Two features of insects that facilitate their great ecological impact are their ability to move over impressive distances and their amazing potential for reproduction. We often speak of reproduction in a single, simple word. The apparent simplicity is misleading. Reproduction entails a great number of successive processe, each of which occur in the correct order one after another. There is not enough time to discuss all aspects of reproduction, but we might list these steps as follows: [1] sex determination, [2] gonial mitoses, [3] meiosis, [4] differentiation of reproductive organs (i.e., gonads, ducts and accessory glands), [5] spermatogenesis, [6] ovarian development, including previtellogenesis and vetellogenesis, [7] accessory gland functioning, [8] pheromone production, [9] sexual behavior, [10] mating, [11] ovulation, [12] oviposition, [13] incubation of embryos or larvae, and [14] parturition. Another important feature of insect reproduction is these many steps are often tightly tied to various environmental cues, such as photoperiod and temperature.

Even though we can reduce a complicated process to a list of intricate steps, it is important to stress that living, real insects orchestrate these steps in harmony with cues from their environment. From this point of view, reproduction necessarily involves sensory physiology, afferant nervous transmission of sensory information, and central nervous system integration of information from environments. These neurobiological aspects are to be conceptually justaposed with anatomy and physiology of organs and glands that directly subserve reproduction.

Sexual characteristics of mammals are influenced by a combination of genetical and hormonal events. We may think mammalian sex determination is a strictly genetical event. The situation is otherwise. There is in humans, for example, a condition called testicular feminization in which individuals with the chromosomal make up of males look like females during juvenile and adult phases of life. This condition is brought on by a failure to produce androgenic hormones or hormone receptors. We take the point that sex determination in mammals is not strictly genetics.

Combined genetic and hormone action was similarly thought to control sexual differentiation of gonads, accessory glands and other structures relating to mating and reproduction in insects. Reproduction is a fundamental aspect of insect biology, and a great deal of research effort has been spent on the question of sex determination. Many experiments consisted of transplanting gonads from male to female and female to male in waxmoths, cross transplanting wing discs in a Gryllus cricket, and several other species, and cross transplanting genital discs in Tenebrio, among several other species. Results from all of these experiments show that hormones do not regulate sex determination in insects.

Of course, there is an counter example. Again, one of the most interesting features of insect physiology is the variety in how things are done. Sex determination in adults of the glowworm Lampyris noctiluca appears to be under hormonal regulation. We will consider this case because it is, so far, the only known example of hormonal influence of sex determination in insects. Sexual differentiation of male and female gonads begins during the 4th larval instar. This differences are subtle at this stage, but in males cells in the apical tissue begin to divide, whereas in females it is cells in basal tissue of the gonads that begin to divide. When testes were transplante into females earlier than 5th instar larvae, they induced a transformation, or a masculinization, in the female recipients. Testes could not produce such effects in female-determinted larvae after late in the 5th instar. Converse experiments did not produce feminization in young males. Hence, ovaries of this species can not feminize males, but testes can transform ovaries of pre-5th instar females into testes.

When ovaries were transplanted into males, they became masculinized; this was so, albeit to a lesser degree, even when ovaries were placed into males that had been testectomized. These results suggest that another tissue is also involved in ovarial transformation. Other experiments with testectomized males that were further treated by ablation of various endocrine organs finally showed that two hormones are involved in male differentiation: an androgenic hormone from the apical tissue of testes and a neurohormone from pars intercerebralis, which triggers apical tissue differentiation.

The contemporary wisdom holds that for most insects sex differentiation is not regulated by hormones, with the single known exception just described.

The figure shows reproductive systems of a locustand the meal worm beetle. There is usually a pair of testes in males, as shown for Tenebrio. Sometimes the testes appear fused as shown for Locusta. Vasa deferentia (pleural of vas deferens) connect testes to seminal vesicles, which are in turn connected to an ejaculatory duct. The figure also gives some sense of variation in accessory glands. In Locusta there are 15 pairs of glands, not counting the seminal vesicles. Some of the accessory glands are given individual names, such as white gland, or opalescent gland. In other species, such as a cricket I am familiar with, there are many accessory glands, and they all look alike, so individual glands can be readily identified. The ejaculatory duct is ectodermal in origin, and therefore cuticle-lined. Some accessory glands are also derived from ectodermal tissue, and these are called ectadenia (pleural of ectadene); ectadenia are found in Coleoptera and some other groups. Other accessory glands derive from mesodermal tissue and are called mesadenia; mesadenia are known in Orthoptera and other groups. Tenebrio is an example of an insect with both sorts of accessory glands. Testes are made up of follicles or testis tubes. These are shown in the next figure. Numbers of follicles varies among insect groups from as few as one to over 100 per testis (singular of testes). Testes of some insects, such as Tenebrio shown earlier are a series of lobes, each of which contains follicles. Some beetles have over a dozen lobes in each testes. Often follicles are connected to the vas deferens by a short tube called vas eferens, as shown. In other cases, particularly in Lepidoptera, follicles are not completely separated and they share a common opening to vas deferens. Usually vasa deferentia dialate, or widen, to form the seminal vesicle just before joining the ejaculatory duct.

