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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).