Spiracular Plate of Nymphal and Adult Hard Ticks (Acarina: Ixodidae): Morphology and Cuticular Ultrastructure

Abstract

Morphology and ultrastructure of the nymphal and adult spiracular plate of five species of hard ticks (Ixodidae) are described using scanning scanning and transmission electron microscopy. Both the surface area and the number of aeropyles increase significantly between the nymphal and adult stages and are greater in females than in males. The spiracular plate is divided into four regions: an outer cuticle with aeropyles, a labyrinth composed of cuticular pedicels and chambers, a thick basal plate, and a cellular layer. The pedicels that separate the chambers of the spiracular plate have a large central channel, surrounded by smaller secondary channels. These channels are found throughout the basal plate cuticle and contain a granular substance, probably secreted by the epidermal cells. The basal plate gland is enclosed by the cuticle of the basal plate and is dispersed randomly in the basal plate. This gland has duct and secretory cells, and the duct has a valve-like portion at its apical end where the gland opens at the base of a chamber. The basal plate gland cells and epidermal cells beneath the basal plate contain many free ribosomes, lipid droplets, rough endoplasmic reticulum, and mitochondria. The glands may be involved in the regulation of water loss and air diffusion. Additional key words: water regulation, gas exchange, glandular cells Ticks are generally rather impervious to desiccation because of their thick cuticle and very few sites from which water may be lost to the external environment, such as the mouth, anus, genital opening, and spiracles (Knulle & Rudolph 1982; Sauer & Hair 1986). The spiracles regulate gas exchange and limit water loss from the respiratory system into surrounding environment (Pugh et al. 1988, 1990). Spiracles of Ixodida are located near coxa IV in all three families, i.e., postero-laterad to coxa IV in Nuttalliellidae (Roshdy et al. 1983), ventro-laterad in Argasidae (Roshdy et al. 1989; Pugh et al. 1991), and posterior to coxa IV in Ixodidae (Pugh et al. 1988, 1990). The spiracle consists of a spiracular plate, an ostium, an atrial cavity, and atrial muscles (Roshdy & Hefnawy 1973; Sonenshine 1991). The surface of the spiracular plate is covered with pores called aeropyles and has a macular region that forms the first part of the ostial region of the spiracle. Internally the spiracular plate is composed of cuticular pedicels that create a complex series of chambers. These pedicels and chambers lie on a thickened cuticular base that overlies the epidermal cells. Most of our information on the morphology of tick spiracular plates is from light microscopy of thick histological sections and scanning electron microscopy (Sixl et al. 1971; Woolley 1972; Roshdy 1974; Roshdy et al. 1983, 1989; Pugh et al. 1988, 1990, 1991; Schol et al. 1995). The only study using transmission electron microscopy is by Sixl & Sixl-Voight (1974) on Haemaphysalis inermis BIRULA 1895. The present study examines the morphology and ultrastructure of the nymphal and adult spiracular plate of Amblyomma americanum (LINNAEUS 1758) and compares it to that of several other species of hard ticks, Boophilus annulatus (SAY 1821), Demacentor variabilis (SAY 1821), Haemaphysalis leporispalustris (PACKARD 1869), and Rhipicephalus sanguineus (LATREILLE 1806).