Testicular physiology

The next figure shows a testis follicle. In the far end of follicles is the germarium. Here germ cells divide to produce spermatogonia. Spermatogonia become encysted in groups of various sizes. As more spermatogonia are produced, they force older ones further down the follicle. Hence, there is a gradient of maturation, with mature sperm in the proximal part of follicles near vasa deferentia, and newly formed spermatogonia in the distal end of follicles. Between these points, people think of threee zone of development along follicles. In Zone I, spermatogonia divide and transform into larger spermatocytes. Zone II is regarded as an area of maturation. Here, each spermatocyte two meiotic divisions to produce spermatids. Finally, in Zone III, spermatids develop into spermatozoa.

The overall path looks something like this:

Germ cells ------> spermatogonia ---------->spermatocytes -------> spermatids ---------> sperm

You may note very little is known about regulation of spermiogenesis. More recently, though, it has been firmly established that ecdysone is necessary for spermiogenesis in many species.

The next figure shows the major features of female reproductive systems, in this case from a locust and from a dipteran. In the general scheme, there are usually two ovaries, from which lateral oviducts join a common genital chamber. In insect species that transfer sperm by direct intromission of a penis, rather than by a spermatophore, the genital chamber is called a bursa copulatrix or vagina. A sperm storage organ, or spermatheca, is connected to the genital chamber by a spermathecal duct. As in males, female reproductive tracts often have accessory glands. Whereas testes are composed of follicles, ovaries are made up of ovarioles. Ovaries have from a few to over 100 ovarioles, depending upon species and, in locusts, on population densities. As usual, there are extremes, and some termites have over 2,000 ovarioles per ovary while certain aphids have only a single ovariole in one functioning ovary.

Three types of ovaries are recognized, based on the arrangement of nurse cells, as shown in the following figure. These are panoistic, telotrophic and polytrophic. Panoistic ovarioles do not have specialized nurse cells. The figure shows distal ends of ovarioles, with terminal filament for attachment with abdomens, oogonium, and layers of external sheath, including tunica propria. Here we see oogonia can develop into young oocytes, and then into primary oocytes. As in testis follicles, there is a spectrum of increasing maturation toward the proximal end of ovarioles. Panoistic ovarioles are regarded as the most primitive type, and they are usually found in more primitive orders such as Odonata and Plecoptera.

Telotrophic ovarioles have trophic tissue in addition to oogonia an oocytes in the germarium. Trophic cells and oogonia derive from germ cells in the germarium. As oocytes mature, they move toward the proximal end of ovarioles. The maturing oocytes maintain contact with the core of tropic tissue by way of a nutritive cord that grows longer as oocytes move toward the proximal end of ovarioles. The nutritive cords break during vitogenesis, and follicle cells form a continuous layer around the oocyte.

Polytrophic ovarioles have trophocytes wrapped up with each oocyte in a follicle. These ovarioles are found in most holometabolous insects. In the germarium, oogonia divide into an oocyte and a trophocyte, but the two cell types maintain a cytoplamic link. Further cell divisions can occur so that a given ooctye may be associated with several trophocytes, as shown in the next figure.

Telotrophic and polytrophic ovarioles, those characterized by having nurse cells, are collectively called meriostic ovarioles. We use the term trophic frequently in this discussion, in telotrophic and polytrophic ovarioles and in trophic tissue and trophocytes. Its enough to make you think these cells feed something to the oocytes, but that is not quite what happens. The principle role of trophic tissue is to supply large amounts of RNA, both ribosomal RNA and transfer RNA, to oocytes. This is thought to support rapid oocyte growth.

An ovariole usually holds a series of oocytes, each further along in development as they near the proximal end. The germarium is at the distal end, and here stem line oogonia derive from germ cells. The oogonia divide into two daughter cells, one of which becomes an oocyte and the other retains it stem cell function. As an oocyte leaves a germarium is becomes invested in a prefollicular tissue that later forms the follicle cell layer surrounding each oocyte. The number of follicle cells increases as oocytes grow. In Drosophila, for example, they increase from about 80 to over 1200. During yolk deposition the follicle cells do not divide, but insteat form a flattened epithelium. The proximal portion of an ovariole is called the vitellarium, which in mature insects is by far the greater part of an ovariole. Yolk deposition, or vitellogenesis occurs in the proximal part. Oocytes vastly increase in size during vitellogenesis. Yolk is categorized as protein yolk and lipid yolk. Juvenile hormone stimulates fat bodies to synthesize and export vitellogens, or yolk proteins. JH also causes gaps in follicular cells, a condition called patency by Ken Davy. This allows free exchange between oocytes and hemolymph, and the oocytes rapidly take up proteins from circulation. Lipid yolk is also derived from fat body. It is thought that early in development only phospholipids are taken up, then triacylglycerol and phospholipids. The lipids probably provide metabolic energy and maternally derived essential fatty acids to developing embryos.

When vitellogenesis is completed the outer layer of the oocytes usually develops. This is called the vitelline membrane which is partly secreted by follicle cells and partly by oocytes. We already know that ovulation refers to the movement of mature oocytes from the ovarioles to the common oviduct.