Review Sheet -- Test 1 (Week 3) Biology 1224 -- Entomology; James Adams
Molting or Ecdysis --
Shedding the cuticle (Chapter 3; pages 56 - 58)
Insects are restricted in size by their cuticle; to grow, the old cuticle must be shed. This also
means that growth is typically stepwise and discontinuous. When a new cuticle is laid down, it is
typically wrinkled to allow expansion room.
Fixing of wounds, change in shape of body, changes in setation patterns, etc. are also
restricted to the time of molting -- it allows the surface of the body to be renewed. Between molts
the epidermis continues to be active in secreting layers of endocuticle and in wax production.
Initiation of molting -- ecdysone (a steroid hormone) stimulates detachment of the cuticle from
the epidermis, termed apolysis, and molting fluids are secreted underneath the old cuticle. The
epidermis thickens and proliferates (produces more cells) just prior to separation of the cuticle.
The New Cuticle -- epicuticle secreted first, followed by procuticle. Once the protein part of
the epicuticle is in place, the molting fluid becomes active and promotes digestion of the old
endocuticle; some of the proteins and chitin are recycled into the new procuticle. Sclerotizing agents
and then an waxy layer and outer cement layer is added to the new cuticle through the pore canals
just before the outer portion of the old cuticle is shed.
Ecdysis -- actual molting; getting rid of the exuvia (the old epicuticle).
Before the new cuticle is completely sclerotized, the insect expands its volume by sucking in air*
or water (depending on the habitat the insect lives in), and also goes through muscular contortions,
both of which split the cuticle along ecdysial lines (thinner regions); the inside of the larger tubes
of the respiratory system is also shed (pulled out through the spiracles).
The new cuticle is expanded even further by the mechanisms mentioned above*. This may
include new body structures, such as the wings at the last molt. The wings must be expanded and
hardened for flight. After expansion, the cuticle hardens and darkens (sclerotizes) rapidly under
the control of another hormone.
Hormonal control of molting (see the handout and pages 72 - 75, chapter 4)
(See also http://alpha.furman.edu/~snyder/snyder/lep/faq.htm , question 10)
Three main hormones you will be responsible for: Prothoracic tropic (brain) hormone, ecdysone,
and juvenile hormones. Bursicon is another hormone that stimulates sclerotization of the cuticle.
During larval life, the insect's corpora allata (extensions posteriorly off of the brain) continuously
produce and release relatively constant quantities of a hormone called Juvenile Hormone (JH),
whose function, as the name suggests, is to keep the larva in the larval stage. Each time the larva
"gets full", however, it needs to shed. The intersegmental grooves getting stretched triggers release
of a hormone called Prothoracic tropic (or brain) hormone, that in turn causes release of another
hormone, Ecdysone, from a gland called the prothoracic gland, which stimulates shedding (ecdysis),
and also differentiation of maturing (adult) tissues. The larva then enlarges (as described above) so
that it can feed for a while before it has to shed again. So, at each molt there is a burst of ecdysone
production, followed by another instar where the larva can feed until it needs to molt again.
During this time, the larva continues producing juvenile hormone. So, with each molt, at least for
a while, the larva molts into . . . another larva. When it comes time for this larva to molt into the
pupal stage, the level of JH appears to drop appreciably just before the molt, which means the larva
will not stay a larva after this molt. End result? This molt leads to an adult in hemimetabolous insects,
and to a pupa in holometabolous insects (now committed to become an adult at the next molt). At
that time the holometabolous larva will produce chemicals that accumulate on the body surface and
harden into the tough pupal surface cuticle.
Although the pupa may seem to be very quiet (except for occasional twitches and contortions),
this is quite misleading. Inside, a radical transformation is occurring. Very early in larval life, several
clumps of cells have been set aside in various parts of the body. These clumps are called imaginal
discs (remember: imago means adult). Each disc, or pair of discs, is composed of the cells that are
the ancestors of adult structures -- two eye discs, four wing discs, six leg discs, etc. These discs
have no function in the larva, but become very active when the animal becomes a pupa.
During pupal life, nearly all other cells die and their contents are recycled to build a rapidly
increasing number of cells in each imaginal disc. Then, each disc unfolds, revealing the adult shape
of a wing or leg or eye, etc. These emerging regions get organized into the adult shape, including a
cuticle, all still hidden under the pupal cuticle. (Some internal body parts, such as the nervous sys-
tem, are retained and re-formed from those of the larva.) After the adult parts have formed inside
the pupa, there is one final molt, or eclosion, which reveals the adult.
Metamorphosis (pages 67 - 72, chapter 4)
Epimorphic development -- immature ecloses from egg with adult number of segments;
true for all but Protura, which have anamorphic development (add segments with molts).
Molt number may be as few as four, or as many as 30, depending on species.
Each stage between molts is called an instar.
Most insects go through a distinct larval stage on their way to the adult stage, or imago.
Eclosion -- hatching (from previous cuticle); typically used for hatching from egg or pupa.
Types of metamorphosis
Ametabolous -- "without change" from larva to adult; all instars look similar. This is typical of
the apterous hexapods (collembolans, diplurans, thysanurans, etc.). Molting continues
Hemimetabolous -- "incomplete"; typical of most exopterygote insects, or insects whose wings
develop on the outside of the body. Larvae, or nymphs, typically have small but enlarging
wing pads with each molt, with the wings enlarging tremendously and becoming functional
with the last molt into the sexually mature adult stage. The one exception to an immature
stage having wings is the subimago stage of mayflies (Ephemeroptera), which flies to leave
the water, then quickly molts into the sexually mature imago. The hemimetabolic changes
between nymph and adult do not involve significant rearrangement of the body, except for
those with aquatic nymphs, or naiads, such as mayflies, odonates, and stoneflies. Includes
the paleopterous, orthopteroid, and most hemipteroid orders.
Holometabolous -- "complete"; typical of endopterygote insects. Distinct stages of
development -- egg, larva, pupa (with a silken cocoon around it in some species), imago;
radical rearrangement of the body during the pupal stage (larvae and adult quite different).