococcus Maritimus (Homoptera, Coccidae)

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The Structure and Development of Wax Glands of Pseudococcus Maritimus (Homoptera, Coccidae) The Structure and Development of Wax Glands of Pseudococcus maritimus (Homoptera, Coccidae). By Priscilla Frew Pollister, Department of Zoology, Columbia University. With Plates 17-20, and 3 Text-figures. INTRODUCTION. ONE of the striking features of the family Coccidae is a uni- versal tendency to form extensive secretions of wax or wax-like substances on the surface of the body. This substance seems to serve a protective role, but in many instances the amount of it is so out of proportion to the necessity for this purpose that many entomologists have held that the exudation of this wax is more accurately to be regarded as a type of excretion of a material that is an inevitable by-product of the metabolism of animals feeding exclusively on plant juices. The wax is secreted through pores in the cuticula, which are the openings from underlying glands. The present study is concerned with the distribution, minute structure, and develop- ment of these glands in one of the mealy-bugs, Pseudococcus maritimus Erhorn. There have been many descriptions of the structure and distribution of the external pores, and a few studies of the histology and development of the underlying glands. Many of these observations have been incidental to a work primarily concerned with the taxonomy of the group. The present study attempts to deal with the problem primarily from a histological and cytological point of view, since in these structures one finds some of the most elaborately differentiated types of glands. The problem was suggested by Professor Franz Schrader, and I am indebted to him for advice and encouragement throughout the work. 128 PRISCILLA FREW POLLISTER MATERIAL AND METHODS. The original stock used in this investigation was obtained from a commercial greenhouse in Falmouth, Massachusetts, where the mealy-bugs were growing on narcissus bulbs. A stock of these was kept on various bulbs during the winter months and on citrus fruits in the summer. The transfer to the fruit each summer was indispensable to the continued reproduction of the stock. Specimens from the stock were identified as Pseudococcus maritimus Erh. through the kindness of Doctor Harold Morrison of the Bureau of Entomology of the United States Department of Agriculture. For the study of the external apertures of the glands pre- parations of the exoskeleton were made by the potash-magenta method as described by Dietz and Morrison, 1916. The fixation of material for sectioning is difficult because of the covering of wax, which must be softened and partially removed by touching the animals with a brush that has been dipped in absolute alcohol. Immediately after the application of the alcohol the animals are dropped into the fixing fluid, which must be hot. Good fixation apparently occurs when the alcohol has affected the wax just enough so that the fixing fluid will pene- trate readily, but has not acted so long that the alcohol has reached the soft tissues of the animal. Many fixing fluids were tried and Allen's B 15 and Kahle's were found to give vastly superior results, so that the study was made entirely from material fixed in one or the other of these mixtures. The material was embedded in paraffin and sectioned at 6 or lOmicra, stained with iron haematoxylin, and in some cases counter- stained with either eosin or light green. OBSERVATIONS. Types and Distribution of Glands. In potash-magenta preparations of adult females of P s e u d o - coccus maritimus one can see hundreds of gland-pores. Each pore is at the top of a small papilla projecting above the level of the surrounding cuticula. As seen from above these derm-pores are of three distinct types, to which, after Ferris WAX-GLANDS OF PSEUDOCOCCUS 129 (1918), I shall refer as the triangular, cylindrical (or tubular), and the multilocular. The triangular derm-pores show four openings, three elongated ones that form a triangle, and a fourth circular aperture within the triangular (Text-fig. 1 B). These pores are the smallest and BECX CELL B TEXT-FIG. 1. A, diagrammatic sagittal section of triangular gland. B, pore of triangular gland as seen from above, c, sagittal section of neck region of triangular gland drawn to the same scale as B. most numerous. They are located on dorsal, ventral, and lateral surfaces of all segments. They are especially concentrated around the bases of the seventeen pairs of large lateral spines, with which they form the so-called cerarian complexes. They form small curled flakes of wax that are scattered over the sur- face of the body and are pushed outward along the large spines to form