Structure and functions of reproductive systems and types of reproduction in insects

In insects male and female sexes are mostly separate. Sexual dimorphism is common where the male differ from the female morphologically. e.g. bee, mosquito and cockroach. The other types are

Gynandromorph: (Sexual mosaic) Abnormal individual with secondary sexual characters of both male and female. e.g. mutant Drosophila.

Hermaphrodite: Male and female gonads are in one organism. e.g. Cottony cushion scale.

Female of reproductive system

The main function of the female reproductive system are egg production and storage of male's spermatozoa until the eggs are ready to be fertilized. The basic components of the female system are paired ovaries, which empty their mature oocytes (eggs) via the calyces (Calyx) into the lateral oviduct which unite to form the common (median) oviduct. The gonopore (opening) of the common oviduct is usually concealed in an inflection of the body wall that typically forms a cavity, the genital chamber. This chamber serves as a capulatory pouch during mating and thus is often known as the bursa copulatrix. Its external opening is the vulva. In many insects the vulva is narrow and the genital chamber becomes an enclosed pouch or tube referred to as the Vagina.

Two types of ectodermal glands open into the genital chamber. The first is the spermatheca which stores spermatoza until they are needed for egg fertilization. The epermatheca is single and sac-like with a slender duct, and often has a diverticulum that forms a tubular spermathecal gland. The gland or glandular cells within the storage 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.

Each ovary is composed of a cluster of egg or ovarian 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 vitellogenesis) and a pedicel. An ovariole contains a series of developing oocytes each surrounded by a layer of follicle cells forming an epithelium (the oocyte with its epithelium is termed a follicle), the youngest oocytes occur near the apical germarium and the most mature near the pedicel.

 

The different types of ovariole is based on the manner in which the oocytes are nourished.

1. Paniostic ovariole: Lacks specialized nutritive cells so that it contains only a string of follicles, with the oocytes obtaining nutrients from the haemolymph via the follicular epithelium. e.g. Cockroach.

Ovarioles of the other two contains trophocytes (nurse cells) that contribute to the nutrition of the developing oocytes.

2. Telotrophic ovariole: (Acrotrophic) The trophocytes are confined to the germarium and remain connected to the oocytes by cytoplasmic strands as the oocytes move down the ovariole,

e.g. bugs.

3. 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 smaller trophocytes. e.g. moths and flies.

Accessory glands of the female reproductive tract are often called as colleterial or cement glands, because their secretions surround and protect the eggs or cement them to the substrate. e.g. egg case production in mantis, ootheca formation in cockroach, Venom production in bees.

Structure of egg

Chorion
Vitelline membrane 

Micropyle 

Periplasm with yolk

Male of reproductive 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 expands posteriorly to form a sperm

storage organ or seminal vesicle. Tubular paired accessory glands are formed as diverticula of the vasa deferentia. Sometimes the vasa deferentia themselves are glandular and fulfil the functions of accessory glands. The paired vasa deferentia unite where they lead into the ectodermally derived ejaculatory duct (the tube that transports the semen or the sperm to the gonopore).

Accessory glands are 1-3 pair, either mesodermal of ectodermal in origin and associated with vasa deferentia or ejaculatory duct. Its function is to produce seminal fluid and spermatophores (sperm containing capsule).

TYPES OF REPRODUCTION IN INSECTS

PHYSIOLOGY OF REPRODUCTION

1. Spermatogenesis: (occurs inside sperm tube)

mitosis

Spermatogonia --------> Primary spermatocytes (2n) Meiosis mitosis

Spermatids (n)<------- Secondary spermatocytes (n) Spermiogenesis--------------> Sperms (n)

2. Oogenesis : (occurs inside egg tube)

Mitosis Meiosis

Oogonia ---------> Primaryoocytes(2n) -------> Secondary oocytes (n)

Oogenesis Mitosis Ovum (n) <----------- Oocytes (n) <---------

3. Sperm transfer

1. Intragenital: Common method, through, aedeagus via vaginal orifice into female genitalb passage.

2. Haemocoelous: Sperms transfered into the body cavity e.g. Bed bug.

3. External: Spermatophores are ejected out into open place by male, while female walk over it and gets inseminated e.g. Silver fish.

4. Fertilization: Sperm entre into egg to produce morphogenesis. Egg nucleous divides meiotically into female gamete nucleus and polar body. Then the fertilization occurs by the fusion of male and female gamete nuclei.
   

Types of reproduction in insects

1. OVIPARITY

Majority of female insects are oviparous that is, they lay eggs. Embryonic development occurs after oviposition by utilizing the yolk, e.g. Head louse moths.

2. VIVIPARITY

Unlike oviparous, here initiation of egg development take place within the mother. The life cycle is shortened by retention of eggs and even developing young within the mother. Four main types of viviparity are observed in different insect groups.

1. OVOVIVIPARITY

Fertilized eggs containing yolk are incubated inside the reproductive tract of the female and hatching of egg occur just prior to or soon after oviposition e.g. Thysanoptera, some cockroaches, few beetles, and some flies-(fleshfly). Fecundity of this group is low.

2. PESEUDOPLACENTAL VIVIPARITY

This occurs when a yolk-deficient egg develops in the genital tract of the female. The mother provides a special placenta-like tissue, through which nutrients are transferred to developing embryos. There is no oral feeding and larvae are laid upon hatching. e.g. aphids, some earwigs, psocids and polytenid bugs.

iii. HAEMOCOELOUS VIVIPARITY

This involves embryos developing free in the female's haemolymph with nutrients taken up by osmosis. This form of internal parasitism occurs only in sterpsiptera and some gall midges.

iv. ADENOTROPHIC VIVIPARITY

This occurs when a poorly developed larva hatches and feeds orally from accessory (milk) gland secretion within the uterus of the mother's reproductive system. The full grown larva is deposited and pupariates immediately (eg) tsetse flies, louse or wallaby flies, bat flies.

3. PARTHENOGENESIS

Reproduction without fertilization is called parthenogenesis. Different types of parthenogenesis  are as follows

1. BASED ON OCCURRENCE

1. Facultative (not compulsory) - e.g. bee.

2. Obligatory or constant (compulsory) - e.g. stick insect

3. Cyclic or spordic : alternation of gamic and agamic population, e.g. aphid.

2. BASED ON SEX PRODUCED

1. Arrhenotoky: Produce male e.g. bee

2. Thelytoky: produce female e.g. aphids

3. Amphitoky or deuterotoky: produce both male and female e.g. cynipid wasp.

3. BASED ON MEIOSIS

1. Apodictic : no meiosis occurs

2. Automictic: meiosis occurs, but diploidy is maintained

4. POLYEMBRYONY

This form of asexual reproduction involves the production of two or more embryos from one egg by subdivision. Mostly observed in parasitic insects (e.g. platygaster). Nutrition for a large number of developing embryo cannot be supplied by the original egg and is acquired from the host's haemolmph through a specialized enveloping membrane called trophamnion.

5. PAEDOGENESIS

Some insect cut short their life cycles by loss of adult and pupal stages. In this precocious stage gonads develop and give birth to young one by parthenogenesis.

1. Larval paedogenesis - e.g. gall midges

2. Pupal paedogenesis - eg Miaster sp.