 Homeostasis

 Growth and Development

 Reproduction
 Energy Metabolism
 Behavior
 Endocrine system

 Nervous system regulate all physiological requirements

of an insect including growth, reproduction, and protein
formation throw the endocrine system via hormones.

 Hormones complement the nervous system, which

provides short term coordination, and the activities of
both systems are closely linked.
 Hormones:
Chemical substances that are transported in
the insect’s body fluids (haemolymph) that
carry messages away from their point of
synthesis to sites that where physiological,
behavioural and developmental processes are
influenced.
The endocrine organs of insects are of

two types (most of which are within

the central nervous system):

1- Specialized endocrine glands

2- Neurosecretory cells
 Endocrine glands
1- Glands producing ecdysteroids

 In the immature stages of all insects, molting hormones

are produced by the prothoracic glands.

 In females, where the same hormones are produced to

regulate embryonic development, the follicle cells in the
ovary are the principal source.

 It may also produced in the abdomen of some insects.

Prothoracic glands

 Diffuse, paired glands located at the back of the head

or in the thorax.
 These glands secrete an ecdysteroid called ecdysone,
or the moulting hormone, which initiates the
epidermal moulting process.
• Additionally it plays a role in accessory reproductive
glands in the female, differentiation of ovarioles and
in the process of egg production.
 Corpora allata

 Small, paired glandular bodies usually one on either side

of the oesophagus.

 They produce the juvenile hormone, which regulate

metamorphosis and yolk synthesis and deposition in the
oocytes of adults.
Corpora cardiaca

 A pair of neuroglandular bodies that are found

behind the brain and on either sides of the aorta.

 The corpora cardiaca store and release hormones

from the neurosecretory cells of the brain, to which
they are connected by one or two pairs of nerves.
Endocrine cells of the midgut

 These are isolated cells scattered a amongst the

principal midgut cells.
 They secrete some peptides which have a hormonal
function relating to digestion and absorption, perhaps
regulating the synthesis of digestive enzymes and post-
feeding dieresis.
 Other cells may have different functions, perhaps
including the regulation of gut motility.
2- Neurosecretory cells

 Occur in the ganglia of the central nervous

system.
 These cells secret hormones that may affect
growth, reproduction, homeostasis and
metamorphosis.
 In the brain, two main groups of neurosecretory
cells on each side.
 The secretions of neurosecretory cells are
usually neruopeptides.
 The most common hormones that are secreted by these cells
are:
- Ecdysiotropin (Protocerebrum secretes
ecdysiotropin or prothoracicotropic hormone
(PTTH) or brain hormone (BH) that acts on ecdysial
glands)
- Bursicon (Tanning hormone): triggers the tanning or
darkening of adult cuticle
- Eclosion hormone: It is stored in the corpora cardiaca and
is released into the blood at the time of switchover from
pupal to adult stage initiate the pre-eclosion behavior.
 Endocrine Control of Growth and
Metamorphosis
• Upon emergence from the egg, the immature insects gradually
increase in size to reach adults through some mechanisms
called moulting.

• Moulting involves the periodic digestion of old cuticle,

secretion of new cuticle (usually with larger surface area than
the older one) and shedding of undigested old cuticle.

• shedding of undigested old cuticle- is commonly referred to as

ecdysis
• Each developmental stage of the insect itself is called an
instar, and the interval of time passed in that instar is referred
to as stadium.

