The Structure and Function of the Caudal Lamellae of the Damselfly: Pyrrhosoma nymphula (Sulzer) [Odonata:Zygoptera]

Brown, David Wilson (1995) The Structure and Function of the Caudal Lamellae of the Damselfly: Pyrrhosoma nymphula (Sulzer) [Odonata:Zygoptera]. PhD thesis, University of Glasgow.

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A characteristic of all zygopteran dragonfly larvae is the presence of three caudal lamellae attached to the last abdominal segment. The structure of these organs varies according to the species, and four basic forms have been recognised, laterally flattened, saccoid, reduced and triquetro quadrate. The most commonly found type is the laterally flattened lamella, which have been further subdivided into the simple unjointed forms and the constricted, nodate, subnodate and denodate jointed forms. It is not certain what role these laterally flattened lamellae have. Superficially they resemble tracheal gills, and historically this has been presumed to be their main function. However, this has never been proven unequivocally for any species. Because of the confusion over the function of these organs there is clearly a need for a comprehensive study of the lamellae and their function, in a single species of zygoptera. The present study aimed to achieve this by investigating their ultrastructure, respiratory physiology, and ecological and behavioural importance of the lamellae of P. nymphula. Previous experimental studies which have compared the oxygen consumption rates of larvae with against those without lamellae have reached conflicting conclusions. For example, some authors conclude they are important as gills and others that they are not. Interpretation and comparison of the results of these studies is problematic due to the variety of different techniques used and deficiencies in their experimental protocol. Other functions have been suggested for laterally flattened lamellae including as sensory, locomotory, defensive and ion uptake organs. However, only limited investigations of the possible locomotory and defensive functions of lamellae have been carried out to date. These studies have found that in one species lamellae may be important for swimming and that in three further species they may function as attack deflecting organs. High frequencies of lamellar loss have been reported in wild larval populations of at least three species, and this has been related to injuries received during fights with conspecifics. The ecological importance of the lamellae of R nymphula was investigated by examining the frequency of lamellar injury in a wild population of larvae. The frequency of injury was then correlated with environmental factors and population parameters. Those that were found to be significantly correlated with lamellar injury were; larval size, sampling season and habitat type. It was concluded that lamellar injury in wild larvae of this species may be due either to interactions with predators or conspecifics. Further evidence for such a function comes from the study of the breaking joint which attaches the lamellae to the abdomen. This joint was found to be specially adapted to allow autotomy of the lamella with little damage to the larva. These results suggest that the lamellae of P. nymphula are used for defensive autotomy as has been suggested for other species of zygoptera. Investigation of the respiratory morphology and physiology of the lamellae of R nymphula confirmed that they do not function as respiratory gills. The lamellae had large surface areas and no tissues with high metabolic oxygen demand such as ion uptake or sensory organs. Both these characteristics suggested a respiratory gill function. However, the arrangement of the tracheoles within the hypodermis is critical for efficient gas exchange by tracheal gills. The arrangement of tracheoles within the lamellae of P. nymphula was not consistent with that required for a tracheal gill suggesting that this is not their main function. The results of the study of lamellar respiratory physiology also support this conclusion. The oxygen consumption rates of larvae with and without lamellae under varying conditions of hypoxia did not differ significantly, showing that lamellar loss had no effect on larval respiration rates. In addition, the lack of evidence of any ion uptake organs or concentrations of sensory organs suggest that these potential functions for the lamellae of P. nymphula must be rejected. The lamellae of R nymphula were found to be important as signals during aggressive encounters with conspecifics. Larvae with and without lamellae were matched in trials to determine which would gain and maintain occupancy of a perch. Larvae were matched to reduce the possibility of interference from other contests asymmetries such as size, fighting experience and familiarity with the area. In these contests, larvae with lamellae were found to be significantly better at occupying perches. However, lamellar loss did not affect the ability of larvae to initially occupy perches nor their non-lamellar related behaviour. Additional support for an aggressive posturing role for lamellae came from the investigation of their morphology. The lamellae had a large surface area and distinctive pattern both of which would increase their effectiveness as signals. The present study therefore concluded that the main functions of the lamellae of P. nymphula were as signals or as attack deflectors during aggressive encounters with conspecifics or predators.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: K H Lockey
Keywords: Zoology, Entomology
Date of Award: 1995
Depositing User: Enlighten Team
Unique ID: glathesis:1995-74886
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 15:40
Last Modified: 27 Sep 2019 15:40

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