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Development Laboratory
Life Cycle and Metamorphosis
General
The insect has a relatively rigid exoskeleton which limits any changes in size to
those periods when the exoskeleton is shed and the new cuticle is soft prior to hardening.
Growth in insects, as all arthropods, is in series of steps or discontinouous stages. The
development of a new cuticle, or exoskeleton, and the shedding of the old is known as
molting. The actual shedding process is usually known as ecdysis. The shed cuticula is
known as the exuviae or cast skin. Each time an insect molts to changes from, sometimes
greatly. The insect between ecdyses is known as an instar (these are usually numbered as
in the 1st instar, 2nd instar, etc.), whereas the period of time between molts is referred to a
stadium.
The process of molting and metamorphosis is under nervous -hormonal control.
Edcysis
Examine and exuviae of a cockroach. This cast skin is only the outermost part of
the old cuticle. Notice the silver-like strands-- these are the linings of the larger tracheal
(respiratory) tubes. The ecdysial line is either T shaped or straight along the dorsal thorax.
Metamorphosis
Your text recognizes two types of metamorphosis- incomplete and complete.
Incomplete is where the young nymphs resemble the adults, develop the wings gradually
as external wings buds, and often are found in the same environment as the adult.
Complete metamorphosis has a larval form that is vastly different from the adult and adds
as pupal stage as a transitional form from the specialized young to the adult. Wing buds or
development is not seen until the pupal or even adult form. The habitat of the young may
be greatly different from the adult.
Many other authorities subdivide these two basic types of metamorphosis into three
of four groups because of some obvious differences. These are summarized below.
Ametamorphosis or Ametabolous
This type of development is restricted by definition to insects which continue
molting throughout adult life and are wingless and whose ancestors never possessed wings.
They also possess paired appendages on the proximal abdominal segments. No special
name is given to the young or immature stages. This is restricted to some of the
apetrygota.
Paurometabolous
The immature stages of insects with gradual metamorphosis are known as nymphs.
The adults of most forms are winged but some have lost them secondarily, particularly
sedentary plant feeders and animal parasites. The nymphs live in the same environment as
the adults, have the same type of mouthparts and eat the same food. The wings develop as
external wings pads, increase in size in successive instars, but are not functional until the
adult stage. What may be the reason why molting does not take place after wings become
functional?
Grasshopper, Cockroach, or Mantid.- Examine the nymnphal instars of a
grasshopper and compare them with the adults. Note particularly the development of the
external wings pads. How many of them and what are their relationships?
Bugs- Examine the nymphs of a bug and compare with an adult.
Hemimetabolous
This is a special type of gradual metamorphosis in which the young stages are
aquatic while adults are aerial. The immature stages are known as naiads, have external
wing pads (as do nymphs), but develop special respiratory organs for their aquatic
existence. The food of the naiads and adults is, of course, different. Examine the
following examples.
Dragonfly or Damselfly (Order Odonata)
a.
Damselfly naiads have prominent apical abdominal gills while dragonfly naiads
have the rectum modified for respiration. In either type of naiads note the labial
mask on the underside of the head. Open this mask carefully and note how it is
hinged. It is used to capture prey. Note the conspicuous wing pads.
b.
Examine the adults. Note the sharp spines on the legs of the dragonfly; these are
utilized in capturing insects on the wing. Compare the mouthparts with those of the
naiads.
Holometabolous
In insects with complete metamorphosis, the extremely specialized growing
immature stage is called the larva following which there is a quiescent pupa. The larva
never develops external wing pads. The pupa, on the other hand, has external wing, leg,
and mouthpart pads. The pupa may be housed in a protective cocoon of silk or various
other substances or may be formed within the hardened cuticula of the preceding larva
instar which develops into a puparium. The immature stages may occur in the same or
different environment as the adult.
The embryonic features that larvae retain are variously specialized for obtaining
food. Morphologically the pupa corresponds to the last nymphal instar of insects with
gradual metamorphosis. The larval instars represent modified earlier nymphal instars.
Examine the following examples of complete metamorphosis.
1.
Beetle (Order Coleoptera). – Examine the larva, pupa, and adult. Note the
differences in all body regions and appendages. Note particularly the complete
absence of wing pads on the larva and their development in the pupa.
2.
Butterfly or Moth (Order Lepidoptera).- Examine the larva, popularly known as a
caterpillar. Note the true thoracic legs and the abdominal prolegs. Observe the
mouthparts. Examine the cocoon and the enclosed pupa and identify its wing pads.
3.
Bee, Ant, or Wasp (Order Hymenoptera).- Examine the larva. Does it have
thoracic legs? Abdominal segmentation? Examine the pupa and adult.
4.
House Fly (Order Diptera).- Examine the larva. Any legs? Note the characteristic
“mouthparts.” Examine the puparium, the hardened cuticula of the third larval
instar.
Larva
It is relatively easy to associate nymphs and even naiads with their respective adult
at least in major groups. Many larvae on the other hand become so specialized that they
appear at first glance as entirely different organisms from the corresponding adults. On the
other hand they show relationship to each other through their retention of primitive
characters and often more conspicuous specific differences than their adults because of
their greater specializations. The study of larvae has contributed a great deal to a more
natural classification of insects.
Eruciform- caterpillar
Examine a caterpillar of a moth. Note the chewing mouthparts. The thorax has
well developed legs. The abdomen has prolegs used for locomotion also. The prolegs are
provided with special muscles. The number of prolegs varies with the family of
Lepidoptera: some have a reduced number and these larvae are called “loopers” or
measuring worms. In the hymenoptera the sawfly larvae have six pair of prolegs without
the row(s) of hooks (crochets) found on the moth and butterfly larvae prolegs.
Scarabaeiform- grublike, white grub
Usually curved (c shaped) with well-developed head and thoracic legs. Relatively
inactive. Examine a larva of the family Scarabaeidae (Coleoptera).
Campodeiform
Body elongate and somewhat flattened. Thoracic legs well developed and the
larvae active and often predatory. Occurs in the Neuroptera, Trichoptera, and many
Coleoptera.
Elateriform- wireworms
Body elongate, cylindrical, and hard shelled. This type occurs in certain
Coleoptera (e.g. Elateridae).
Veriform- Maggot
Body elongate and wormlike. Legless (apodous) and often without a well
developed head. This type occurs in the Diptera, Siphonaptera, many Hymenoptera, and
some Coleoptera and Lepidoptera. A good example of this type is the house fly larva.
Pupa
The pupa is a transitional stage and is usually quiescent but some active pupae are
known (e.g. the mospuito). Three types are usually recognized:
Obtect- with appendages more of less glued to the body. This type occurs in the
Lepidoptera and some Diptera. The pupa of many Lepidoptera is covered by silken
cocoon formed by the larva before it molts to the pupal stage.
Exarate- Appendages free and not usually covered by a cocoon. Occurs in most
insects with a complete metamorphosis except the Diptera and most Lepidoptera.
Coarctate- essentially an exarate pupa but remain covered by the hardened exuviae
of the larva, which is called a puparium. This type occurs in the Diptera (suborders
Brachycera and Cyclorrhapha).