• The whole developmental process by which the first instar

immature stage of an insect is transformed into the adult
insect is called metamorphosis

• Hormones required: Brain hormone, Ecdysone, Juvenile

hormone.
 Brain hormone (prothoracicotropic hormone
(PTTH)):
• Protocerebrum secretes brain hormone (BH) or
prothoracicotropic hormone (PTTH) or ecdysiotropin
which accumulate in the carpora allata and
subsequently released into the haemolymph (except in
Lepidoptera, in other insects BH is stored in corpora
cardiaca). Through the haemolymph, PTTH reach to
prothoracic gland and stimulate its secretory activity
• The prothoracic glands secrete α-ecdysone or moulting
hormone (MH) which through haemolymph reach the
target (epidermis)
• which initiates the growth and moulting activities of the cells.
• Ecdysone fovours the development of adult structures and
favours the moulting processes that terminate into successive
larval instars
• The corpora allata secrete juvenile hormone (JH), which
promote larval development and inhibit development of adult
characteristics
• In fact, JH interacts with MH to stimulate larval maturation
during each stage of development. The concentration of JH
evidently decreases toward the end of a larval instar, allowing
the ecdysone to cause moulting.
• The total picture here should be one of balanced interaction-
synergism—between these two hormones to induce normal
growth and differentiation, rather than a simple antagonism.

• During the last immature instar, two separate and distinct

peaks of ecdysone are present in both the holometabola and
hemimetabola. The first one is low and in absence of JH, the
epidermal cells are reprogrammed from larval to pupal
commitment in holometabolous insects, and from nymphal to
adult stage in hemimetabolous insects.
 Eclosion

• Eclosion hormone or EH is released from brain by a circadian

clock and declining ecdysteroid titers. If ecdysone titer is
artificially kept high, the release of eclosion and its activity are
inhibited.
• This hormone influences many aspects of pupal-adult ecdysis,
including the behavior associated with ecdysis and subsequent
degeneration of abdominal inter-segmental muscles used in
the act of ecdysis
 Ecdysis triggering hormone

• It is the most recent hormone discovered that plays an

important role in ecdysis. This 26 amino acid peptide
hormone is synthesised by the epitracheal glands that are
located segmentally in larvae, pupae and adults of Manduca
sexta. According to Zitnan (1996), this hormone may act
upstream from the eclosion hormone in a series of cascade
events leading to ecdysis.
Bursicon (Tanning hormone)

• Bursicon, commonly found in neurohaemal organs associated

with the ventral chain ganglia is suggested to stimulate
tanning and sclerotisation of the cuticle following ecdysis.
 Hormonal control of reproduction

 Like other higher multicellular organisms, reproduction in

insects is a complex process.

 Different stages of reproduction, starting from the production

of male and female gametes to oviposition, are seem to be
influenced by several hormones.
  Spermatogenesis

• Ecdysone controls the permeability of the testis walls to the

humoral factor differentiating the spermatocytes.

• Juvenile hormone is shown to have some inhibitory effects

on spermatogenesis in many insects.
 Oogenesis

• Hormones from corpora allata help in egg maturation through

the incorporation of yolk into the oocyte.

• In addition to secretions from brain cells and corpora allata,

ecdysone has been found to be involved in control of
oogenesis in female mosquitoes. Following a blood meal,
lateral neurosecretory cells secrete egg development
neurosecretory hormone, which in turn, induces the ovary to
secrete ecdysone. Ecdysone, in turn triggers the synthesis of
yolk protein vitellogenin in the fat bodies.

• Juvenile hormones secreted by corpora allata also activate fat

body and ovaries.
 Fertilization
• In many insects studied, ovulation (the passage of egg from
the ovary into the oviduct) and oviposition, (passage of
fertilized eggs to the outside, are closely linked. Both these
events are affected by some peptides secreted by female
accessory glands and neurosecretory products of brain.
• The process of reproduction involves both the nervous and
endocrine systems. The major centers are the neurosecretory
cells of brain and the major events are the secretion of juvenile
hormone by corpora allata, and either ecdysone production by
ecdysial gland in immature insects or ecdysone biosynthesis
by the ovary in adult insects. Both hormones act either
independently or together in association with nervous system
to make reproduction success.
 Vitellogenesis

• Vetellogenesis or egg yolk synthesis is also known to depend

on JH from the corpora allata. In mosquitoes, juvenile
hormone is required for egg development only during the
early previtellogenic stages of development of the follicles.
Endocrine System