the long wax filaments projecting from their tips— a feature very characteristic of the genus Pseudococeus. The tubular pores have a single large opening at the top of the papilla (Text-fig. 2). Like the first type, they are very numerous and are found on all surfaces. They are especially abundant on the ventro-lateral parts of the body, and within NO. 317 K 130 PRISCILLA FREW POLLISTBR each segment are more numerous at the anterior and posterior limits. These glands form large solid cylinders of wax. The multilocular type of pore is much larger than either of the others and consists of a shallow cup at the top of a low papilla. The margin of the cup overhangs the cavity, within TEXT-PIG. 2. A, diagrammatic sagittal section of tubular gland, B, pore of tubular gland as seen from above, c, sagittal section of neck region of tubular gland drawn to same scale as B. which is a peripheral circle of ten triangular openings surround- ing a single central aperture. These pores are sometimes called circumgenital in allusion to the fact that they are especially concentrated in a ring around the vulva—although they are also found, in sparser number, on adjacent regions. The multi- locular pores are much less numerous than the other types and are restricted to the anterior and posterior regions of the ventral surfaces of the last five segments. These glands form irregular masses of wax. In Pseudococcus citri Matheson (1928) seems to have made out a fibrillar texture to this wax, but such a finer structure is not identifiable in Pseudococcus mari- timus. WAX-GLANDS OF PSEUDOCOCCUS 131 Histological Structure of the Glands. Triangular Glands. The triangular type of gland is composed of five cells. As a whole the gland is roughly pear-shaped, with the narrow stem-like portion ending in a heavily chitinized triangular external pore at the top of a small papilla (fig. 13, PI. 17). The stem or neck of the gland extends from the base of the epidermis (hypodermis) to the outer surface of the cuticula, while the body of the gland lies mainly below the epidermis (fig. 12, PL 17). The neck portion is formed from a single cell, the neck-cell or terminal cell (fig. 17, PL 17). The main body of the gland is made up of four elements—a large pear-shaped central cell partially surrounded by three more slender peripheral cells. The latter are fitted into deep grooves in the former (figs. 1 and 12, PL 17). The central cell contains three nuclei, a large spherical one in the distal end of the cell and two small ones at the proximal end nearer the epidermis (figs. 2-4, 5, 6, and 18, PL 17). Above the distal nucleus one finds a large, globular, clear space, the reservoir, and a system of tubes lined with chitin. This branched system of tubes leads into a single efferent duct which emerges from the top of the reservoir and traverses the narrow upper part of the central cell, from which it is continued through the centre of the neck-cell to terminate in the central aperture of the external pore (figs. 15, 16, and 17, PL 17). The three peripheral cells are uninucleate. Directly above the nucleus are clear vacuoles (fig. 21, PL 17) in the cytoplasm, and farther up these vacuoles run together to form a single space, the duct, which extends spirally through the upper end of each cell to the neck-cell (figs. 7-10,12,14,18, and 19, PL 17). From the base of the neck-cell the ducts of the peripheral cells are continued as three tubes, which pursue a slightly spiral course through the neck-cell to end in the three peripheral openings that form the sides of the triangular ex- ternal derm-pore of the gland (figs. 10,11, and 13, PL 17). The structure of the triangular gland is indicated diagrammatically in Text-fig. 1 and PL 20. 132 PRISCILLA FREW POLLISTEE. Tubular Glands. The tubular type of gland is composed of twelve cells, and, as a whole, is flask-shaped. The short neck portion is composed of a single cell that is located in the epidermis and contains a large central chitinous duct which opens to the exterior where it is continuous with the exoskeleton. The bulk of the gland is below the level of the epidermis and consists of ten peripheral cells completely surrounding a much larger central cell. The latter has essentially the same shape and internal structure as the central cell of the triangular gland. It contains three nuclei, one large and two small (figs. 55, 56, 57, and 64, PI. 18). In the larger tubular glands the large distal nucleus of the central cell is shaped like a cup with one part of the rim higher than the rest (fig. 65, PI. 18). The duct of the central cell emerges at its upper end and opens into the side of the large tube that is the main duct of the gland as a whole (figs.
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