Summary

Almost all species of insects use internal fertilization and lay eggs that contain much yolk. The female reproductive system basically includes paired ovaries and lateral oviducts, a common oviduct, bursa copulatrix, spermatheca, and accessory glands. Each ovary includes ovarioles, which may be panoistic (lacking nurse cells) or meroistic, where nurse cells either are enclosed in each follicle with an oocyte (polytrophic type) or form a syn-cytium in the germarium and connect with an oocyte via a trophic cord (telotrophic type).

592 The male system includes paired testes, vasa deferentia and seminal vesicles, a median ejaculatory duct, and accessory glands. Each testis is composed of follicles in which zones CHAPTER 19 j , „

of development of germ cells occur.

In most insects a period of sexual maturation is required after eclosion. In females maturation may include vitellogenesis (formation of yolk), development of characteristic body coloration, maturation of pheromone-producing glands, growth of the reproductive tract, and an increase in receptivity. Sexual maturation is affected by quality and quantity of food eaten, population density and structure, mating, temperature, humidity, and photope-riod. These factors exert their influence by modifying endocrine activity. The two primary endocrine components in most insects are the cerebral neurosecretory system and corpora allata. The neurosecretory system liberates various peptides, including allatotropic and al-latostatic hormones that regulate corpus allatum activity. The corpora allata release juvenile hormone (JH), which has both gonadotropic effects (priming the follicular epithelium, then stimulating its differentiation so that it becomes permeable to proteins) and a metabolic effect (promoting synthesis of vitellogenins in the fat body.) However, in mosquitoes and probably other Diptera, JH acts only during previtellogenesis and it is ecdysone, synthesized by the follicle cells in response to ovarian ecdysteroidogenic hormone, that promotes vitellogenesis in the fat body. In Lepidoptera vitellogenesis is regulated by ecdysone in species that develop eggs in the larval and/or pupal stages, while JH takes on this role in species that develop eggs as adults.

Spermatogenesis occurs principally in the last juvenile instar, under endocrine control, but may continue at a reduced rate in adult males. Sexual maturation in adult males is largely devoted to synthesis and accumulation of accessory gland secretions, development of pheromone-producing glands and mature body coloration, and courtship behavior. As in females, maturation is controlled hormonally, especially by juvenile hormone, though ecdysteroids may be important in some species.

Mating behavior serves to ensure that sperm is transferred from male to female under the most suitable conditions, and, perhaps, to prevent interspecific mating. It includes mate location and/or recognition, courtship, copulation, and postcopulatory behavior. Mate location and recognition are achieved through visual, chemical, auditory, and tactile stimuli. Courtship may synchronize the behavior of male and female and appease a normally aggressive female. Copulation may depend on the receptivity of a female and may occur only under specific conditions, for example, at a set time of the day, near the food plant, or immediately after feeding.

Primitively, sperm are transferred in a spermatophore produced from secretions of the accessory glands and formed within the male genital tract. At a more advanced stage, the spermatophore is formed in the female genital tract and may simply serve as a plug to prevent loss of semen. Some species do not form spermatophores but use a penis for depositing sperm in the female genital tract. Sperm normally reach the spermatheca as a result of peristaltic movements of the genital tract, though in some species active migration may occur in response to a chemical attractant. Seminal fluid includes a variety of chemicals that may induce major changes in the behavior and physiology of the mated female. These changes, which are designed to ensure paternity of the eggs laid, include: induction of refractoriness, reduction in attractiveness, plugging of the reproductive tract, acceleration of egg development, stimulation of egg laying, and displacement or incapacitation of sperm from previous matings.

Both male- and female-driven postinsemination strategies have evolved to ensure that some sperm are used in preference to others for egg fertilization. Males primarily take the "last in, first out" approach (i.e., sperm from the most recent mating will be used), 593

though examples of stratification (packing), loading, incapacitation, and removal of sperm are known. Females may discard sperm, not transport them to storage sites, or simply remate quickly.

In almost all insects sperm enter eggs via micropyles as the eggs move down the common oviduct during oviposition. Release of sperm from the spermatheca is closely synchronized with movements of the eggs. Eggs may be precisely oriented in the oviduct or the micropyles may be arranged in a cluster or a groove to ensure location of the micropyles by sperm. Polyspermy is common, though usually only one sperm undergoes transformation into a pronucleus. Fertilization (fusion of male and female pronuclei) does not occur until after eggs are laid.

Insects may show great selectivity in their choice of oviposition sites. Eggs may be attached to surfaces by secretions of the female accessory glands or buried using an ovipositor. Eggs may be covered with an ootheca, again formed from accessory gland secretions, which may prevent desiccation and/or parasitism.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment