In biology , the imago ( Latin for "image") is the last stage an insect attains during its metamorphosis , its process of growth and development; it is also called the imaginal stage ("imaginal" being "imago" in adjective form), the stage in which the insect attains maturity. It follows the final ecdysis of the immature instars .
11-522: In a member of the Ametabola or Hemimetabola , species in which metamorphosis is "incomplete", the final ecdysis follows the last immature or nymphal stage. In members of the Holometabola , in which there is a pupal stage, the final ecdysis follows emergence from the pupa , after which the metamorphosis is complete, although there is a prolonged period of maturation in some species. The imago
22-585: A single instar when sexually mature. Apterygotes possess small unsegmented appendages, referred to as "styli", on some of their abdominal segments, but play no part in locomotion. They also have long, paired abdominal cerci and a single median, tail-like caudal filament, or telson. While all members of winged insects ( Pterygota ) has a closed amniotic cavity during embryonic development, this varies within Apterygota. In Archaeognatha, species like Petrobius brevistylis and Pedetontus unimaculatus have
33-417: A wide open cavity, whereas Trigoniophthalmus alternatus does not have an amniotic cavity at all. In Zygentoma, the cavity is open through a narrow canal called the amniopore in the species Thermobia domestica and Lepisma saccharina , but in other species like Ctenolepisma lineata it is completely closed. The composition and classification of Apterygota changed over time. By the mid-20th century,
44-407: Is imagines , and this is the term generally used by entomologists when a plural form is required –however, imagoes is also acceptable. Apterygota Pterygota The name Apterygota is sometimes applied to a former subclass of small, agile insects , distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are
55-457: Is the only stage during which the insect is sexually mature and, if it is a winged species, the only stage that has functional wings. The imago often is referred to as the adult stage. Members of the order Ephemeroptera (mayflies) do not have a pupal stage, but they briefly pass through an intermediate winged stage called the subimago . Insects at this stage have functional wings but are not yet sexually mature. The Latin plural of imago
66-636: The monophyly of Thysanura+Pterygota is the absence of intrinsic antennal muscles, which connect the antennomeres in entognaths , myriapods , and crustaceans . For this reason, the whole group is often termed the Amyocerata , meaning "lacking antennal muscles". However, the Zygentoma are now considered more closely related to the Pterygota than to the Archaeognatha, thus rendering even
77-539: The silverfish , the firebrat , and the jumping bristletails . Their first known occurrence in the fossil record is during the Devonian period, 417–354 million years ago. The group Apterygota is not a clade; it is paraphyletic , and not recognized in modern classification schemes. As defined, the group contains two separate clades of wingless insects: Archaeognatha comprises jumping bristletails, while Zygentoma comprises silverfish and firebrats. The Zygentoma are in
88-514: The capability for powered flight. Apterygotes also have a number of other primitive features not shared with other insects. Males deposit sperm packages, or spermatophores , rather than fertilizing the female internally . When hatched, the young closely resemble adults and do not undergo any significant metamorphosis , and lack even an identifiable nymphal stage. They continue to molt throughout life, undergoing multiple instars after reaching sexual maturity, whereas all other insects undergo only
99-548: The clade Dicondylia with winged insects, a clade that includes all other insects, while Archaeognatha is sister to this lineage. The nymphs (younger stages) go through little or even no metamorphosis , hence they resemble the adult specimens ( ametabolism ). Currently, no species are listed as being at conservation risk. The primary characteristic of the apterygotes is they are primitively wingless. While some other insects, such as fleas , also lack wings, they nonetheless descended from winged insects but have lost them during
110-427: The course of evolution. By contrast, the apterygotes are a primitive group of insects that diverged from other ancient orders before wings evolved. Apterygotes, however, have the demonstrated capacity for directed, aerial gliding descent from heights. It has been suggested by researchers that this evolved gliding mechanism in apterygotes might have provided an evolutionary basis from which winged insects would later evolve
121-643: The subclass included four orders ( Collembola , Protura , Diplura , and Thysanura ). With the advent of a more rigorous cladistic methodology, the subclass was proven paraphyletic . While the first three groups formed a monophyletic group, the Entognatha , distinguished by having mouthparts submerged in a pocket formed by the lateral and ventral parts of the head capsule, the Thysanura ( Zygentoma plus Archaeognatha ) appeared to be more closely related to winged insects . The most notable synapomorphy proving
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