Abstract Arthropod photoreceptors are versatile sense organs. Any investigation of these organs has to consider that their structure and functional limitations at the moment of fixation depend on many factors: species, sex, developmental and nutritional state of the animal, time of day and ambient light. The microscopic image of an arthropod photoreceptor is always a sample frozen in time and space. Quite often publications on arthropod photoreceptors only provide the name of the species studied, but nothing beyond that. At least the developmental status of the study animals ought to be noted, possibly even the sex and body size. Forty publications on insect and 54 on crustacean photoreceptors were checked for the information that was given about the investigated animals: Out of these papers 40% provide only information on the name of the studied species and nothing else. The aim of this thesis, thus, was to investigate, to what extent the developmental state and the sex of the animal as well as the ambient light conditions affect the structure of the eye of a given species. Five species of arthropods were chosen: (a) the semi-terrestrial isopod Ligia exotica and two aquatic Branchiuran fishlice, Argulus foliaceus and A. coregoni, to represent the Crustacea, and (b) the stick insect Carausius morosus and the spittle bug Philaenus spumarius, both terrestrial, to represent the Insecta. The addition of new ommatidia was studied in a paper on L. exotica, which also dealt with the site of newly added ommatidia. It was found that all of these species had two sessile, large compound eyes firmly positioned on their heads (but fishlouse compound eyes were bathed in haemocoelic liquid). In all species, the compound eye was found to be of the apposition type. The gross structural organization of the ommatidia stayed approximately the same during the whole post-embryonic development. Lateral ocelli of the A. coregoni nauplius eye changed from elongated to spherical between the metanauplius and the 8th stage pre-adult. The sex of the specimens was not found to affect the structure of the eye. In all species, it turned out that the larger the animal and hence the eye, the better its sensitivity. The addition of new ommatidia in the L. exotica compound eye was concluded to take place in the anterior and ventral marginal areas of the eye.

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MicroRNAs (miRNAs) are short, non-coding RNA sequences that are generated from longer primary transcripts (pri-miRNA). These pri-miRNAs are processed by the endonuclease Drosha into a hairpin secondary structure (pre-miRNA), exported from the nucleus and cleaved by the enzyme Dicer to form a duplex RNA molecule. This miRNA:miRNA* duplex is subsequently further processed to form a single-stranded, mature miRNA. miRNAs have been extensively characterized and are known to play important roles in various physiologic and pathologic pathways. One hallmark of miRNAs function is their ability to modulate the downstream activities of protein-coding genes, as well as various other aspects of gene expression, by acting as post-transcriptional repressors of their messengerRNA (mRNA) targets. miR-184 is a highly conserved miRNA gene expressed in the Drosophila nervous system throughout development; and has been shown to target key regulators of differentiation, proliferation and apoptosis. Here we identify a novel role for miR-184 in regulating the development and maintenance of the Drosophila melanogaster post-embryonic nervous system. We present evidence which suggest miR-184 targets (i) paralytic (para), a voltage-gated sodium channel, shown to control neuronal excitability; and (ii) tramtrack69 (ttk69), a transcription factor known to regulate glial cell number and fate determination during embryonic development. In the absence of miR-184, homozygous loss-of-function mutant adult flies demonstrate hyperactive episodes in response to mechanical shock, indicative of increased susceptibility to seizures. Homozygous loss-of-function mutants also exhibit shortened lifespan, as well as reduced group longevity. Additionally, miR-184 deficient mutant larvae exhibit abnormal development of glia and glial progenitors; while expression of miR-184 exclusively in glia - reversed polarity- (repo) expressing cells - up-regulates development of glial cells. Phenotypes of the adult loss-of-function mutant are suppressed by genetic loss of para function; while larval phenotypes are rescued by reducing the genetic dosage of ttk69. These data imply that miR-184 functions to control post-embryonic gliogenesis, as well as in maintaining neuronal excitability and integrity of the Drosophila aging brain. / Biology