Comparison was made of original and rebuilt cases of mature Glyphotaelius pellucidus (Retzius) (Trichoptera: Limnephilidae) larvae, and the procedure of case building was observed. A long-term case-building observation (1973 hours) was made to discover how cases change after the time in which building of a new case appears to cease. Ontogenetic changes in building procedure were observed in a sample of larvae reared to maturity from eggs. Examination was made of the technique of cutting single leaf discs. Finally, changes to cases prior to pupation were found through examination of pupal cases. The building trials led first to knowledge of the case design and building technique of mature larvae. Definitive cases have two walls of leaf discs covering side walls of small leaf panels. Like other case-building caddis, all larvae made naked will first construct a rough preliminary case of panels, which is replaced by the typical case of discs and panels. This definitive style occurs in all larval instars. Across larval instars, size is the only major difference between cases. By stadium two (of five), the quality of leaf pieces cut by larvae has virtually reached its peak. During the building of a single case by an at first naked larva, it is observed that both leaf piece size and quality improve steadily, with an upward step coinciding with the point at which provisional style changes to the definitive style. For a naked larva, the need for a case evidently has a high priority. The rate of building is most rapid during the initial body covering. Not until after the change to definitive style does the rate decline and are activities such as feeding allowed. These case-building experiments shed light upon development of the building control system in Glyphotaelius throughout larval ontogeny. Case attributes can be broken down into three features–size, quality and procedure. Procedurally, the building mechanism is constant, but apparent adjustments to that part of the control scheme that directs qualitative aspects of building results in size increase and a gradual improvement in the quality of leaf pieces cut.
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