part of the spermatheca provide nourishment to the contained spermatozoa. The second type of ectodermal gland, known collectively as accessory glands, opens more posteriorly in the genital chamber and has a variety of functions depending on the species (see section 5.8).

Each ovary is composed of a cluster of ovarian or egg tubes, the ovarioles, each consisting of a terminal filament, a germarium (in which mitosis gives rise to primary oocytes), a vitellarium (in which oocytes grow by deposition of yolk in a process known as vitel-logenesis; section 5.11.1), and a pedicel (or stalk). An ovariole contains a series of developing oocytes, each surrounded by a layer of follicle cells forming an epithelium (the oocyte and its epithelium is termed a follicle); the youngest oocytes occur near the apical germarium and the most mature near the pedicel. Three different types of ovariole are recognized based on the manner in which the oocytes are nourished. A panoistic ovari-ole lacks specialized nutritive cells so that it contains only a string of follicles, with the oocytes obtaining nutrients from the hemolymph via the follicular epithelium. Ovarioles of the other two types contain tropho-cytes (nurse cells) that contribute to the nutrition of the developing oocytes. In a telotrophic (or acrotrophic) ovariole the trophocytes are confined to the germarium and remain connected to the oocytes by cytoplasmic strands as the oocytes move down the ovariole. In a polytrophic ovariole a number of trophocytes are connected to each oocyte and move down the ovariole with it, providing nutrients until depleted; thus individual oocytes alternate with groups of successively smaller trophocytes. Different suborders or orders of insects usually have only one of these three ovariole types.

Accessory glands of the female reproductive tract often are referred to as colleterial or cement glands because in most insect orders their secretions surround and protect the eggs or cement them to the substrate (section 5.8). In other insects the accessory glands may function as poison glands (as in many Hymenoptera) or as "milk" glands in the few insects (e.g. tsetse flies, Glossina spp.) that exhibit adenotrophic viviparity (section 5.9). Accessory glands of a variety of forms and functions appear to have been derived independently in different orders and even may be non-homologous within an order, as in Coleoptera.

3.8.2 The male system

The main functions of the male reproductive system are the production and storage of spermatozoa and their transport in a viable state to the reproductive tract of the female. Morphologically, the male tract consists of paired testes each containing a series of testicular tubes or follicles (in which spermatozoa are produced), which open separately into the mesodermally derived sperm duct or vas deferens, which usually expands posteriorly to form a sperm storage organ, or seminal vesicle (Fig. 3.20b). Typically, tubular, paired accessory glands are formed as diverticula of the vasa defer-entia, but sometimes the vasa deferentia themselves are glandular and fulfill the functions of accessory glands (see below). The paired vasa deferentia unite where they lead into the ectodermally derived ejaculatory duct - the tube that transports the semen or the sperm package to the gonopore. In a few insects, particularly certain flies, the accessory glands consist of an enlarged glandular part of the ejaculatory duct.

Thus, the accessory glands of male insects can be classified into two types according to their mesodermal or ectodermal derivation. Almost all are mesodermal in origin and those apparently ectodermal ones have been poorly studied. Furthermore, the mesodermal structures of the male tract frequently differ morphologically from the basic paired sacs or tubes described above. For example, in male cockroaches and many other orthopteroids the seminal vesicles and the numerous accessory gland tubules (Fig. 3.1) are clustered into a single median structure called the mushroom body. Secretions of the male accessory glands form the sper-matophore (the package that surrounds the spermatozoa of many insects), contribute to the seminal fluid which nourishes the spermatozoa during transport to the female, are involved in activation (induction of motility) of the spermatozoa, and may alter female behavior (induce non-receptivity to further males and/or stimulate oviposition; see sections 5.4 & 5.11, and Box 5.4).

0 0

Post a comment